Let's say you are planning a flight and your destination airport does not have a terminal aerodrome forecast (TAF) issued for that airport. Can you simply use the closest airport that has a TAF? It is not unusual for a pilot during their preflight planning to simply use that closest airport. What's disturbing is that if you look at the most popular aviation apps, they actually encourage this by listing the closest airport that has a TAF. This can be very dangerous and lead to a poor decision.
It's important to acknowledge that the size of the terminal area is incredibly small. The definition is defined by the National Weather Service (NWS) as...
"...the area within five (5) statute miles (SM) of the center of an airport’s runway complex."
When a meteorologist is constructing this forecast, they are sensitive to this definition. In other words, a TAF is a point forecast and not a zone or area forecast. Therefore, forecasts for adverse weather such as thunderstorms, freezing rain and even windspeed or gusts, for example, are often issued with that tiny forecast area in mind. The expected weather just 10 miles away (outside of the terminal area) may be entirely different. This may be due to terrain such as mountains or even a body of water. Airports just 10 miles further inland may not experience the marine layer that may occur at airports right along the coast.
Here's a perfect example. As shown above, the Essex County (KCDW) and Teterboro (KTEB) airports are just 10 miles apart. However, the wind at these two airports may be completely different. Teterboro has a TAF, but Essex County does not. In fact, the TAF for Teterboro on October 27, 2021 forecast the winds in the early morning hours to be somewhat gusty as shown below.
KTEB 270905Z 2709/2812 36013G21KT P6SM SCT030 OVC080
FM271000 35016G26KT P6SM BKN030...
And, in fact, that's what occurred at Teterboro as shown by these surface observations.
KTEB 271251Z 35015G26KT 10SM OVC040 15/08 A2959 RMK AO2 KTEB 271151Z 35012G22KT 10SM OVC043 14/11 A2954 RMK AO2 SLP002 7 KTEB 271051Z 35009G16KT 10SM OVC044 14/11 A2952 RMK AO2 SLP995 KTEB 270951Z 34013G20KT 10SM OVC043 14/11 A2951 RMK AO2 SLP992 KTEB 270851Z 34013G17KT 10SM OVC085 13/11 A2950 RMK AO2 RAE12 SLP989 KTEB 270751Z 35009G17KT 10SM -RA OVC080 13/11 A2949 RMK AO2 SLP987
However, just 10 miles to the west, the winds at Essex County (below) were mostly light and variable over the same time period.
KCDW 271253Z VRB06KT 10SM OVC041 16/11 A2962 RMK AO2 SLP033
KCDW 271153Z VRB06KT 10SM OVC047 16/11 A2957 RMK AO2 SLP017
KCDW 271053Z VRB03KT 10SM FEW120 16/11 A2954 RMK AO2 SLP008
KCDW 270953Z AUTO VRB05G15KT 10SM OVC100 16/11 A2953 RMK AO2 SLP005
KCDW 270853Z AUTO 33005KT 10SM OVC090 15/11 A2953 RMK AO2 RAE30
KCDW 270753Z AUTO VRB03KT 10SM -RA OVC090 14/11 A2953 RMK AO2 SLP004
The weather at the two airports were fairly close to include ceiling height, surface visibility and even present weather. Surface windspeed and gusts, on the other hand, were not all that similar between these two neighboring airports.
What does one do if the NWS doesn't issue a TAF for an airport you are flying to? Well, that's a topic for another discussion.
Most pilots are weatherwise, but some are otherwise™
Dr. Scott Dennstaedt
Weather Systems Engineer
CFI & former NWS meteorologist