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WAFC

(abbr) World Area Forecast Centre - there are two such centres covering high-level / intercontinental air routes across the globe: "London" and "Washington" (actually located at Exeter, UK & Kansas, USA respectively). In addition, RAFC (Regional Area Forecast Centres) are provided by various major weather forecasting services around the world to cater for medium-level work on a continental/sub-continental scale, though increasingly even this work is being concentrated on just a few centres. Purely low-level work (roughly below 10000ft) is the prerogative of national weather services (NWS's), or equivalent military formations. All charts issued by WAFC's, RAFC's & NWS offices are produced to standards agreed between member states of the International Civil Aviation Organisation (ICAO). For more on SIGWX charts see here.

Warm advection

The replacement of cold air by warm (usually) due to horizontal movement of air-masses. The process can be gradual or abrupt: the latter often being marked by a 'warm front' on a synoptic chart.

Warm anticyclone

This feature has a warm core (relative to surrounding air), with cold / dense air dominant in the upper part of the troposphere / lower stratosphere, contributing to the net high pressure observed at the surface. Has a deep circulation (i.e. 'high' / ridge found on charts extending to tropopause levels), with that tropopause high/cold etc. Often associated with major long-wave blocking patterns. However, note that 'warm' anticyclones can have cold air trapped in the lowest layers under the surface inversion, and can of course be the means of advecting cold air (if it was there in the first place) around it's periphery. (See Anticyclones; Cold anticyclone)

Warm occlusion

If air (at the surface) behind an occluded front is warmer than the air it is displacing (a typical example being when a winter-time Polar Continental 'block' is being eroded by an incursion of Polar Maritime air), then the front is known as a warm occlusion. The occlusion is shown on synoptic charts as a linear extension of the warm front; the cold front leaving the occlusion/warm front at a fairly sharp angle. (see also Cold occlusion).

Warm-front wave

A secondary disturbance, often accompanied by a shallow closed-low circulation, that forms at some point on a marked warm frontal boundary a good way (at least 1000 km) from the parent (occluding) depression. Once formed, it moves quickly away from the parent depression (in the Northern Hemisphere east or southeastwards). Although not common, they are often responsible for considerable forecast errors, and are of particular importance in winter (snow-situation) forecasting as mild, maritime air attempts to displace a cold, continental blocking anticyclone. (See also cold-front wave).

Warm sector

The area between a warm and cold front in the classical Norwegian model frontal system.

Warnings - UK Public Service

The Met Office is responsible for providing advice and warning of forthcoming Severe Weather within the United Kingdom. There are various categories; see here.

WBPT

See Wet Bulb Potential Temperature

WDSPR

(abbr) Widespread, as used in aviation weather reports, forecasts etc.

Wet Bulb Freezing Level

Because evaporative cooling is so important in rain versus snow forecasting (see the main FAQ entries, here and here), the wet-bulb 0degC level is a better guide to snow-risk level than the actual ZDL. The wet-bulb is the lowest temperature to which air can be cooled by evaporating water into a sample, which is a mechanism often important in determining whether it will rain or snow at or near the surface. However, note carefully that advection, both cold and warm must be taken into account - the former is particularly important in marginal situations in the vicinity of a well-marked cold front. (see also "Downward penetration of snow" in this Glossary).

Wet Bulb Potential Temperature

(often abbreviated to WBPT, or 'theta-W') A relatively conservative property within any one air mass that is derived from the temperature and humidity values of a particular air sample for a particular level: usually 850 or 500 hPa. Very warm/very humid southerlies for example (in NW Europe) would have typical 850hPa WBPT values well in excess of 16 degC, and perhaps as high as 20 degC or more; polar maritime air streams would have values typically 5 to 10 degC, but these values would be much lower in the depths of winter.

Wet day

A period of 24hr, conventionally beginning at 09UTC, during which precipitation of 1.0mm or more has been recorded. (See also Rain day).

WF

Abbreviation for warm front.

Wind shear

The change in wind direction, or speed, or both, either in a vertical or horizontal plane. Vertical shears are important in the study of convection, particularly for severe storm development; horizontal shear, particularly speed shear, contributes to relative vorticity terms in synoptic development.

Wind waves (sea)

Waves generated by the wind blowing over the surface of the ocean. (See also Swell).

Wintry precipitation

(often used as 'wintry showers') When the air temperature is close to zero deg.C (either side), it is sometimes easier to use this shorthand term for showers producing soft hail, sleet, snow, 'sleety-rain' etc. Frowned upon by proper meteorologists but a useful term nonetheless. (However, we try to avoid it when the showers are much heavier, and are expected to give significant snowfall.)

WKN

(abbr) Weakening, as used in aviation forecasts, particularly SIGMETs.

WR

(abbr) Used in the uk.sci.weather newsgroup to report a 'weather event'; entered in the subject line thus: [WR] snow becoming heavy at Tilehurst 1540UTC. The use of this abbreviation allows those who do not wish to read such reports to ignore them during 'busy' weather periods. (See also OBS)

WS

(abbr) Wind shear, as appended to METAR reports etc. This abbreviation should only be used to indicate actual (or forecast) conditions experienced by aircraft in the atmospheric boundary layer.

WV

Water vapour imagery...see the FAQ entry on satellite imagery.