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Are you using MOS?

Updated: Sep 5, 2021

Say you are flying to Effingham Count Airport (1H2) in southern Illinois within the next eight hours. However, there isn't a Terminal Aerodrome Forecast (TAF) issued for that airport. What can you reasonably do to understand the weather you might face during your arrival at 2200 UTC? Use the nearest TAF? How about the Graphical Forecast for Aviation (GFA)?

Effingham County Airport sits in a hole (show to the right) with terminal forecasts located on all sides for the most part. Decatur, Illinois (KDEC) is the closest airport that has a scheduled TAF issued. It is about 50 nautical miles to the north of Effingham County. Fifty miles isn't all that far, so can a pilot expect the weather at Effingham County to be the same as Decatur? It is certainly easy for a pilot to leap to that conclusion.

Don't count on the nearest TAF to be accurate in all cases, especially one that is 50 nm away. A TAF is a point forecast that is valid only five (5) statute miles from the center of the airport's runway complex. This leaves the pilot with the unfortunate job of extracting site-specific weather data from the GFA (see below). That is not an easy task considering that the GFA tool doesn't present all of this information in one view, so you have to hunt for it.