Here we go again. Pursuant to a new International Civil Aviation Organization (ICAO) requirement, the National Weather Service (NWS) is changing the Terminal Aerodrome Forecast (TAF) format. From November 5, 2008, all TAFs will include new data and time abbreviations. Morever, the TAFs issued for 32 large airports in the United States—as well as overseas—will cover a 30-hour period. The rest of the United States’ TAFs will cover a 24-hour forecast period.
ICAO says we need 30-hour coverage because of the growing number of long-haul flights. They reason that with many flights now lasting 14 hours or more, there’s a need for longer-range forecast guidance. While most of us don’t make flights anywhere near this length, we still have to learn the new TAFs if we’re to understand the valid times at the other 593 TAF sites. And student pilots will have to learn the new TAFs if they expect to pass knowledge exams.
The main thing to remember is that only the valid dates and times are changed in the new TAFs. The abbreviations for weather phenomena, visibility, wind, and sky conditions stay the same as before.
You’ll see the new valid dates and times in five places within the new TAFs. At each place, there will be a date/time group that identifies the forecast information by date as well as time—because the 30-hour TAF can cross over into a new day.
Valid date. This is the third data entry in a TAF. The new TAF will say, for example, 2418/2524. This means that the TAF is valid from day 24 of the month at 1800Z to day 25 of the month at 2400Z.
FROM designator. Again, there will be a date and time identifier preceding the forecast information. Where FM0900 once appeared, you’ll now see FM-180900, meaning from day 18 of the month at 0900Z. The FM change group is for significant and rapid change to a new set of prevailing weather conditions.
PROB group. You’ll now see a date after this probability forecast, e.g. PROB50 1812/1820, meaning a 50-percent probability of the forecast weather from day 18 between 1200Z and 2000Z.
TEMPO group. Again, there’s a date and time breakdown for temporary weather conditions. TEMPO 1908/1912 means that the forecast conditions are valid on day 19 of the month, between 0800Z and 1200Z. Just to review, TEMPO refers to any change in wind, visibility, weather, or sky condition that’s expected to last for “generally less than an hour at a time, and are expected to occur during less than half the time period,” according to the NWS. So for the above time period—0800Z to 1200Z—the temporary weather is expected to last for less than an hour, and take place during less than two hours.
BECMG group. The “becoming” designator is used for gradual changes in weather taking place over a longer time frame—usually two hours. The changes, like the elements we’ve already discussed, are bracketed by date and time identifiers. So BECMG 1914/1918 VRB35G50 1SM TSRA BKN010CB means that strong, gusty winds to 50 knots, visibilities of one mile, and thunderstorms with 1,000-foot broken skies and cumulonimbus clouds are expected to appear between 1400Z and 1800Z on day 19 of the month.
That’s the gist of the latest changes. The BECMG group is still used at various international forecast stations, but seems to becoming—pardon the pun—less popular among forecasters in the United States.
In any event, get ready for November 5, and don’t say we didn’t warn you! See AOPA Online for additional weather resources. Now we’ll have to watch the publishers of educational texts and videos scramble to include the new format.
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