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THE POINT FORECAST
by Dr. Scott C. Dennstaedt
As a flight instructor and former NWS meteorolo- gist, I’ve accepted that pilots like to rag on meteo- rologists for issuing bad forecasts. After hearing the backstory behind the pilot’s dissent, there was nothing inherently wrong with the forecast; it was how the pilot was trying to use the forecast that was problematic. This is not to say that forecasts are always accurate, but pilots tend to not appreciate the hard limitations these forecasts demand.
By far, a Terminal Aerodrome Forecast, simply known as a TAF, is perhaps the most difficult forecast any meteorologist will ever make. Think about the challenge these forecasters face. A TAF is essentially an hour-by-hour forecast for conditions significant to aviation at an airport over the next 24 or 30 hours. This includes a forecast for details such as wind speed and direction, cloud coverage, ceiling height, prevailing visibility and precipitation type.
When you think of a TAF, size matters. The forecast is difficult because of the relatively small diameter of the terminal area. The U.S. definition of a terminal area is the region within five statute miles of the center of the airport’s runway complex. That’s a tiny region to get the forecast elements correct over the forecast period. Thus, meteorologists refer to a TAF as a point forecast and it’s critical to understand its limitations and how they affect the forecasts pilots ultimately use every day.
Here’s a way to visualize why it’s so hard to issue
these forecasts. What are the chances that there will be
a thunderstorm somewhere in the United States in the month of July? The answer is 100 percent, of course. If that’s not the case, we’ve got bigger problems to worry about. So that’s an easy forecast and you don’t even need a meteorology degree to answer. Now, how about the chance of a thunderstorm sometime during July in the state of Oklahoma? Given a month is a long period of time and Oklahoma is a state with a lot of thunderstorms during the summer, again, the answer is easy, nearly 100 percent. Now, how about the chance of a thunderstorm on July 14th at the Oklahoma City airport at 8 a.m.?
When you narrow down the time and the size of the location of interest, forecasting for that small five statute mile area is incredibly difficult if not fundamentally impossible at times, but forecasters at the local weather forecast offices are asked to carry out the impossible many times each day.
There’s no doubt that TAFs are used by all pilots because of the significant detail they provide. Everyone from general aviation pilots to commercial air carriers utilize
The terminal area is a tiny region that is within five statue miles (4.3 nm) of the center of the airport’s runway complex as shown by the red circle around the Charlotte Douglas Interna- tional Airport. The terminal area’s vicinity (not shown) is the donut shaped region from 5 to 10 statute miles and does not include the terminal area itself.
The colored regions on this map represent the NWS county warning areas (CWAs). There is one weather forecast office in each of these areas and they are responsible for issuing the TAFs for airports that fall within the CWA.
34 MMOPA MAGAZINE MAY / JUNE 2021