"My destination does not have a terminal forecast (TAF). Is it okay to use the TAF for an airport that's 20 miles away when planning for my IFR alternate airport?"
- Bill W.
“Legally, you cannot. If there isn’t a terminal aerodrome forecast or TAF for your destination, the area forecast (FA) is what you are required to use.
A TAF is a forecast for expected weather significant to aviation within the terminal area. The terminal area is defined as the region within 5 statute miles from the center of the airport’s runway complex. Therefore, the terminal area is actually quite small and a TAF is considered by meteorologists as a point forecast. Consequently, the forecast is not valid outside of this area.
The reason for this isn’t obvious. Many pilots feel that the weather is normally quite homogeneous over a fairly large area. That may be true in some situations, but not all. Local effects can make all the difference in the world.
For example, the onset of fog is usually delayed in urban areas. The warmer ground often keeps fog from forming. On the other hand, if the airport is more rural, fog can often form much earlier. As a result, it is not unusual to see a huge difference in the weather between two airports that are in close proximity.”