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TAFs to anticipate weather conditions in the airport terminal area. Without question, TAF content can have a strong impact on fuel loads, the need for alternates, and other operational aspects due to their stringent regulatory nature.
Each weather forecast office in the conterminous United States is typically responsible for issuing a TAF
for up to 10 airports within its region of coverage called a county warning area or CWA (not to confuse this with a Center Weather Advisory). For example, the Greenville- Spartanburg forecast office in Greer, South Carolina has the responsibility for preparing TAFs for six local terminal areas to include the Charlotte Douglas International Airport (KCLT).
It’s quite important that the TAFs are prepared and issued by local forecasters instead of forecasters sitting in some Washington D.C. office. They often take into account sub-synoptic local effects and they are tuned into the local weather patterns since they deal with it every day. The difference between a low IFR ceiling and a clear sky can be just a matter of 10 miles.
Therefore, the size of the terminal area is a point that should not be overlooked. The TAF may or may not always be representative of a zone forecast. Additionally, locally derived forecast rules and outside pressure from the FAA or even the airlines can cause the TAF to be quite different than the “local” forecast.
Scheduled TAFs are issued four times a day (every six hours) at 00Z, 06Z, 12Z and 18Z. In most circumstances, the TAF is normally transmitted between 20 minutes
and 40 minutes prior to the beginning of these times. Moreover, for high impact airspace such as Atlanta, Chicago and New York, TAFs may be routinely issued every three or even two hours. For now, those off-schedule issuances will still be released as amendments. So if you
see an amended forecast in these regions, it may not be due to a poorly aligned forecast with respect to the weather; instead, it just may be a new and improved forecast.
Precipitation events, especially thunderstorms, give meteorologists the most trouble. Forecasting convection in the terminal area is all about the (un)certainty of the event. Even in reasonably dynamic situations with traveling weather systems meteorologists can find it challenging to predict if and when it will impact the terminal area over the forecast period.
Unfortunately, forecasters don’t have a convenient
way in a TAF to quantify their uncertainty. In the public forecast you’ll see a forecast like, “a 30 percent chance
of thunderstorms.” Sure, forecasters can throw in a PROB30 forecast group into a TAF, but by NWS directives PROB30 groups can’t exist in the first nine hours of the forecast period. By the way, the NWS only uses PROB30 although you may see PROB40 in other countries or in military TAFs.
If you visit weather.gov and in the upper-left corner type in a location such as an airport, city and state or ZIP code, you will be presented with a forecast that includes a link to the area forecast discussion for that area. That link will contain the entire discussion that includes the aviation section.
Each area forecast discussion has an aviation section like the one shown here. It is a plain Eng- lish discussion that allows forecasters to quan- tify their uncertainty. It may be a little techni- cal at times, but well worth the read especially when thunderstorms, fog or freezing precipita- tion is of a concern.
36 MMOPA MAGAZINE
MAY / JUNE 2021