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A weather forecast is a statement of expected meteorological conditions for a specified time or period, and for a specified area or portion of airspace (ICAO Annex 3 - Meteorology)
A weather forecast is compiled by a weather forecaster or meteorological office based on a number of factors, which include the following:
- Recent weather conditions,
- Understanding of the processes that contribute to changing weather conditions,
- Statistical data,
- Computer prediction,
- Climatic factors.
Aeronautical weather forecasts contain some or all of the following information:
- Wind velocity (strength, direction, gusting), low level wind shear),
- Amount and type of precipitation,
- Cloud type, amount and vertical extent,
- Atmospheric characteristics (Pressure, temperature, dew-point),
- In-Flight Icing,
- Extreme weather e.g. Thunderstorm, Tropical Revolving Storm, Mountain Waves, Sand Storm, Volcanic Ash,
- Expected change to forecast conditions.
Weather reports (METARs) are reports of actual weather conditions pertaining at specific locations, e.g. aerodromes, at specified times.
Types of Forecast
The following types of forecast are regularly used in aviation:
- Aerodrome Forecast,
- Area or Route Forecast,
- Special Forecasts.
An aerodrome forecast (TAF) consists of a concise statement of the expected meteorological conditions at an aerodrome for a specified period (ICAO Annex 3).
Aerodrome forecasts are issued in the TAF code form and include the following information in the order indicated:
- a) Code name TAF/TAF AMD;
- b) Location indicator (ICAO 4-letter code);
- c) Date and time of origin of forecast;
- d) Date and period of validity of forecast;
- e) Surface Wind;
- f) Visibility;
- g) Weather;
- h) Cloud; and
- i) Expected significant changes to one or more of these elements during the period of validity.
Route of Area Forecasts
Area and route forecasts contain upper winds, upper-air temperatures, significant en-route weather phenomena and associated clouds. Other elements may be added as required. This information covers the flight operations for which they are intended in respect of time, altitude and geographical extent.
A special report or forecast (SPECI) is issued whenever significant changes have occurred or are expected to occur to the published report or forecast. These are published in the SPECI code format. The SPECI code is similar to the METAR/TAF code.
Distribution of Weather Forecasts
Two principal methods are used for the distribution of weather forecasts:
- Written Forecasts are available on request from meteorological offices and typically contain the following information, valid for the period of the expected flight:
- Map showing weather forecast in specified are or along specified route;
- Map showing forecast upper air winds and temperatures;
- Aerodrome forecasts for departure and destination airports as well as selected en-route and alternate aerodromes;
- Actual weather reports for departure, destination and en-route aerodromes.
- Electronic Transmission of aerodrome actual weather reports and forecasts. These are transmitted on published RTF frequencies using the TAF/METAR code and are updated at regular intervals or when significant changes (SIGMETs) occur.
- ICAO Annex 3 Appendix 4: Technical specifications for aerodrome forecasts in the TAF code form.
- Aircrew Quick Reference to the METAR and TAF Codes is an easily understood explanation of the METAR and TAF codes with examples, produced by USAF.