Models

This section provides a brief explanation of some of the output seen on the most popular sites available on the WWW that store, or otherwise make available NWP output.

Information on the various models

(NB: information presented with no warranty!)

Although a nominal data initialisation time is shown (00, 06 UTC etc.), modern NWP analysis routines 'assimilate' data over a period either side of these times. With integration of asynoptic data (i.e., not at standard times), plus the use of previous operational (or special pre-analysis) 'runs' to provide background fields, the use of such DT (data time) indicators is more a convenience for us mortals!

A computer completes its calculations quite quickly with the modern 'super numbercrunchers' in use, and the computations are in any case performed using code that is written to optimise speed: however, always bear in mind that there will be a delay in availability to us on the Internet due to the post-processing of the raw NWP data into a format that can be squirted onto data-networks.

 Model  UKMO- GLOBAL
 Data times: (UTC)
(i.e. the nominal time for initialisation, raw data ingest etc.)
 00 & 12 (but other runs are performed to 'update' the analysis)
 Number of full 'runs' per 24hr:  2
 Out to T + ..... (hr)  144
 Usually available:
(on the Internet .. the output will be available in a more timely fashion on 'closed' networks. )
 0500 - 0800; 1800 - 2100
 Other information:  Currently (winter 2005 / 06) the UK Global model has the following specifications:
~40 km horizontal resolution & 50 levels vertical resolution: a change in late 2005 put several additional levels into the stratosphere (and extended the 'top' of the model too) as well as reducing the grid-length from 60km: I understand that it is intended to go to 70 levels (twenty more in the troposphere) sometime in 2006.
 Note that a limited-area, finer-resolution (~12km) model [ North Atlantic European / NAE ] is run 4 times daily, with an earlier cut-off time, out to T+48. However, this is not (generally) available on the Internet. It currently takes it's boundary conditions from the previous main Global model run, updated by a 're-analysis' phase.
Other models, at 'mesoscale' resolution (~11km or less) are also run, using the above for 'boundary' conditions - these are also NOT available publicly. Of particular note is the future operational running of the UK 4KM model, expected late 2005 or early 2006. (see HERE for more on UKMO models.)

 

 Model  NOGAPS
 Data times: (UTC)
(i.e. the nominal time for initialisation, raw data ingest etc.)
 00, 06, 12 & 18
 Number of 'runs' per 24hr:  4 (but 2 via some Internet portals)
 Out to T + ..... (hr)  144
 Usually available:
(on the Internet .. the output will be available in a more timely fashion on 'closed' networks. )
 On the US Navy site .. 0500 - 0600 & 1700 - 1800; On other sites, circa 0800 & 2000.
 Other information:  On the US Navy site (if you get past the various security checks etc.), you can get intermediate hour runs as well.

 

 Model  NCEP - GFS (formerly: AVN)
 Data times: (UTC)
(i.e. the nominal time for initialisation, raw data ingest etc.)
 00, 06, 12 & 18 (not all 'runs' are carried by all Internet sites.)
 Number of 'runs' per 24hr:  4 (but 2 only to some Internet sites)
 Out to T + ..... (hr)  384 (high resolution to T+180/7.5 days; lesser resolution thereafter .. to 16 days) [ see note below re: resolution ]
 Usually available:
(on the Internet .. the output will be available in a more timely fashion on 'closed' networks. )
 about 4.5 to 5 hr after initial time (e.g. 00Z issue usually available by 05Z), but perhaps earlier on the US Navy site. Note that the first 180hr will be available fairly quickly; the remaining files (at courser spatial and temporal resolution), a little way behind.
 Other information:  NB: Now (since April 2002) the primary model run from NCEP - anything labelled 'MRF' are essentially 'look-alike' output based on the full Global AVN run as above.
[ Historical: Up to April, 2002, the 'MRF' (medium-range forecast) run was output once daily, to T+384, with the same physics, mathematics etc., as the (then) AVN, (now) GFS, but with a later cut-off time to allow extra data to be used - supposedly to achieve a better end result. Tests showed that it was no longer necessary to do this, hence the standardisation on the 4-times daily GFS (AVN) as above. (to emphasise, although these models run out for over 2 weeks, don't take each time-step slavishly; major errors can occur beyond even 3 or 4 days lead time.)
Resolution: Currently (Winter 2005/06), the GFS has an approximate horizontal resolution of ~40km up to T+180h, and ~80km thereafter; there are 64 vertical levels. However, bear in mind that model development may render these figures obsolete: visit this site for updates.

 

 Model  ECMWF
 Data times: (UTC)
(i.e. the nominal time for initialisation, raw data ingest etc.)
 00 & 12
 Number of 'runs' per 24hr:  2
 Out to T + ..... (hr)  240 (i.e. 10 days) ... but limited output to Internet users .. either 6 or 7 days according to site, fields etc.
 Usually available:
(on the Internet .. the output will be available in a more timely fashion on 'closed' networks. )
 On the EC site, and those carrying such data (e.g. Wetterzentrale), then about 6 to 7 hours after data-time (e.g., 00Z run available between 06 and 07Z). However, where additional processing is required, then a considerable delay from this time may be encountered.
 Other information:  This model runs out to 10 days, but only the first 6 days (7 days for mslp fields) are freely available via these sites.

 

 Other models  DWD: GME (Global Modell Ersatz) (or Global model replacement)
 Data times: (UTC)
(i.e. the nominal time for initialisation, raw data ingest etc.)
 00, 12 (& a short update run at 18)
 Number of global 'runs' per 24hr:  2 (for full output)
 Out to T + ..... (hr)  174 (i.e. just over 7 days) ... but limited output to Internet users.

 

 Other models  MétéoFrance: "ARPEGE" (Arpege is the acronym for Action de Recherche Petite Echelle Grande Echelle: Research Project on Small and Large Scales. This is the successor model to the EMERAUDE baseline model.)
 Data times: (UTC)
(i.e. the nominal time for initialisation, raw data ingest etc.)
 00, 12
 Number of global 'runs' per 24hr:  2
 Out to T + ..... (hr)  240 (i.e. up to 10 days) : NOTE: French model output to Internet users very limited.

 

NWP basics

NWP prediction is performed by solving mathematical equations which describe the behaviour of the atmosphere. The principal equations involved, and the purposes for which they are evaluated are:-

Three equations of motion:  Under Newtonian laws, the acceleration of a particle is proportionate to the forces acting upon that particle. These equations handle the 'balance' between the pressure gradient force (horizontal & vertical), the Coriolis 'force' (primarily horizontal) & acceleration due to gravity (principally in the vertical).  Fundamental to the analysis & forecast of wind flow, hence advection of air, convergence / divergence, vorticity development, vertical motion (in concert with the continuity equation) etc.
Continuity equation  A defined 'parcel' of air retains the same mass, regardless of how it moves about, or how it is deformed: 'conservation of mass'. Using this statement, a link is established between vertical and horizontal convergence.  Used to define vertical motion (not measured directly) from fields of horizontal motion ('the wind'), and hence key to describing formation / dispersal of clouds, precipitation etc. Indirectly involved with diagnosis of development due to vorticity changes.
Thermodynamic (or heat) equation  Relates heat transformation due to atmospheric processes to 'parcel' volume & internal energy changes as air ascends / descends: the 'bicycle pump' analogy.  Required to describe the the various adiabatic processes mathematically, key to understanding the degree of stability of an air mass.
Moisture equation  In similar fashion to the continuity equation (above), this describes the fact that moisture is not gained or lost, but can change phase (with consequent latent heat exchange), or is diffused via mixing actions (i.e. exchange with water / land surfaces).  Describes the changes in humidity content of an air mass (hence dew point, vapour pressure & indirectly to wet-bulb variants), which in turn are used implicitly to handle cloud / precipitation processes in the 'model' atmosphere.
Equation of state  Relates the density, pressure & temperature of a 'parcel' in a perfect gas; the atmosphere is not a perfect gas, but can be assumed to behave as such.  Used to describe mathematically changes in these fundamental properties of a parcel as they move vertically, and in concert with the thermodynamic equation, are used to describe the behaviour under adiabatic transformation.
Hydrostatic equation  In the vertical, there are two (main) forces which are roughly in balance: the upward-directed force ("buoyancy") due to the pressure gradient [ pressure falls with height ] & the downward force due to gravity. Any disturbance of this balance following atmospheric development leads to vertical accelerations.  Describes the relationship between height & pressure (the 'barometric equation') and is used in the algorithms to diagnose atmospheric stability; also important when considering the explicit modelling of small-scale vertical motion.

And the variables usually 'carried' within the model schemes are:-

  • Three components of wind velocity: two horizontal & one vertical;
  • Air density;
  • Temperature (or Potential temperature);
  • Pressure;
  • Humidity (usually in terms of the humidity mixing ratio):
    [ In addition, some models (certainly small-scale models) will also 'hold' an explicit representation of liquid water and ice fraction, surface wetness / character (i.e. ice/snow cover), vegetation development etc. ]

We can't possibly analyse each field at the level the atmosphere produces: it is difficult enough to describe the broader (or synoptic) scale motions. This means that instabilities are already 'built-in' from the point of analysis - which if not 'damped' mathematically can feed through to the ensuing forecast in an unstable fashion. This used to be a common problem with early NWP, but is a rare occurrence now.

All analyses are approximate (human or model); the key is obtaining the most consistent such starting point. NWP 'runs' don't start with a 'blank chart' - they integrate short-period forecasts from a previous solution (either the 'main' run, or a special 'update' calculation) and by careful injection of new data, a "best solution" starting point (or initialisation stage) is achieved.

There are broadly two types of atmospheric computer model in use around the globe:

In a grid-point model, the variables above are held at intersections of a regular matrix, or at fixed points between the intersections, and it is at these points that the equations outlined above are applied. Finite-difference calculations are used to solve the equations, i.e. solutions for the variable concerned 'nudge' the calculations a small fraction of time (a "time-step"). A grid point model (such as the UKMO Global) is usually defined in terms of the number of vertical levels, and the horizontal (approximate) grid-length.

In a spectral model, the variables are represented by periodic functions which are the sum of several waves, the calculations being performed via various methods such as Fourier series, spherical harmonics etc. The fields are therefore represented by wave functions of differing wavelengths. To take a 'real world' example, it is possible to model the behaviour of a cork bobbing on the surface of a river by considering the individual energy strands carried within that flow, then integrating them at one point - the cork. Spectral models (such as the GFS, NGP & EC) are regarded as computationally more efficient, and the solutions are available for every point on the globe, rather than tied to a regular grid array. These models are usually defined by the numbers of vertical levels and the wave-number truncation: thus T382L64 would indicate 'triangular' truncation at wave-number 382, with 64 levels in the vertical. To convert the wave-number 'horizontal resolution' to an approximate grid-length, divide 360 by the wave-number, divide by 3 (it takes 3 grid-points to define a wave), then multiply by 111.1km (per deg. latitude).

Grid spacing (or wave numbers) will determine computational resources required - shorter grid-length (or more wave representation), then the greater the amount of calculating power required: also remember that the calculations are performed both horizontally and vertically. Vertical levels are not evenly spaced: there are more levels near the ground (the boundary layer) & around the tropopause, to capture the often high degree of variability in the altitude bands, but even then, there are never enough!

Some elements are not even attempted, nor can they be attempted e.g. CB/TS, turbulence, fine-detail of precipitation processes etc. To use computing time more efficiently, approximations and assumptions are used [ parameterization or parametrisation ], both when dealing with the application of the primary equations and the mathematical methods used.

The numerical simulation, both in it's analysis and forecast output can NEVER be a perfect representation of the real atmosphere - and it is difficult to conceive of a time when it might be. Users should be aware that small-scale processes (e.g. thunderstorms, sea-breeze circulation) cannot be explicitly handled in the modern generation of global models. However, these same important processes can have a significant effect on the broader-scale evolution; errors generated via the parameterization (or parametrisation) routines can have significant 'downstream' consequences.

The real atmosphere is highly complex; it is fluid-flow performing in four dimensions, which numerical models can only approximate to the processes involved. Some of the motions / processes (such as the micro-scale movement of the air itself) cannot be handled, and even fairly large 'blocks' of weather features, such as individual thunderstorms or local turbulent flow can only be 'parametrized' (or 'fudged' if you prefer).

But this introduces problems much as if you 'nudge' a supermarket trolley: it is not always possible to predict the outcome! It is no wonder that the trough disruption process for example, which is taking place throughout a large vertical slice of the atmosphere, and involves a skewing of the thermal profile at differing rates, is often poorly handled. And get the disruption wrong - everything following on is going to be in serious error.

You'll read old soaks like me bang on about how the basis of a good forecast is a good, detailed analysis. This adage applies particularly to NWP routines: a human can sometimes 'correct' or allow for a poor or inadequate analysis through experience & 'gut instinct': a computer will simply use the data it has and employ the routines written for it without mercy! Consider the analogy below:-

I sometimes think predicting the various weather variables numerically is like trying for work out what the EXACT time of arrival of a bus on a typical 'town-circular' route might be, as it is about to leave the terminus (T). You would need to model a large number of variables as below:-

bus_round_loopA To predict the time-of-arrival at stop 1 is fairly straightforward; the distance will only be a matter of a few hundred metres, and a knowledge of the average speed, instantaneous traffic flow etc., between T & 1 will yield a prediction within 60 seconds of the actual time, perhaps below 30 seconds. The short time-step, plus the restricted impact of 'external' variables allows a fairly accurate prediction. [ In meteorology, this is analogous to a 'nowcast' routine, with results over half-an-hour to 6 hour periods. ]
bus_round_loopB To predict the time at stop 2 is complicated by several factors: we need to predict the numbers of passengers getting on / off at the previous stop & whether they tender cash, the exact fare or use some form of pre-pay card / concession: we also need to predict the traffic flow between 1 & 2, and also make allowance for being held (or not) at the light-controlled junction. All these variables can be predicted in advance, but require a much greater amount of information than for the previous time-step. The accuracy of prediction at stop 2 could drop dramatically, though for the majority of cases, it will still be within 60-90 seconds. [ In meteorology, this would simulate the problem for the short-range forecast - say the first 48 hours of a run. ]
bus_round_loopC Predicting the time-of-arrival at 6 (and later stops) to any sort of accuracy (before it has left T remember) is now fraught with difficulty. Passenger loading (on/off), ticket types / difficulties, road-traffic density, timing of light-controlled crossings etc., all need to be 'built-in' to this model, either explicitly (by monitoring actual traffic / passenger behaviour) or implicitly by using 'notional' numbers based on an previous experience (analogous to parametrisation). To achieve an accuracy of +/- 3 minutes for stops 6, 7 and 8 on a typical town-circular bus route on any given trip is very difficult. [ In meteorology, this typifies the problems when trying to forecast 5 or more days out. ]

So it is with the weather: a computer model of the atmosphere must have information about a wide variety of variables that might impact upon the outcome - and the longer down-stream the forecast, the more information & mathematical processing is required.

. . . and there is the added problem that whereas the bus tends not to modify it's environment as it runs on it's course, in meteorology developments can / will significantly modify the broadscale pattern, which in turn will affect the way the forecast 'pans out'!


So, NWP models are trying to predict an outcome given the information they have (which can be limited) & using the current understanding of atmospheric physics / thermodynamics (which is developing all the time); at short 'lead times', they will do well in the main, but further 'down stream', errors can be large.

To try and handle the uncertainty almost inevitable in modern-day computer output, the concept of the 'ensemble' is used: a control run is performed (same model physics as the 'main run' but at lower spatial & temporal resolution), and then the initial field is 'perturbed' (or nudged) slightly, and a series of further forecast runs performed. Again, reference to a 'real-world' example might help. Here we have a ski slope, where the skier starts a downhill run from point 'A' and ends up at point 'D'. Each time he or she 'pushes-off' from the top, the starting point is very slightly different.

ski_slope_basic From the same starting point (analysis), and using the same physical structure (basic equations), we get a different result (forecast).
ski_slope_A At the top of the slope (B) such deviations are of little consequence - this might represent the first 6 to 12 hours of a weather forecast;
ski_slope_B . . . but at 'C' & 'D', the deviation becomes somewhat more significant. It may be that the upper height contours (or mean-sea level pressure) is the parameter to be forecast: it can be seen that early model deviation at B & perhaps C would have little impact upon the final forecast: at 'D' though, considerable variation can be found, which might make the difference between a roaring 'Storm-Force 10' and just a good 'Force 6/7 blow'.
ski_slope_C In an ensemble prediction system (EPS), several 'runs' are made using the 'control' as the core 'driver', and the results can be clustered together into like patterns. The human analyst will have high confidence in a solution where there are a large number of ensemble members (the individual ski-slope runs) giving a similar solution; the confidence would be poor where the results are peppered over a wide span of outcomes.

To see an example of ensemble output (with explanation) from the GFS EPS . . . click HERE.

The ECMWF ensemble prediction system (EPS) is regarded as one of the best in the world. It uses 51 members in its scheme: one 'control' run (a half-resolution horizontal analysis / two-thirds vertical resolution - based on the main operational output) & 50 members with an analysis slightly perturbed from the control member. Unfortunately, they aren't available on the Internet!


One question that is often asked is .. if all models use the same data, why do they come up with different results?

There are so many variables involved with atmospheric modelling, that I'm surprised when models do agree: some of the factors that distinguish one centre's output from another are:

  • Most importantly, the initial condition (the 'analysis') on which the subsequent forecast calculations are based, may be assessed in different ways. This is much as if you had two forecasters faced with the same set of observations, but who produce a slightly different analysis - location of front, depth of low, sharpness of trough etc.
  • The various centres will (usually) have available all the same raw data, but what they do with it may be different; they may not assimilate all the data, or may be selective about which datasets are used - this applies particularly to satellite-derived data.
  • A different 'weighting' may be applied to data: for example, an isolated ship in a data-sparse area might have greater influence in one centre's scheme than another.
  • The methods used in numerical analysis are now highly complex and as has been noted elsewhere, data are not just used at the 'primary' DT's (00, 06, 12 & 18 UTC), but are used asynoptically - but the methods of doing this are different from centre-to-centre, hence different analyses on any one occasion, and therefore a different forecast outcome.
  • Then of course there are different grid lengths (or wave numbers for spectral models), and a variation in the number of vertical levels used: these later will be spaced differently.
  • It is impossible for every nuance of the world's orography (depth / shape of valleys, height profiles of hills & mountains etc.) to be 'carried' within a numerical simulation: even coastlines & islands must be smoothed - think of the ins & outs of the Norwegian fjords or the clusters of small islands in the Aegean. So each centre will have a slightly different representation of the physical earth - it may not matter when output are averaged over lengthy periods, but for an individual situation, the interaction between land surface and the atmosphere may be crucial.
  • Some models are 'tuned' to produce results at higher definition for specific purposes - e.g. the tropical models, GCM's used for climate simulation work or mesoscale models used for short-range, high spatial resolution work.
  • The time-steps (or mathematical procedures used) will be different one from another, even if the actual equations [ see above ] are the same.
  • The way some processes are parameterised will be different - some may have an explicit (and 'near-real-time') representation of the surface type, whilst others will use climatology, or have a mixture of methods.
  • Models will have different ways of coping with 'polar convergence' - the physical distance for a given degree of longitude will vary from equator to pole - not only at the surface but aloft as well. Some centres will skew the grid they use to make sure that the primary area of interest is over them, so 'notional' poles will result which may affect, even if only slightly, the forecast over the domain further away.
  • Another variable that is not always obvious is that modellers may add 'fudge' factors (or correction factors) to offset know model biases. For example, no matter how many levels are squeezed in at the top of the troposphere / lower stratosphere, capturing the 'top-strength' of a jetstream (comparatively shallow vertically) is difficult. Once identified, a percentage correction may be added to the output at the post-processing stage, but in some situations this may not give an appropriate result.

All centres will have available the same raw data, but what they do with it will be different - hence the large variations on occasion, particularly at the longer lead-times.


Bibliography:

Dynamical meteorology: an introductory selection; Ed: B W Atkinson
(Methuen 1981 / The Royal Meteorological Society)

and visit the Met Office web site HERE for a lot more on numerical modelling, and in particular the way the UK national weather provider goes about it.

Ensemble: 'Meteogram'

This is an example of a 'Plume' or 'Meteogram' (or Meteogramme in German), that is available on the Wetterzentrale site. For each site, selected from the 'drop-down' menu, output from the Ensemble run of the NCEP GFS model can be viewed, specifically the temperature at 850 hPa ("850 hPa Temperatur") and 12hr accumulated precipitation ("Niederschlag"). [ Although not stated on the page, I assume the precipitation is rainfall-equivalent, as there is no allowance for explicit phase-change within the diagram. ]

Example Meteogram for Manchester

AT THE TOP OF THE DIAGRAM (TEMPERATURE AT 850 hPa):-
The thick red line shows the 30 year average value.
The thick blue line shows the output from the Control Run of the Ensemble model (lower resolution than the operational!)
The thick white line shows the ensemble mean (i.e., very roughly, an 'eyeball' guide to the trend over the period).
The thin differently coloured lines are individual ensemble members (11).

AT THE BASE OF THE DIAGRAM (PRECIPITATION TOTAL FOR PREVIOUS 12HR):-
The thick white line shows the ensemble mean.
The thin differently coloured lines are the individual ensemble members.

Cautionary notes:
+ ensembles are best used to detect future trends rather than to define the precise outcome on a particular day, particularly beyond about 3 days.
+ where the ensemble members cluster tightly together (as they should in the first couple of days of any output), then confidence in the outcome will be high.
+ where many ensemble members cluster tightly together, with only a single 'outlier' to either side, then confidence in the mean of the dominant cluster can be regarded as reasonably high (but not a 'perfect' solution).
+ the greater the 'spread of the fan' as forecast lead time increases, the more uncertain is the outcome. This will be the normal pattern in mid-latitudes.

[ For more on Ensembles, Operational runs, Control runs etc., see the Glossary ]

Sembach

Example of output

This section is simply describing what the various lines, coloured areas are, what interval/units the contours, isobars, colour-filled areas use etc. The use, or meteorological significance of the fields are dealt with in a separate section here.

The Sembach site is here.

IMPORTANT: I am not going to attempt a description of the full range of products, just the ones that might be most useful to those who want to 'dip-in' without much knowledge.


UKMO (with forecaster interpretation)

NB: The output viewed on this site is based on 'raw' Met Office model products, with forecasters at Sembach interpreting same by adding the fronts - the cloud representation is based upon model relative humidity fields. Bearing in mind that UKMO 'FAX' output is sometimes heavily modified as seen on other sites (e.g. Wetterzentrale, Westwind), there may be significant differences seen at times from (apparentley) the same centre's output.

 Example: Brief description of parameters (etc.) on charts shown.
 
sembach output example
Yellow lines: MSLP isobars (units = hPa / interval = 4 hPa)
Conventional frontal symbology: click HERE for examples of types of fronts etc.
Grey shading: Cloud-amounts (roughly for the layer 6000 to 14000 ft [ 1800 to 4300 metres ] /i.e. 800 to 600 hPa), inferred / integrated from upper level relative humidity fields. See caption at foot of each chart for explanation.
Blue/Green circles (varying size) or crosses (varying size & shapes), or triangles: precipitation types, intensity etc. See matrix at foot of each diagram for more details.

 


 

Unisys

The Unisys site is here. It is blessed with an excellent guide to the use of its products here.

Westwind

This section is simply describing what the various lines, coloured areas are, what interval/units the contours, isobars, colour-filled areas use etc. The use, or meteorological significance of the fields are dealt with in a separate section ... here

The Westwind site is here.

IMPORTANT: I am not going to attempt a description of the full range of products, just the ones that might be most useful to those who want to 'dip-in' without much knowledge. Also, the fields presented below are not necessarily from the same situation, run-time etc. They are there simply to aid a quick identification for each product. Many of these products 'mirror' those on the Top Karten site - so refer to the notes for that site to get the complete picture.

 


Georg Mueller (relay from Wetterzentrale site of GFS output (formerly AVN & MRF products)

 Example: Brief description of parameters (etc.) on charts shown.
 500 hPa, Bodendruck (MSLP), Re-Top (Thickness) (500/1000)
mslp_tthk_500
White lines: MSLP isobars (units = hPa / interval = 5 hPa)
Dark grey/dash & double-dot lines: thickness isopleths for the layer (500-1000) hPa. (units = dekametres [dam] / interval = 18 dam)
Colour shading: 500hPa height, colour coded as at scale by diagram. (units = dam / interval = 4 dam). Orange/Red high values - blue/indigo etc., low values.
Black line: the 552 dam contour (of the 500 hPa surface) - highlighted as the approx. median value to trace major troughs/ridges in pattern.
 Bodendruck (MSLP) und 850 Aeq.PT (Theta E)
mslp_850ThetaE
White lines: contours of 850hPa isobaric surface. (units = dekametres [dam]/ interval 4 dam)
Black solid/dashed lines: temperature at 850 hPa (solid >= 0degC; dashed < 0degC: units = degC / interval = 5 degC.)
Colour shading: value of Theta E (equivalent potential) temperature at 850 hPa as per shading chart at side/interval = 2 degC.) A better tracer than temperature alone, because it is based upon the dew-point as well as air temperature.
 Richtung (direction) & Betrag (strength/force) 10m Windes (winds)
10 metre wind fields
White 'arrows': 10m (i.e. 'surface') wind using conventional wind-barbs: e.g. arrow points in direction FROM which wind is blowing (NOT with the wind); a long feather = 10knots, a short feather = 5 knots, a triangle = 50 knots & speeds are built up using these combinations.
Colour shading: Wind speed (at 10m) colour shaded as scale at side of diagram (units = knots / interval = 2 knots). Blue/green light winds - yellow strong winds - red gales or stronger.
 2m Temperatur
2m temperature
Black/thin contours: 2m (i.e. 'screen' - level) temperature. (units = degC / interval = 10degC).
Colour shading: 2m temperature as scale at side of diagram. (units = degC / interval = 3 degC). Orange/Red high values - blue low values.
 6h - Niederschlag (Precipitation)
6 hrly ppn accum
Colour shading: 6 hourly precipitation (rain, snow etc.) in the period up to the time of the chart. See scale at side of chart - the deeper the blue the more precipitation - purple/violet etc., exceptional rainfall/snowfall accumulations. (units = mm): also displayed is a field labelled 'rot=Konvektion' (red dashed lines) which appears to indicate areas of PPN with a high convective / instability element.
 700hPa und Vertikalbew. (Vertical motion)
700 vertical velocity
White lines: contours of 700hPa isobaric surface. (units = dekametres [dam]/ interval 4 dam)
Black line: highlighted 700hPa contour at 300 dam. Useful to trace major changes in the pattern.
Colour shading: vertical motion as scale at side of diagram - yellow to red to violet: upward values; green to blue to violet: downward values. (units = hPa/h; interval 4 hPa/h)
NB: negative is upward motion!
 Bodendruck (MSLP), Wolken (Cloud)in %, ReTop (Thickness) 500/1000
mslp_cloud_tthk
White lines: MSLP isobars. (units = hPa / interval 5 hPa)
Coloured lines: 500-1000hPa thickness isopleths (units = dam; interval 18 dam)
Monochrome shading: % cloud cover based on medium level moisture (i.e. 'frontal') - see scale by side of diagram (black low % cloud values, light grey/white high % cloud values)
 850 hPa und Temperatur
850 tt etc.
White lines: contours of 850 hPa isobaric surface (units = dekametres [dam] / intervals = 4 dam).
Black lines: isotherms of 850 hPa temperature. (units = degC / interval = 5 degC).
Colour shading: temperature at 850 hPa- see scale at side of chart. (units = degC / interval = 2 degC, blue = cold, orange/red = warm).
 500 hPa, T (Temperature) und Bodendruck (MSLP)
mslp_500
White lines: MSLP isobars (units = hPa / interval = 5 hPa).
Dark grey/dash & double-dot lines: temperature at 500 hPa (units = degC / interval = 5 degC).
Colour shading: 500hPa height, colour coded as at scale by diagram. (units = dekametres [dam] / interval = 4 dam). Orange/Red high values - blue/indigo etc., low values.
Black line: the 552 dam contour (of the 500 hPa surface) - highlighted as the approx. median value to trace major troughs/ridges in pattern.
 200 (or 300, 500 etc.) hPa streamlines ("stromlinien") & wind strength ("windgeschwindigkeit").
200hPa output
 White lines (with arrows): streamlines following the wind flow at the level given. (Levels based on pressure levels - such as 500, 300, 200 hPa)
Colour shading: speed of wind - see the scale at the side of the chart; nearer to red / purple, high values; nearer to green/blue lower speeds.
NOTE: 300 or 200 hPa good approximation to jet level winds.

 


GME (German Weather Service, DWD), 'Global Modell' [ output via wetter.com ]

 Example: Brief description of parameters (etc.) on charts shown.
 10m Wind GME (kt) [ also isobaric pattern ]GME MSLP and Winds Blue lines: isobars (mslp) at 5 hPa intervals: (NB: as this is a German chart, High's are labelled 'H' (Hoch or Hochdruckgebeit - area of high pressure) and Low's are labelled 'T' (Tief or Tiefdruckgebeit - area of low pressure.)
Thick blue line: 1015 hPa isobar (nearest value to ISA standard.)
Black wind arrows: 10m wind forecast in knots (usual convention - see elsewhere).
Colour shading: Beaufort force groups - see scale at bottom of each chart.
 Tiefe (low) or Mittel (medium) Bewölkung (cloud cover)Low cloud and MSLP  Black lines: Isobaric pattern, every 5 hPa and 1015 hPa isobar highlighted in bold (see above).
Grey shading: from light to dark grey, according to scale (usually bottom/left), for 'FEW', 'SCT', 'BKN' & 'OVC' cloud amounts. (See the main FAQ here.)
 Example of an analysis with 'named' pressure systems (ex- Free University of Berlin)FUB analysis  Usual monochromatic chart symbology, but the major pressure centres are named: BLUE names for high centres, and RED names for low centres.

 


Wetter3 output based on GFS model (selection only .. many of the ideas are covered above).

 Example: Brief description of parameters (etc.) on charts shown.
 850hPa Fronten (objective frontal zones), Bodendruck (mslp) & 700hPa Vertikalbewegung (vertical velocity)
850Fr_mslp_700VV
White lines: MSLP isobars. (units = hPa / interval = 3 hPa (NOTE: unusual choice of standard isobars).
Colour-fill/shading: 700hPa vert. velocity - orange > red > purple implies upwards / negative values; green > blue implies downwards / positive values (see scale at right; units = hPa/hr.
Grey/thin lines: based on model parameters - close 'packing' indicates near-surface frontal zones using objective techniques.
 6 - stuendiger Niederschlag (6hr precipitation total in mm to hour given)6hr rainfall totals  Blue colour shading: light blue, low precipitation totals, dark blue (or purple), higher accumulation: see scale at right of diagram.
White lines (thin): contouring of same at 1, 5, 10, 25 and 50 mm.
 Niederschlagsstaerke (precipitation intensity) [mm/hr at hour given: 6hr time-steps] und Niederschlagsform (precipitation type)rain_snow output  Blue (Blau)/ Purple (Rosa) shading: blue shading for rain (regen) or freezing-rain (eisregen); purple shading for snow (schnee) or soft hail / snow pellets (Graupel). Intensity is given by the depth of shading - see scales at right-hand side (in mm/hr).
White (thin) lines: intervals of 0.1, 1, 2 (etc.) mm/h; Note that for the snow, the PPN is 'rainfall equivalent' not snow depth. (See notes HERE.)
 850hPa aequivalent potentielle Temperatur (ThetaE), Bodendruck (mslp)
850ThetaE_mslp
White lines: mean sea level isobars (see above) [hPa]/ interval = 5 hPa
Colour-fill/shading: 850hPa ThetaE field- see scale at right of chart. (blue = low, orange/red = high).
Light-grey lines: isotherms of 850 hPa ThetaE/interval=3 degC.
 2m Temperatur (2m temperature/degC)
[ similar conventions used for:
6-stuendige 2m Minimumtemperatur (min. temp), 6-stuendige 2m Maximumtemperatur (max. temp & 2m Taupunkt (dew point)].2m temperaturesW3_Tmax
Thin grey (with intermediate thicker white) lines: 2 metre (i.e. 'screen - level') temperatures. (units = degC / interval = varying 2 to 5 degC)
Colour-fill/shading: 2 m temperatures (actual, maximum in 6hr, minimum in 6hr or dew-point: see scale at right of chart. (purple/blue = low values, orange/red = high values). [NB: the dew point is a measure of the absolute humidity of the air - i.e. how much water vapour is carried at the particular level given - in this case 2m above ground level.]
 Tiefe (low), Grenzschicht (boundary-layer), Mittelhohe (medium) or Hohe (high) Bewoelkung (cloud) Isolinien (% probability)
LowCloud
Colour-fill/shading: shading (green) according to the legend at right-hand side of chart - deeper green, higher-probability of cloud. (I'm not sure of the cut-off for 'boundary layer' versus 'low' cloud.)
Blue/grey dashed (30,45), or solid (60+): lines of equal % probability of cloud cover at the appropriate level.
 10m wind (in knots/kn)
SFC wind
Standard wind barbs: where the barb points to the direction FROM WHICH the wind blows: with half-feather representing 5kn, a full-feather 10kn and a triangle 50kn.
The barbs are also colour-coded, where the sequence deep orange > red > violet indicates stronger to stormy winds: see the scale at right-hand side.
 850 (and 700, 500, 300, 200) hPa Geopotential (height fields) and Temperatur (temperature/degC)
850ht and T
 Black lines: contours of the pressure level given (i.e. 850, 700 etc.); intervals = 4 or 8 dam, with one contour highlighted as follows: 850hPa (144dam), 700hPa (300dam), 500hPa (552dam), 300hPa (912dam) and 200hPa (1176dam). The highlighting of one 'standard' contour allows broadscale pattern changes to be seen on image-loops.
Colour-fill/shading: 850 (or 700, 500 etc.) temperature fields - see scale at right-hand side of diagram.
White lines: on some charts, isotherms in degC, intervals usually 5degC .. see labels.
 850hPa FQn Divergenz von Q-normal (divergence of the component of Q, normal to the isotherms) und Fronten (objective frontal zones).
850FQnDiv
 Colour-fill/shading: deep red negative FQn, deep blue positive FQn: units = fractional degs. Kelvin per sec*m^2. (NB: some areas 'blank' data usually seen.)
Thin grey contours: frontal zones inferred from 850hPa model parameters.
 700hPa relative Feuchte (relative humidity %).
700RH
 Green shading: 60% or higher relative humidity at 700 hPa;
Lines (dashed low values, solid high values): 15, 30, 45, 60, 75 & 90% relative humidity at 700 hPa.
 500hPa Geopotential (height field) and Vertikalbewegung (vertical velocity in hPa/h).
500 and VV
 Black lines: contours of the 500hPa surface [ interval=8dam ]: 552dam contour thicker (used as a 'tracer').
Colour-fill/shading: see the legend at the right of the diagram. (yellow > orange > red > purple .. increasing vigour of upward motion [negative values]; green > cyan > blue .. increasing vigour of downward motion [positive values].
 500hPa Geopotential (height field) and absolute Vorticityadvektion (absolute vorticity advection/ per h^2)
500 and VortAdv
 Black lines: contours of 500hPa field, with 552dam highlighted - 8dam intervals.
Colour-fill/shading: absolute vorticity advection - for values see scale on right side of diagram. (Green > cyan > blue > purple: increasingly negative vorticity advection [ NVA / non - developmental ]; yellow > orange > red: increasingly positive vorticity advection [ PVA / developmental ]
 
500hPa Geopotential (height field) Bodendruck (mslp) & Relative Topographie (Total thickness).500 and TTHK
 White lines: mean sea level pressure isobars (in hPa or mbar); intervals=5 mbar.
Black lines: 500hPa contours in dam; intervals=8dam, with 552dam highlighted (thick black line).
Colour-fill/shading: 500-1000 hPa total thickness (TTHK) field (green > cyan > blue > purple: increasingly cold; yellow > orange > red: increasingly warm. See scale at right of diagram.
Light grey lines: TTHK contours at 4 dam intervals.
 500hPa Geopotential (height field) and Schichtdickenadvektion (layer-thickness advection)
500 and TTHKadv
 Black lines: 500hPa contours (see notes above: interval 8dam)
Colour-fill/shading: thickness (500-1000hPa layer) advection: - see scale at right-hand side, expressed as a fractional change in Kelvin (or degC) per hour.
(green > blue > purple: cold advection; yellow > orange > red > purple: warm advection: the deeper the colour, the stronger the advection. Crudely: strong WARM advection coupled to FALLING contour height=> strong SURFACE DEVELOPMENT.)
 500hPa Geopotential (height field) & Divergenz des Q-Vektors (divergence of Q-vectors).
500 and DivQ
 Black lines: 500hPa height field (see notes elsewhere - similar ideas with one contour 552dam highlighted, thick black).
Colour-fill/shading: FQ values as at the scale on right-hand side of diagram ... yellow > orange > red > purple: negative; green > blue > purple: positive. (See notes elsewhere).
 KO-Index & 500hPa Vertikalbewegung (vertical velocity).
KOIndexVV
 Varying width blue-black lines: KO-Index, every 2 degC (or K); one of many instability indices, based on Theta E at 1000, 850, 700 and 500. Broadly, higher negative values imply greater potential instability.
Colour shading: vertical velocity (or "V V" negative, upward only) - deeper red/purple, then more vigorous upward motion implied.
A combination of high V V (highly negative values) and high KO value=> strong risk of vigorous convection in frontal situations.

 

 UKMO products (modified output: 'FAX')

 Example: Brief description of parameters (etc.) on charts shown.
 FAX output (final product)
UKMO FAX product
These charts are simply standard 'FAX' charts which depict MSLP isobars (4 hPa intervals), fronts, pressure centres etc., to conventional symbology. For more on these, see the FAQ entry here) and for Conventional frontal symbology: click here for examples & further notes.

 

 ECMWF products.

 Example: Brief description of parameters (etc.) on charts shown.
 500 hPa & Bodendruck (MSLP)
EC image
White lines: MSLP isobars (units = hPa / interval = 5 hPa)
Colour shading: 500hPa height, colour coded as at scale by diagram. (units = dekametres [dam] / interval = 4 dam). Orange/Red high values - blue/indigo etc., low values.
Black line: the 552 dam contour (of the 500 hPa surface) - highlighted as the approx. median value to trace major troughs/ridges in pattern.

 

Wetterzentrale

This section is simply describing what the various lines, coloured areas are, what interval/units the contours, isobars, colour-filled areas use etc. The use, org meteorological significance of the fields are dealt with in a separate section ... here.

The Wetterzentrale site is here.

IMPORTANT: I am not going to attempt a description of the full range of products, just the ones that might be most useful to those who want to 'dip-in' without much knowledge. Also, the fields presented below are not necessarily from the same situation, run-time etc. They are there simply to aid a quick identification of each product. You will note that the products from this site are mirrored on the 'Westwind' site, so many of the notes are applicable to each.


 

WETTERZENTRALE: 'TOPKARTEN'

 

UK Met Office (UKMO)

NB: scales referred to in the table below only appear on this site for individual frames, NOT on composite or looped imagery.

 Example: Brief description of parameters (etc.) on charts shown.
 500 hPa & Bodendruck (MSLP)EC 500 and mslp White lines: MSLP isobars (units = hPa / interval = 5 hPa)
Colour shading: 500hPa height, colour coded as at scale by diagram. (units = dekametres [dam] / interval = 4 dam). Orange/Red high values - blue/indigo etc., low values.
Black line: the 552 dam contour (of the 500 hPa surface) - highlighted as the approx. median value to trace major troughs/ridges in pattern.

 

UKMO modified (FAX) product

 Example: Brief description of parameters (etc.) on charts shown.
 FAX output (final product)
UKMO FAX output
These charts are simply standard 'FAX' charts which depict MSLP isobars (4 hPa intervals), fronts, pressure centres etc., to conventional symbology. For more on these, see the FAQ entry here) and for Conventional frontal symbology: click here for examples & further notes.

 

ECMWF products (ex EC site)

 Example: Brief description of parameters (etc.) on charts shown.
 MSLP isobars and 850hPa predicted wind speed
EC mslp pattern
Black lines: MSLP isobars. (units = hPa / interval = 5 hPa: every 20 hPa, e.g. 980, 1000, 1020, 1040 hPa the lines are bolder - this aids detection of major changes in the pattern.)
Yellow/Green infill: 850hPa wind speed - see scale on right-hand side.
 500 hPa contours
EC 500 hPa pattern
Blue lines: 500 hPa Contours. (units = dekametres [ dam ] / interval = 6 dam.)

 

 ECMWF products (on the Wetterzentrale site)

 Example: Brief description of parameters (etc.) on charts shown.
 500 hPa & Bodendruck (MSLP)EC 500 and mslp White lines: MSLP isobars (units = hPa / interval = 5 hPa)
Colour shading: 500hPa height, colour coded as at scale by diagram. (units = dekametres [dam] / interval = 4 dam). Orange/Red high values - blue/indigo etc., low values.
Black line: the 552 dam contour (of the 500 hPa surface) - highlighted as the approx. median value to trace major troughs/ridges in pattern.

 

GFS (formerly known as AVN / MRF) products

(if any products are missing from this list, see the display at the 'Westwind' site ..... here

 Example: Brief description of parameters (etc.) on charts shown.
 500 hPa height, T (Temperature) & Bodendruck (MSLP)
500 hPa HTB
White lines: MSLPisobars (units = hPa / interval = 5 hPa).
Dark grey/dash & double-dot lines: temperature at 500 hPa (units = degC / interval = 5 degC).
Colour shading: 500hPa height, colour coded as at scale by diagram. (units = dekametres [dam] / interval = 4 dam). Orange/Red high values - blue/indigo etc., low values.
Black line: the 552 dam contour (of the 500 hPa surface) - highlighted as the approx. median value to trace major troughs/ridges in pattern.
 850 hPa height & Temperatur
850 ht & T
White lines: contours of 850 hPa isobaric surface (units = dam / intervals = 4 dam).
Black lines: isotherms of 850 hPa temperature. (units = degC / interval = 5 degC).
Colour shading: temperature at 850 hPa - see scale at side of chart. (units = degC / interval = 2 degC, blue = cold, orange/red = warm).
 Bodendruck (MSLP), Wolken (Cloud) in %, ReTop (Thickness) 500/1000
Wolken,BodenD
White lines: MSLP isobars. (units = hPa / interval =  5 hPa)
Coloured lines: 500-1000hPa thickness isopleths (units = dekametres [dam]; interval 18 dam)
Monochrome shading: % cloud cover based on medium level moisture (i.e. 'frontal') - see scale by side of diagram (black low % cloud values, light grey/white high % cloud values)
 6h - Niederschlag (Precipitation)
6hr Niederschlag
Colour shading: 6 hourly precipitation (rain, snow etc.) in the period up to the time of the chart. See scale at side of chart - the deeper the blue the more precipitation - purple / violet etc., exceptional rainfall / snowfall accumulations. (units = mm): note that on this display there is NO discrimination between rain and snow.
Red dashed-lines: labelled as 'Konvektion', which I interpret as the model detects a high degree of instability in the area.
 2m Temperatur
2m temperature
Black/thin contours: 2m (i.e. 'screen' - level) temperature. (units = degC / interval = 10degC).
Colour shading: 2m temperature as scale at side of diagram. (units = degC / interval = 3 degC). Orange/Red high values - blue low values.
 700hPa und Vertikalbew. (Vertical motion)
700 ht&VB
White lines: contours of 700hPa isobaric surface. (units = dekametres [dam]/ interval 4 dam)
Black line: highlighted 700hPa contour at 300 dam. Useful to trace major changes in the pattern.
Colour shading: vertical motion as scale at side of diagram - yellow to red to violet: upward values; green to blue to violet: downward values. (units = hPa/h; interval 4 hPa/h)
NB: negative is upward motion!
 Bodendruck (MSLP) und 850 Aeq.PT (Theta-E)
Bodendruck & 850TE
White lines: contours of 850hPa isobaric surface. (units = dekametres [dam]/ interval 4 dam)
Black solid/dashed lines: temperature at 850 hPa (solid >= 0degC; dashed < 0degC: units = degC / interval = 5 degC.)
Colour shading: value of Theta E (equivalent potential) temperature at 850 hPa as per shading chart at side/interval = 2 degC.) A better tracer than temperature alone, because it is based upon the dew-point as well as air temperature.
 2m Taupunkt (Dew Point temperature)
2m dewpoint temperature
Black lines (thin): dew point temperature at 'screen' level. (units = degC / interval = 10 degC, but selected values only.)
Colour-shading: See scale at right of diagram: blue, cold & orange/red, warm. ( units = degC / interval = 2 degC.)
 CAPE und Lifted Index
GFS_CAPE and LI output
Grey or White-lines: Lifted Index (units =  degC / interval = 2 degC for values above zero, 1 degC values below zero); solid grey lines > 0 (poor LI values), dashed white lines<=0degC (moderate or 'good' LI values)
Colour-shading: See scale at right of diagram: blue/green - low values of CAPE; orange/red - high values of CAPE (units  = J/kg / interval  = variable, but generally 100 J/kg over much of range.)
 Nds in mm (rot=konvektiv) + 0° - Grenze in m
Freezing Level example
Brown lines: height of freezing level (in metres, above mean sea level).
Colour shading: indication of rainfall (or equivalent rainfall) accumulation per 3 hr.
Individual figures: PPN accumulation in mm (remember, if snow, then it is the equivalent water).

 

NCEP (GFS) Ensemble products (ENS)

 Example: Brief description of parameters (etc.) on charts shown.
 500 hPa mean contour pattern
500 hPa ensemble mean
Colour-fill: Mean 500 hPa pattern for the entire ensemble - see scale at right. Blue, low values to orange/red high values. (units = dam / interval  = 4 dam)
Black line: the mean position of the 552 dekametre [dam] contour at this time step - useful when comparing with other products with same highlight, and to check run-to-run consistency.
 500 hPa contour Spaghetti plot
500 hPa spaghetti plot
Blue, red, yellow, green etc. lines: Individual members of a particular ensemble run for 500 hPa contours at 516, 552 and 576 dekametre [dam]. (Click HERE for a brief explanation of Spaghetti plots)
 850 hPa mean temperature pattern
850 ensemble mean temperature
Grey lines: the ensemble meanpattern of 850 hPa temperature. Dashed below zero degC, solid at and above freezing. (units = degC / interval = 5 degC)
Colour-fill: see scale at right. Blue, cold to orange/red warm. (units = degC / interval  = 2 degC)
 850 hPa isotherm Spaghetti plot
850 spaghetti plot of temperature
Blue, red, yellow, green etc. lines: Individual members of a particular ensemble run for 850 hPa isotherms at -15, 0, +15 degC. (Click HERE for a brief explanation of Spaghetti plots)
 Meteogramme
Meteogram
 For explanation of these diagrams (also known as 'plumes'), see HERE

WXMAP

This section is simply describing what the various lines, coloured areas are, what interval/units the contours, isobars, colour-filled areas use etc. The use, or meteorological significance of the fields are dealt with in a separate section ... HERE.

The WXMAP site is here.

IMPORTANT: I am not going to attempt a description of the full range of products, just the ones that might be most useful to those who want to 'dip-in' without much knowledge. Also, the fields presented below are not necessarily from the same situation, run-time etc. They are there simply to aid a quick identification for each product.

 


 NOGAPS products (US Navy model)

 Example: Brief description of parameters (etc.) on charts shown.
 MSLP, thickness & 12hr precipitation
MSLPtthkPPN
Gold lines: MSLP isobars. (units = hPa / interval = 4 hPa)
Alternate blue & purple lines: thickness (500-1000 hPa) blue 528 & 552, purple 540 & 564 dekametres [dam])
Colour-fill regions: 12 hour precipitation rate (units=mm/12hr / interval=variable, see colour quadrant top/right of diagram) [ It's not entirely clear if this is the accumulated rainfall in the previous 12hr, or the instantaneous rate at some point in the last 12hr - my thoughts would be the former. ]
 300 hPa height & isotachs
300 ht and isotachs
Green lines: 300 hPa contours. (units = metres / interval = 120 m)
Black (thin) lines: isotachs of the 300 hPa wind. (units = knots [kn]/ interval = 10 kn)
Wind arrows: conventional plots of wind-barbs at 300 hPa (units = kn)
Colour-fill: strongest winds at this level (>= 50kn), according to scale at top/right of diagram. (Use to pick out approximate alignment of jet core).
 500 hPa height & relative vorticity
500 height and rvort
White lines: 500 hPa contours. (units = metres / interval = 60 m).
Colour-fill: Relative Vorticity (at 500 hPa .. roughly the level of non-divergence)
(units=10-5s -1/ interval=2 : see colour quadrant at top-right of diagram - blue implies negative (non-developmental) values & red, orange positive (or developmental) values.
 850 hPa temperature, relative humidity & wind
850Trhwind
Purple/solid lines: 850 hPa temperatures above 0degC. (units=degC / interval=3 degC)
Blue line: the Zero degree C isotherm at 850 hPa (labelled 'FR')
Indigo/dash lines: 850 hPa temperatures below 0degC. (units=degC / interval=3 degC)
Wind arrows: conventional plot for wind barbs at 850 hPa.
Grey-fill: 850 hPa relative humidity. (units = % / interval = 20%: see grey-scale quadrant at top/right of diagram).
 Wave height & surface wind
wave forecasts
Wind arrows: conventional plots of the 'over-sea' wind-barbs (units = knots [kn]).
Colour-fill: Wave height. (units=feet / interval = 3 ft: see colour chart at top/right of diagram - blue - low values, orange-to-red high values.)
[ This would appear to be the 'total' wave height, i.e. wind + swell wave. ]

 

AVN/MRF products (NCEP models)

(These output are in fact in identical format to that above for the NOGAPS model, which makes comparison easy. Only the wave / sea-surface wind field is not included in this set.)

 Example: Brief description of parameters (etc.) on charts shown.
 MSLP, thickness & 12hr precipitation
MSLPtthkPPN
Gold lines: MSLP isobars. (units = hPa / interval = 4 hPa)
Alternate blue & purple lines: thickness (500-1000 hPa) blue 528 & 552, purple 540 & 564 dekametres [dam])
Colour-fill regions: 12 hour precipitation rate (units=mm/12hr / interval=variable, see colour quadrant top/right of diagram) [ It's not entirely clear if this is the accumulated rainfall in the previous 12hr, or the instantaneous rate at some point in the last 12hr - my thoughts would be the former. ]
 300 hPa height & isotachs
300 ht and isotachs
Green lines: 300 hPa contours. (units = metres / interval = 120 m)
Black (thin) lines: isotachs of the 300 hPa wind. (units = knots [kn]/ interval=10 kn)
Wind arrows: conventional plots of wind-barbs at 300 hPa (units = kn)
Colour-fill: strongest winds at this level (>= 50kn), according to scale at top/right of diagram. (Use to pick out approximate alignment of jet core).
 500 hPa height & relative vorticity
500 height and rvort
White lines: 500 hPa contours. (units = metres / interval = 60 m).
Colour-fill: Relative Vorticity (at 500 hPa .. roughly the level of non-divergence)
(units=10-5s -1 / interval=2 : see colour quadrant at top-right of diagram - blue implies negative (non-developmental) values & red, orange positive (or developmental) values.
 850 hPa temperature, relative humidity & wind
850Trhwind
Purple/solid lines: 850 hPa temperatures above 0degC. (units = degC / interval = 3 degC)
Blue line: the Zero degree C isotherm at 850 hPa (labelled 'FR')
Indigo/dash lines: 850 hPa temperatures below 0degC. (units = degC / interval = 3 degC)
Wind arrows: conventional plot of wind-barbs at 850 hPa.
Grey-fill: 850 hPa relative humidity. (units=% / interval=20%: see grey-scale quadrant at top/right of diagram).

Explanations

MSLP patterns

Isobars join points of equal atmospheric pressure. On one side of the isobar pressure is smaller than the isobar value, on the other side greater. A common metaphor is a topographic map with it's lines denoting height: mountains are equal to areas of high pressure, dales equal to lows. Similarly, we can talk about ridges and troughs (or valleys). Wind follows roughly the direction of the isobars so that if you walk along an isobar and the wind is blowing onto the back of your neck, the lower pressure is at your left hand side (in the Northern Hemisphere .. reverse the rule for the Southern Hemisphere) *. Due to friction, the wind is not exactly parallel to isobars, but turned about 10-30 degrees towards lower pressure.

Furthermore, there are several local effects (e.g. the sea breeze) which affect wind and make this general rule not valid in such cases.

The closer the isobars are to each other, the higher the wind speed. (Meteorologists talk about steep or strong pressure gradient.) There are rules and even scales of how to determine the wind speed from distance between the isobars. When using them, be sure to check that the isobars are between the intended steps (usually 5 hPa but sometimes 4, 8 or 10 hPa) and the scale and projection of the underlying map.

Between tropical and arctic areas, at so called mid-latitudes, the isobars tend to form closed circles round the lowest pressure. These are called frontal depressions, and most of meteorological interest at our areas is concentrated in them. Tracking the frontal depressions is the main reason to look at MSLP chart.

(* this was first defined & explained by a Dutch meteorologist, Buys Ballot in 1857, and the 'Law' is named in his honour.)


500 hPa patterns

Level of 500 hPa is roughly dividing the mass of the atmosphere in two. It lies near 5 km, and it's height is typically analysed at intervals of 40 or 80 m, corresponding to MSLP isobars at intervals of 5 or 10 hPa.

The shapes of 500 hPa isohypses are similar to those at the MSLP isobars: even here you can find lows, highs, ridges and troughs, though the latter two are more usual than closed circles. Formations are much smoother because the underlying surface (except in the high mountain areas of the world) doesn't have an effect at these high altitudes (and because observations are sparse).

If you compare 500 hPa and MSLP charts, you often find a family of small surface lows below an upper level low. Also, the weather under a 500 hPa low or trough tends to have more precipitation, at least showers, even is there are no fronts present.

For numerical models, 500 hPa height (and especially the stream pattern it reflects) is much easier to get right than the exact location of individual surface lows and fronts. So we look at it to get a general outlook of weather, especially to talk about 5-10 days ahead.


Spaghetti plots

So named because of their resemblance to a certain Italian food. Spaghetti plots are another way of viewing Ensemble forecasts. Each of the individual forecasts are merged into one image. To reduce the confusion a bit, only two or three contours are drawn. The purpose of a spaghetti plot is to give the user some idea of the uncertainty in the forecast. When the contours are drawn in the same place, a high level of confidence can be used in conjunction with the Ensemble forecasts; when the plot resembles it's namesake (a plate of spaghetti) then the forecast has a very low confidence level, and it is probably best not to make any plans based on this forecast. Generally the ensemble members diverge with time, making forecasts further into the future much less certain.


Cloud forecasts

The Cloud cover forecasts give an output relating to the percentage of sky covered with cloud. Hence 100% would be overcast, 50% (equivalent to 4 oktas) predicts a half cloudy, half blue sky. This is useful for picking out the frontal systems and determining where they are most likely to be the most intense from the higher percentages. Low cloud (roughly below 6000 ft / 1800 m) is notoriously difficult to forecast and global NWP models do not usually attempt this output. Therefore forecasts on these sites should only be used to infer cloudiness at medium and high levels. There are other representations of cloud forecast, which may show overlays of cloud at different levels, and some centres output 'boundary layer' cloud forecasts - not sure how accurate these are though, or how defined.


Precipitation (PPN) forecasts

[ NB: Precipitation is a collective term which includes rain, snow, sleet, hail etc. ] These charts are usually of rainfall accumulation (or the amount of rain that would have fallen if dealing with snowfall). That is, the amount of rain predicted to have fallen in the preceding 12 hours. Hence a value of 6 on a forecast chart for 1200 means that 6 mm of rain should fall between midnight and midday of the day in question. [NB: 1 mm of rain depth equates to 1 litre of rain per square metre - some centres will in fact output rainfall forecasts in this manner, i.e. 40 litres/m2.]

NOTE: Some of the forecasts are of 6 hour accumulations, 24 hour accumulations, or even of 1 hours accumulation. The charts are usually superimposed on another field, usually surface pressure, although the COLA-MRF images (in Wetterzentrale site) are shown with vertical velocity.


850hPa temperature fields

850 hPa level is roughly at 1.5 km, usually above the atmospheric boundary layer. That means there is no diurnal temperature variation, and the underlying surface such as cool sea doesn't affect it's temperature. That is why 850 hpa temperature is used to distinguish air masses and thus to locate cold and warm fronts.

Because the models have had several problem in surface parametrisation, 850 hPa temperature forecasts have been more accurate than those for lower levels. So we used to bring the 850 hPa to surface by adding 15 (or 10) degrees, and use it instead of surface maximum temperature. [***]
WARNING: all this works only at low altitudes, not in the mountains. It works only when the sun is heating the ground: not at the sea or windward coast, not at night-time, not at winter. 15 is the continental value (for dry adiabatic lapse rate); in Ireland they use 10 (for moist adiabatic lapse rate) and I guess at the Alps the 850 hPa temperature is more or less equal to the surface temperature.

You can see from all these warnings, that using the 850 hPa temperature to infer the screen temperature is problematic at best!

[ *** To a first approximation, you can also use T850 to assess the surface temperature in persistent precipitation, provided the layer 850 - surface is near-saturated: in this case, add ~7 degC (i.e. obeying the Saturated Adiabatic Lapse Rate) to the forecast temperature at 850 hPa. ]


Vertical motion fields

The magnitude and sign (i.e. whether "upward" or "downward") of motion in the vertical is obviously important in operational meteorology. All NWP models output such guidance - usually expressed in terms of the variable 'omega' (w), which is defined as the rate of change of pressure with time. Pressure is used, rather than something more intuitive like vertical velocity, because upward/downward motion in the atmosphere due to dynamic forcing (e.g. frontal ascent), comes about as a result of a disturbance of the "hydrostatic equilibrium" - or the balance of forces in the vertical between gravity trying to collapse the atmosphere & the upward, opposite - directed pressure gradient force.

The 'Omega equation' which is used to calculate vertical motion is made up of two terms: an element due to the rate of change of vorticity advection with height, and an element due to horizontal thermal advection. It turns out that solving these terms gives upward motion (ascent) a NEGATIVE value, and downward motion (subsidence) a POSITIVE value: this can be confusing when first looking at these charts, but it might be easier to remember that 'minus' values are tied to falling pressure values or '-ve' pressure tendency (i.e. potential development) & 'positive' values with rising pressure values or '+ve' pressure tendency (i.e. weakening systems, or building anticyclones). (A little more on this HERE).


Vorticity fields

We live on a spinning planet and air in motion tends to pick up a 'turning' moment due to this rotation at every point, except right on the equator (where the local horizontal component of the earth's angular rotation is zero). This natural 'twisting' motion, or local vorticity, is due to the Coriolis acceleration at any particular latitude (f).

Any developments in the atmosphere which act to enhance this effect will increase the likelihood (other factors being right - e.g. enough humidity, sufficient thermal contrast) of development leading to wind, rain etc. Any developments which act to reduce this action negate development. Vorticity due to atmospheric developments, or flow patterns, is termed relative vorticity(z) [ relative to earth 'normal' ].

The sum total of vorticity at any one point (Absolute Vorticity or zA) = f + z. For the vast majority of cases, Absolute Vorticity is positive, but Relative Vorticity can be positive or negative, and it is usually (though not always) this latter quantity that is seen on WWW sites.

... and why are these changes in vorticity important? Well, anything that encourages air to spin more quickly (Advection of Positive [relative] Vorticity, or PVA) will lead to a narrowing of an atmospheric column, and it's expansion upwards, leading to formation of cloud & precipitation .. if of course there is sufficient humidity. The opposite mechanism (Advection of Negative [relative] Vorticity, or NVA) will lead to a broadening of a column, allowing it to contract vertically, with the air subsiding - a non-developmental state.


Thickness **

Thickness (as used on many of these sites) is a useful tracer of warm and cold air in the lower 'half' of the troposphere (surface to about 5.5km), where a large part of the 'real weather' is to be found. Bulk movement of warm air (high values) and cold air (low values) can be followed by looping the fields. In particular, note carefully areas where the thermal contrast is increasing as these are regions of potential major baroclinic development.

However, be aware that working out things like maximum temperatures, snow-level etc., from this rather crude measure is fraught with difficulty and best avoided.
For more, you can follow the links in the FAQ entry for Thickness ... here

** NB: Some will know this quantity at 'Relative Topography', hence the 'Re Top' seen on sites using the German language.


Ensembles

More and more you will read / hear the term 'ensemble' in modern-day weather forecasting. Beyond about 3 days (and in some circumstances less than that), it is not very useful to treat a particular model frame as the answer to the forecast for that particular time-step, but much better to adopt the ensemble techniques as briefly described in the FAQ Glossary, the relevant entry for which you can find ... here. You might also like to read the entry on the 'Poor Man's Ensemble Technique' ... here which explains how YOU can effectively use the various models here described in this fashion.


Jet Streams

On the broad-scale, these are 'key' to deciding the overall complexion of the weather-type in any one geographical region in the mid-latitudes, i.e. roughly between 35 and 75 degrees N/S. The various entries in the FAQ & Glossary are useful starters ... for a basic definition, follow the link: here, and to delve even more deeply into the mysteries of upper air patterns etc., try this page .... here


Equivalent Potential Temperature ('Theta - E')

A useful quantity because it is calculated taking the humidity content of the air into account - and as most 'interesting' meteorology is to do with atmospheric moisture, Theta E (QE) is a better 'tracer' of air-mass property (at 850 hPa**) than temperature alone. It is a conservative property (doesn't change much) during both dry and moist adiabatic processes.

> The actual value of Theta-E can be useful: it can be used in algorithms to calculate daytime maximum temperature and snow-risk for example (as for Theta-W#). Remember though that this variable couples temperature AND absolute humidity: so high values show areas of warm, potentially more humid air (e.g. tropical maritime or modified tropical continental air-masses); lower values pick out colder, lower-humidity content air (e.g. polar maritime air-mass).
> Where there is a marked or 'sharp' discontinuity in values (best seen on colour-enhanced output), then some attempt at frontal placement can be made.
> The location of axes of plumes of high (orange/red) Theta-E, act as a "focus" for significant thundery activity: a plume of such air running northwards over NW Europe ahead of an upper trough coming eastwards may herald the initiation of a ' Spanish Plume' (q.v.); event.
> Although not strictly appropriate to these charts, it is pertinent to note here that the vertical distribution of Theta-E (and Theta-W) with height is a useful diagnostic of whether an air-mass possesses Potential (or Convective) Instability (q.v). If a column of air is lifted bodily, the temperature decrease varies from level to level, particularly because some layers become cloudy (and acquire the released latent heat) sooner than others. Therefore, certain layers can become unstable simply because they have been 'mechanically' lifted - these can be found by inspecting a thermodynamic diagram (or tabulated list of data), and finding layers [roughly between 900 and 400 hPa] where Theta-E (or Theta-W) decreases with height.
#[ in the UK service, the Wet-bulb potential temperature (Theta-W) is more usually used for these purposes, but is not readily available on external web sites - Theta E will do the same job though.]

** [ Beware of using these parameters at 850 hPa as tracers of air-mass change slavishly. Provided the air in the lowest 2 km of atmosphere is well-mixed, then they are indeed excellent for this purpose. However, in stagnant situations, or where there is marked 'de-coupling' between the quasi-frictionless flow above about 900 hPa and the surface, (or boundary-level) air, changes at 1500 m (i.e. 850 hPa) may not tell the whole story. ]


CAPE & Lifted Index

I've 'bracketed' these two parameters, because they are often considered together:

CAPE: it would be better if this parameter was always referred to by it's full expansion: "Convectively Available Potential Energy"; unfortunately, this doesn't happen, so we have the situation where newcomers to the world of 'severe convection' are bemused by all the talk of a quantity 'CAPE' which is not adequately defined. I'll have a go here:.....

When a parcel is lifted by whatever means from the surface (or another level), it will cool according to the basic laws of thermodynamics. The rate of cooling is dependent upon whether the parcel is 'dry' (i.e. unsaturated) or 'moist' (saturated). The rate of cooling is known as the Dry Adiabatic Lapse Rate (DALR) in the former case, and the Saturated Adiabatic Lapse Rate (SALR) in the latter. By inspecting the temperature of the parcel at any point in its upward travels, and comparing with the 'environment' through which it is travelling (best done on a thermodynamic diagram), it will either be cooler (denser) than the environment (stable), neutral (the same temperature), or warmer (less dense) than the environment (unstable). In this latter case (unstable/parcel warmer than environment), then the parcel (packet, bubble whatever), has Potential Energy, manifest as continued upward motion, which was initiated, and made Available due to Convective activity: C-onvectively A-vailable P-otential E-nergy - CAPE. [ For more on stability, lapse rates etc., see the main FAQ.]

The greater the excess in temperature (parcel warmer than environment), then the greater the energy available - and all other things being equal (e.g. no compensating descending currents due to broadscale dynamic effects), this should result in stronger upward motion, potentially more 'severe' conditions (downdraught gusts, heavy precipitation, larger hail, higher tornadic threat etc., etc.).

CAPE is simply the integration (sum) of the energy that a parcel would have throughout its vertical 'life', once convection is released. It is calculated on a thermodynamic diagram by assessing the area between the Environmental Lapse Rate (ELR/actual temperature trace) and the Parcel trace, provided the temperature of the latter exceeds the former. [ See a crude example HERE ]
The units of CAPE are J/kg (energy per unit mass of atmosphere), and some typical values are:
> ~ 150 - 300: heavy/intense shower-rainfall with/without thunderstorm - mainly 'slight' TS
> ~ up to 1000: 'moderate/severe' thunderstorm
> ~ up to 2500: 'severe/intense' thunderstorm.

(Note carefully: many so-called 'critical' values for CAPE & LI - see below, have been defined based on spring/summer US experience and may not be applicable to European/'all-season' storms - do NOT treat these figures as a 'threshold' or a 'requirement'!)
Some other figures for CAPE are outlined in the main FAQ.

Lifted Index (LI): In many respects, this can be viewed as a very crude measure of the above - i.e. CAPE. It is very easy to assess (probably why it was devised in the first place), and so has gained widespread currency in the operational / severe weather world. It is defined simply as:-
LI=T(500) - T(P);
where LI=Lifted Index (degC), T(500) is the temperature at 500 hPa as found from an actual radio-sonde ascent (or computer/model output) and T(P) is the temperature that a parcel would have, if encouraged to reach the 500hPa level (about 18000 ft or 5600m) by whatever means. [ See a crude example, based upon a Skew T, logP diagram: HERE ]

It will be readily seen that when the parcel is cooler (denser) than the environment (i.e. a 'stable' state), LI will be +ve; when the parcel is warmer (lighter) than same, (i.e. the 'unstable') state, LI will be -ve. The more 'negative' the number, the greater the excess of energy the parcel has at this point, hence it's crude likeness to CAPE (above).

Over the years, some rough figures have been developed which relate the value of LI to expected conditions: note that they must NOT be used slavishly in this respect:
LI +ve: stable, non-convective.
LI=0: neutral - non-severe convection may be possible.
LI -ve: unstable .... and the following are offered as a guide ....IF CONVECTION IS INITIATED!
LI -1 to -4: moderate thunderstorms/mainly small hail,
LI <-4: possible severe thunderstorms/large hail,
LI <-10: possible severe/intense thunderstorm, whirlwind phenomena (e.g. tornadoes), 'giant' hail etc. (see cautionary note above re: applicability to European conditions)


Precipitation type forecasts

(i.e. rain or snow phase)
These charts show precipitation intensity (Niederschlagsstaerke) and precipitation type (Niederschlagsform). For all meteorological NWP output, it is important not to "follow the dots" slavishly, that is, not to believe every twist and turn of the output at every time-step. This is particularly so for this product.

The output is decidedly 'blocky', and the output resolution (i.e. what you actually see), is certainly rather course; a single 'block' appears to cover one or two medium-sized English counties. Whether the model resolution is equally course is not known: indeed, if orography is used (and it would be unusual if it isn't), then it must be very crude.

The output is in mm per hr (mm / h); this is fine for liquid precipitation (e.g. rain), but snow output (rosa / purple / schnee) is also shown as mm / h .. these aren't mm of snow on the ground: this is the rainfall equivalent. As a very rough conversion, multiply the intensity by 10, so 1.5mm / h rainfall-equivalent=> 1.5 cm / h. But remember, that not only is this a highly variable quantity, it assumes that the model has got a perfect representation of the lower-tropospheric temperature / humidity profile, and the precipitation intensity is correct, and the model 'knows' about the surface (snow covered, frozen etc.), and that all the snow settles!

In my view, it would be best to regard the 'snow' phase colours as scales of probability in any one situation: the deeper the shade of purple, the higher the probability of snow reaching the surface .. be very suspicious of the lower-decimal (roughly < 0.8 mm/h) figures for snow rate.


Freezing levels

(or 0°C isotherm levels)
On the Topkarten site, there is a section where you can choose from various fields with 3 hr time-steps (though over a limited domain); in particular, and most useful currently, the field labelled:- " 3h Niederschlag " will show precipitation amounts in millimetres. (I assume where the phase is snow then it is 'liquid water equivalent', though I can't see any legend to that effect), and also shown are contours of the height of the zero degC level above mean sea level (in metres): remember to make an allowance for your location - so if you are 200 m altitude, and the forecast is 500 m amsl, the zero deg.C level will be 300 m above you.

This latter shouldn't be taken as the 'snow-level' as such, because of course snow can descend (depending upon the relative humidity in the boundary layer & intensity) well below this level, but it is a useful guide & provided it doesn't change too much with time, it can be used to infer the 'usable' (for winter sport purposes) snow-line in hilly / mountain areas.

If the freezing level is less than ~300 m above your elevation (remember that the output heights are amsl - adjust for your location), then it is reasonable to assume that there is a >50% probability of PPN falling as snow inland. If it is <100m then there is a >=90% probability of snow. (However, in coastal areas with an onshore wind off a relatively warmer ocean, then these figures will not apply so well if at all). [ Ref: Boyden Met.Mag 1964 ]

The horizontal resolution of the GFS is approx. 40 km out to 180hr (~80 km beyond that), and the output resolution of these charts, according to the site information, is 50km, so for this display you are seeing near-full model resolution, as opposed to the main charts, which are 'thinned' to ~100 km. The GFS is still a crude model to use for 'local' work though, bearing in mind that modern mesoscale models are working down to the ~10km or better level.


The Omega Equation

There is an equation in atmospheric physics known as "the Omega equation": it allows synoptic-scale vertical motion to be accurately estimated by analysing temperature and wind fields above the boundary layer, leading to a value for vertical velocity (allocated the greek letter 'omega'), in pressure co-ordinates (dp/dt).
[ By the way, because pressure decreases with increasing height in the atmosphere, ascending air gives rise to a negative value: therefore on computer output, -ve values imply ascending air, +ve values descending air. ]

In it's broadest exposition, the Omega equation brings together the forcing due to vertical variation of absolute vorticity advection, the thermal advection terms (various) and components due to diabatic warming. These factors can be viewed subjectively (e.g. a PVA area engaging a focus of strong warm advection will lead to strong upward motion and vigorous surface development).

Q-Vectors

A numerical model though needs to have all this in terms of rigorous mathematical formulae (albeit with some approximations) that can be solved. On the way to calculating all this development, the models spew out all sorts of diagnostics that can be used by forecasters: one such are 'Q-vectors'. These are found by using a 'cut-down' form of the Omega equation developed by Hoskins, Draghici & Davies in 1978.

The vector "Q" is dependent upon the rate of change (with time) of the potential temperature gradient brought about solely by horizontal change in the wind-field (derived using model contour fields, geostrophic.)

Q-vectors are not a measureable quantity like the actual wind or the temperature - they are more akin to the 'thermal wind', with which they are in fact allied.
They are derived, as indicated above, by manipulation of the Omega equation. In brief, the concept of the Q-vector is a mathematical representation of the atmosphere's attempt to restore the thermal field once it has been disturbed from its 'equilibrium' state of both hydrostatic (1) and geostrophic (2) balance. This 'restoring force' is deemed to be via ageostrophic motions, which are most important in determining where development is occurring.

[Notes:
(1): hydrostatic balance: the 'struggle' between the pressure gradient acting upwards, due to decreasing pressure with height, and the force of earth's gravity acting in the opposite direction - the hydrostatic equation represents this process.
(2): geostrophic balance: the 'struggle' between the pressure gradient (all planes, though usually taken as horizontal) and the deflection due to the earth's rotation - the equations of motion represent these.]

Where Q-vectors are convergent (i.e. negative divergence), the forcing is normally associated with tropospheric ascent (or low-level development / falling pressure.
Where Q-vectors are divergent, the forcing is normally associated with tropospheric descent (or low-level decay / rising pressure ).

Also, where Q-vectors are directed from cold to warm air, (at 850 hPa) this implies frontogenesis (a tightening-up of the thermal field); the opposite implies frontolysis (or frontal decay).

On various charts available via the Westwind site:-

FQ :=Forcing due to Q-vector convergence / divergence.

FQ: +ve divergence==> descent / non-developmental
FQ: -ve divergence==> ascent / developmental (or convergence)
The higher the number (+ve or -ve), the deeper the colour and the greater the forcing term. These data are usually shown in association with the 500 hPa field, roughly at the level of non-divergence (or maximum vertical velocity).

FQn

:=the component of Q 'normal' (i.e. at right-angles) to the isotherms and for most work, the isotherms used are those at 850 hPa; for mountainous terrain though, 700 hPa should be used.

 

FQn: strongly -ve in cold air=> frontogenetic

 

FQn: strongly +ve in warm air=> frontogenetic

 

Best used to find areas of 'strong blue' (positive) immediately adjacent to 'strong red' (negative); this implies synoptic-scale processes in place enhancing frontal development / ascent etc (or frontogenesis).

 

 

Example of CAPE & LI

 Example CAPE & LI on SkewT, LogP  On this example (of a Skew T, log P diagram), CAPE is the area enclosed by: X - A - TOP - B - Y - X (shaded green); Lifted Index is given by the difference (observing the sign) between the temperature (B-A): see explanation below:....
 A  The point (and associated temperature) where a saturated parcel given this particular ascent crosses the 500 hPa isobar=T(P)
 B  The actual temperature, T(500), from the radio-sonde ascent in this particular situation. The difference B-A [ or T(500) - T(P) gives the Lifted Index.
 X  The temperature at the surface (SFC) which initiated this particular parcel's ascent. From X to LFC, ascent-cooling will be at the Dry Adiabatic Lapse Rate (DALR).
 Y  The lowest 100 m or so of the ascent as given in the radio-sonde ascent; this will become heavily modified due to afternoon heating.
 LFC  The 'Level of Free Convection', above which the parcel, rising from the surface, will be saturated, and cool at the Saturated Adiabatic Lapse Rate (SALR), in this case from LFC to TOP.
 TOP  The point at which the temperature of the parcel=temperature of the environment, and buoyancy is neutral; the parcel theoretically ceases to rise of it's own volition - but given enough positive energy below this point, tops may (& often do) 'overshoot'. (also known as the 'Equilibrium Level')

 

ThetaE, ThetaW & derived parameters

The relationship between Theta E, Theta W and theoretical maximum temperature and snow probabilities (latter based on Bradbury, 1970).

 

Notes:

1. The relationship for day maximum temperature was developed specifically for SE England. In mid-winter in particular, at latitudes further north, the expected maxima will be lower. Also note that the figures are not necessarily based on a plentiful data-set over the whole range of expectations; for example, at the very high and very low end of the scale, the numbers of events used to find the theoretical 'base' maxima will have been much smaller than for the 'middle', or most-likely area of the data-set. With experience, you may indeed find your own figures are better than these, so keep a record of what happens in each occasion.
2. The figures will not apply over a full snow-cover, as insolation gives a different heating response over such surfaces.
3. The techniques use the data from 850 hPa (or mbar), which is very roughly at 1500m (or 5000 ft) amsl. Under conditions of strong anticyclonic subsidence, this level may not represent the air mass in the lowest 50 hPa of the atmosphere. Cold-undercutting (and indeed warming over sea surfaces) should be allowed for. Also, in cases of very high pressure (over 1035 hPa / mbar), the 850 hPa / mbar level will be even higher up, and will represent the near-surface even less.
4. Once you have found the basic working figure for maximum temperature, 'correct' for cloud cover using the table given. Note that basic figures & corrections are only given to the nearest 1degC and the results should not be used slavishly - arguments involving the odd degree between one situation and another are pointless given the approximations involved & the accuracy of the raw input data (whether model or ascent-based).
5. In the case of the snow risk, remember it is for mean sea level. You need to adjust for altitude using the adjusting table given.

Method of use:
(a): Find expected ThetaE (Equivalent Potential Temperature) over the area in question at around the middle of the day - from latest NWP output, or from radio-sonde ascents.
(b): Against the appropriate value of ThetaE in the table below, look up the value for that month - interpolating / adjusting as necessary.
(c): Look at the list of corrections given underneath and choose appropriate value: apply.
(d): For snow probabilities, find the base (i.e. msl) figure from the extreme right-hand column.
(e): Work out the altitude of the site in question, and using the table of adjustments, find the % risk of snow for that altitude. (IMPORTANT NOTE: In my experience, these figures are too pessimistic and should be used as a very rough guide only. I find that you need persistent, moderate or heavy precipitation before these figures are relevant. What I do is use the 50% equivalent isopleth at msl ... 3degC ThetaW / 16 degC ThetaE ... as a 'tracer' for 'snow-worthy' air at lower levels, then use other means to decide on the precise risk.)

 850 thetaE
(degC)
 850 thetaW
(degC)
 Jan.  Feb.  Mar.  Apr.  May  Jun.  Jul.  Aug.  Sep.  Oct.  Nov.  Dec.  Snow
Prob. (at msl)
 54  18  19  20  21  23  25  26  26  25  24  22  20  19  
 46  16  17  19  19  21  23  25  25  23  23  21  19  17  
 40  14  16  17  18  20  22  23  23  22  21  19  17  16  
 36  12  15  15  17  19  21  21  21  21  19  17  15  15  -
 32  11  13  15  15  18  20  21  21  20  19  17  15  14  -
 30  10  13  14  15  17  19  20  20  19  18  16  14  13  -
 26  8  11  13  13  15  17  19  19  17  17  15  13  11  1%
 22  6  9  10  11  13  15  16  16  15  14  12  10  9  14%
 18  4  8  9  10  12  14  15  15  14  13  11  9  8  34%
 14  2  6  7  8  10  12  13  13  12  11  9  7  6  78%
 10  0  4  5  6  8  10  11  11  10  9  7  5  4  96%
 8  -1  3  4  5  7  9  10  11  10  8  6  4  3  100%
 6  -3  1  3  4  6  8  9  10  9  7  5  3  2  100%
 4  -4  -1  1  1  4  6  7  7  6  5  3  1  1  100%
 2  -5  -3  -1  0  2  4  5  5  4  3  1  0  -1  100%
 0  -6  -5  -3  -1  0  2  3  3  2  1  -1  -2  -3  

Corrections to be applied (for the maximum temperature figures):

A day of heavy overcast with precipitation: apply correction to above of at least -2 to -3degC, and for air-masses laden with cloud and having high low-level humidity (spring / summer), then the correction may be as high as -5degC.
A day of bright, virtually uninterrupted sunshine (but no snow cover) apply + 1degC. On days of strong sunshine over a dry ground (in a relatively warm air-mass for the season), then correction is at least +2degC, and may be up to +3degC.

Use the following table to adjust the % snow risk (at mean sea level) in the above to the required elevation:

 Elevation (in metres)  % PROB at msl >>>>  20%  40%  60%  80%
 50m    28%  50%  68%  84%
 100m    39%  59%  73%  89%
 150m    50%  68%  78%  93%
 200m    59%  73%  83%  98%
 250m    66%  78%  88%  100%
 300m    72%  83%  92%  100%
 350m    77%  88%  98%  100%