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So what can a forecaster do when there’s a chance of thunderstorms in the public forecast, but certainty is low? In most cases, the forecaster will leave out any mention of thunderstorms. It’s just too uncertain and the likelihood is small that a thunderstorm will roll through that tiny forecast region. Moreover, forecasters are also pressured by the airlines to avoid placing thunderstorms in a TAF in these situations. A forecast for thunder requires the flight to file an alternate and take on more fuel.
Perhaps these are just the scattered variety of afternoon thunderstorms, referred to by meteorologists as pulse thunderstorms. In this case, the forecaster has two possible solutions, neither of which will appear in your aviation textbook or taught in your aviation ground school. First, they can add rain showers (SHRA) or showers in the vicinity (VCSH) instead of thunderstorms. Showery precipitation is inherently a convective process. It’s
not unusual to see forecasters include one of these two precipitation forecasts into the TAF when uncertainty of thunderstorms is high. Essentially it becomes a placeholder for thunderstorms when conditions eventually begin to evolve and it becomes clear thunderstorms will impact the terminal area. At that point, they will likely amend the TAF to replace –SHRA with -TSRA.
Second, forecasters can also explain their reasoning
in the area forecast discussion or AFD. No, it’s not a discussion about the retired aviation area forecast. Instead, it’s a discussion about the weather expected in the local county warning area (CWA) for that particular weather forecast office. Every AFD contains an aviation section that discusses the TAFs for airports within that CWA.
It is here that a forecaster can express why they didn’t include a forecast for thunderstorms or perhaps fog or even freezing rain.
In fact, most forecasters will do a pretty good job trying to quantify their uncertainty. In the case of thunderstorms you may see words in the AFD like, “including light rain showers to cover the unlikely threat of thunderstorms.” The AFD isn’t something you’d get in a standard briefing, but certainly should be part of your own preflight brief. I’ve always said that if you are not reading the AFDs, you are missing half the forecast. You can find the full AFD for each CWA from visiting the weather.gov website or visiting my new progressive web app, ezwxbrief.com.
So the next time you pour over the TAFs along
your route, remember these two points. First, never assume the weather forecast at one airport applies to a nearby airport. On some occasions when the weather is homogeneous across a region, it very well may be that a TAF is representative of the weather at airports close by. Forecasters have local knowledge and often make forecasts that take into consideration how terrain or the previous day’s weather can impact any particular airport.
Second, TAFs are not an area or zone forecast and should never be used as such. It’s often easy to look at all of
the TAFs along your route and make a hasty decision. Just because the three or four TAFs along your proposed route do not mention thunderstorms, doesn’t mean you won’t encounter them while en route. Use TAFs for what they
are intended to show. Therefore, if you have an emergency and need to land, knowing the potential weather at those airports along your route is important, but don’t try to use the forecast to fill in the gaps between those terminal areas.
Lastly, keep in mind that a forecast for –RA (light rain) or –DZ (light drizzle) doesn’t imply there’s no chance of running into FZRA (freezing rain) or FZDZ (freezing drizzle). The precipitation forecast is based on what’s expected at the surface. If the temperature is forecast
to be a degree or two above freezing at the surface, you will see a forecast for rain, but you may find that just 500 feet above the surface there’s a nasty freezing rain event waiting for you.
Dr. Scott Dennstaedt is a CFI and former NWS meteo- rologist and founder of EZWxBrief (ezwxbrief.com). He is the co-author of the newest weather book, entitled, Pilot Weather: From Solo to the Airlines.
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