Explains how to understand Aviation Weather Forecasts such as the terminal aerodome forecast (TAF), terminal forecast (FT), area forecast, sigmet, airmet, CWA, winds and temperatures aloft (FD), and TWEB.
(1) Type of report
(2) Location identifier - ICAO 4-letter
(3) Date and time of issue
(4) Valid period - TAF valid time, two digit date followed by begin &
*forecast elements: wind/visibility, weather/sky conditions
(5) Wind - 3-digit direction, two or three digit speed in knots
(6) Visibility - expected prevailing visibility in statute miles
forecast visibility >6 miles coded as P6SM (plus 6 sm)
(7) Weather - signifigant weather and obstructions to visibility
(8) Sky condition - clouds, ceiling height or vertical visibility
(9) Temporary conditions - fluctuations expected to last less than an hour in each instance, two digit begin time in hours and two digit end time in hours
(10) forecast change group - signifigant, permanent changes in conditions; from (FM) four-digit hour & min the change is expected to begin; becoming (BECMG) with four digit time group to indicate begin and end time in hours
(11) Probability Forecast - of thunderstorms or precipitation (in %) along with associated weather conditions (wind, visibility, sky condition); 4-digit begin and end hour follows abbreviation PROB40
(12) End of report seperator (=)
Terminal forecasts contain information for specific airports on expected ceiling, cloud heights, cloud amounts, visibility, weather and obstructions to vision, and surface wind. They are issued three times a day, amended as needed, and are valid for up to 24 hours. The last six hours of each forecast period are covered by a categorical statement indicating whether VFR, MVFR, IFR, or LIFR conditions are expected (L in LIFR and M in MVFR indicate "low" and "marginal'). Terminal forecasts are written in the following form:
CEILING: identified by the letter "C" (for lowest layer with cumulative sky cover greater than 5/10)
CLOUD HEIGHTS: in hundreds of feet above the station (ground).
SKY COVER AMOUNT: (including any obscuration)
CLOUD LAYERS: Stated in ascending orders of height
VISIBILITY AND OBSTRUCTION TO VISION: Standard weather and obstruction to vision symbols are used
SURFACE WIND: in tens of degrees and knots (omitted when less than 6 knots)
DCA Forecast 22nd Day of month - valid time 10Z-10Z
10SCT C18 BKN 5SW- 3415G25 OCNL C8 X 1/2SW:
Scattered clouds at 1000 from 340 degrees at 15 knots, Gusts to 25 knots, occasional ceiling 8 hundred feet, sky totally obscured, visibility 1/2 mile in moderate snow showers.
12Z C50 BKN 3312G22:
By 12Z becoming ceiling 5000 feet broken, surface wind 330 degrees at 12 knots. Gusts to 22.
04Z MVFR CIG:
Last 6 hours of FT after 04Z marginal VFR due to ceiling.
Area forecasts are 12 hour aviation forecasts plus a 6 hour categorical outlook prepared 3 times/day, with each section amended as needed, giving general descriptions of potential hazards, airmass and frontal conditions, icing and freezing level, turbulence and low-level windshear and significant clouds and weather for an area the size of several states. Heights of cloud bases and tops, turbulence and icing are reported ABOVE MEAN SEA LEVEL (MSL); unless indicated by Ceiling (CIG) or ABOVE GROUND LEVEL (AGL). Each SIGMENT or AIRMET affecting an FA area will also serve to amend the Area Forecast.
SIGMENT, AIRMET, and CWA messages (in-flight advisories) broadcast by FAA on NAVAID voice channels warn pilots of potentially hazardous weather. SIGMET's concern severe and extreme conditions of importance to all aircraft (i.e. icing, turbulence, and dust storms/ sandstorms or volcanic ash). Convective SIGMET's are issued for thunderstorms if they are sufficiently strong, wide spread or embedded. AIRMET's concern less severe conditions which may be hazardous to aircraft particularly smaller aircraft and less experienced or VFR only pilots. CWA's (Center Weather Advisories) concern both SIGMET and AIRMET type conditions described in greater detail and related to a specific ARTCC area.
Winds and temperatures aloft forecasts are 6, 12, and 24 hour forecasts of wind direction (nearest 10 degrees true N) and speed (knots) for selected flight levels. Forecast Temperatures Aloft (degrees Celsius) are included for all but the 3000 foot level.
FD WBC 121645 BASED ON 121200Z DATA VALID 130000 Z FOR USE 2100-0600Z. TEMPS NEG ABOVE 24000 FT. 3000 6000 9000 18000 4000 30000 34000 39000 BOS 3127 3425-07 3420-11 3421-16 3512-38 311649 292451 283451 JFK 3026 3327-08 3324-12 3120-27 2923-38 284248 285150 285749 At 6000 feet MSL over JFK wind from 330 degrees at 27 knots and temperature and temperatue of -8 degrees Celsius.
TWEB is individual route forecasts covering a 25 nautical mile zone either side of the route. By requesting a specific route number, detailed enroute weather for a 15 hour period plus a synopsis can be obtained.
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