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OAT

(abbr) Outside air temperature: often used in connection with aviation weather reports.

OCF

Abbreviation for occluded front (or occlusion).

OBS

In general use, often used as an abbreviation for observations; in the specific case of the newsgroup uk.sci.weather, used as part of the 'Subject' line where the body text contains observations in some structured (perhaps semi-coded) way and in a time sequence. The (suggested) format for the Subject is:

[OBS] < location >< date >

(this entry may be amended with experience and validation of the current trial - see also WR)

Occluded front

The merging of two fronts, as when a cold front overtakes a warm front.

OCNL

(abbr) Occasional (well separated, as in CB in aviation forecasts.)

OF

Abbreviation for occluded front (or occlusion).

OP

(abbr.) 'operational' model

OPER

(abbr.) 'operational' model

Operational model

A term used to differentiate the primary NWP output from a particular centre from any ensemble products from the same source. The operational model will almost always be run at a higher resolution than that used for ensemble output. It must not, however be assumed that the 'Op/OPER' is necessarily the best outcome, particularly beyond 3 days or so. (see Ensemble)

Orographic forcing

An airstream encountering a barrier to its passage is forced to go around or over the obstacle. The upward deflection of the airflow is sufficient to give rise to adiabatic cooling, and if the air is moist enough, the formation of clouds, precipitation etc. In addition, convergence of the flow on the windward side (due to a rapid decrease in velocity) when the air encounters a sharply graded barrier not only enhances the vertical motion, but also leads to a deformation of the flow which in turn alters the vorticity of the air particles. Thus, hill and mountain ranges are most important in a study of meteorology.

Orographic rainfall/snowfall

For precipitation to occur, other conditions being satisfied (i.e. enough humidity, required temperature structure, sufficient depth of cloud etc.), there must be a supply of upward motion through the cloud producing the rain, snow or whatever. In orographic precipitation, the forcing agent is provided by large ranges of hills/mountains blocking the flow of humid air in such a way that vertical (upward) currents of air are produced, leading to adiabatic cooling >> condensation >> cloud formation/enhancement >> precipitation element growth. Orographic forcing OF ITSELF usually only produces small amounts of precipitation, but can be the means of enhancing or triggering other mechanisms (e.g. convective activity), and is one of the important elements in the 'seeder-feeder' model (q.v.). Computer models in operational use do now have sufficiently realistic orography and vertical resolution to model such, but the output (usually) does not explicitly define orographic precipitation.

Orographic waves

See Mountain waves.

Outlier

(in an NWP ensemble suite) When considering a collection of solutions at a particular lead time (from a single-centre, or as part of a 'Poor Man's ensemble), some 'clustering' is usually observed - i.e. a large number of members pointing to a similar outcome. However, as lead times get longer (especially beyond 72 hours), one or two members may depart significantly from the ensemble mean, (and/or mode of larger clusters) - these are termed 'outliers'; such indications carry small weight, but cannot be totally ignored. In particular, at extended range (beyond about 7 days), there may be no clear 'clustering' signal, and an 'outlier' is just as likely to be right as a solution nearer the mean/median of the output. (See also ensemble, clusters, ensemble mean etc.)

OVC

(abbr) Overcast 8 oktas (cloud amount, as used in aviation reports, forecasts etc.)

Overconvection

The promise of a fine, sunny day is sometimes spoiled because cumulus cloud builds and spread out into an almost unbroken sheet of stratocumulus by late morning - which then refuses to break up for the rest of the day*. For this to occur, there must be a marked inversion (see "What is an inversion?") within 100 to 300 hPa of the surface, which must be intense enough to stop convective currents 'breaking - through' the inversion even at maximum temperature; the convective condensation level (CCL) must be at least 60 hPa below the inversion level, and the layer between CCL and inversion must have a reasonably high relative humidity. For some rather obscure reason, this phenomenon has come to be called 'overconvection' (at least in the UK, probably originating within the gliding community) - possibly because convective cumulus 'spills-over' to cover the sky? (* though, other conditions being right, the cloud may disperse around or just after dusk.)

Over-running trough

An active, mid-latitude frontal system is associated with a marked short-wave trough. The 'active' weather associated with the front lies forward of the trough, driven by the dynamics associated with it. At some stage in its life though, the trough (or a portion of it) will 'relax' (and effectively weaken), allowing the trough to run well ahead of the lower-tropospheric portion of the frontal system - it 'over-runs' the (surface) location of the front, and the activity at that position will decay. Note however that the upper trough will still have 'weather' associated with it - and may be the means of driving an upper/split frontal structure well away from the classical surface front as drawn on conventional analyses. See also "Why fronts die".