Updated: May 1, 2019
As you might imagine, I get a lot of questions from pilots about weirdness in the weather. They see something that doesn't look quite right or doesn't make logical sense and want to know more. With a little forensic weather sleuthing, I can usually explain what's going on. Here's one such request for more information...
One of my customers saw the following two METARs from Albany, New York and Glens Falls, New York; the two airports are just 37 nm apart. However, the winds were significantly different. Given their close proximity he was curious why such a large difference in wind speed, 25 knots versus 3 knots?
KALB 300151Z 17013G25KT 3SM BR OVC006 09/08 A2981 RMK AO2 SFC VIS 4 SLP097 T00940078= KGFL 300212Z AUTO 29003KT 3/4SM -RA BR SCT003 OVC011 06/06 A2980 RMK AO2 RAB12 P0000=
Some of the reasons for this are related to terrain. But I'm not going to focus on that element. Instead, let's drill down at each location and see what's going on. But first, you might think there's a front in between these two airports. Not the case as you can see from this surface analysis chart valid at 00Z. The red circle shows the locations of both KALB and KGFL. Certainly no front in the immediate vicinity of either airport.
First and foremost, to see significant differences in weather between two airports in close proximity isn't all that uncommon. In some places around the country it's quite predictable. Pilots that frequently fly out of these airports can usually describe these local anomalies. But the devil is in the details. And yes, that means we cut to the Skew-T diagram to see those details.
Gusty winds are usually a result of air "mixing down" from faster air above. An unstable atmosphere near the surface (in the planetary boundary layer) is a good example of when the atmosphere is mixy (yes, not a real word, but what seemed appropriate here). This is quite easy to see in a thermodynamic diagram called a Skew-T log (p). Below is the Skew-T analysis from Albany valid at 02Z (highly zoomed in).
Notice how the temperature (shown in red) is nearly dry adiabatic (lapse rate of 3 degrees C/1000 ft). That's an unstable situation. Unstable air can easily move up and down. If you look at the same Skew-T analysis** below, notice the wind at the top of the boundary layer (about 1300 ft MSL in this case) is moving along at 31 knots. The unstable air allows the winds at the top of the boundary layer to "mix down" toward the surface creating gusts. Using the wind at the top of the boundary layer is a great way to estimate the wind gusts at the surface when there's no TAF available.
What about Glens Falls? In the Skew-T analysis below also valid at 02Z, you can see the atmosphere is very stable. This essentially decouples the surface from the winds aloft. In this stable environment the stronger winds aloft can't mix down to create surface gusts.
The moral of the story is that we should be very careful assuming that two airports in close proximity will have the same weather. So using the "closest" TAF isn't always a good idea. As you can see from this example, the winds can be significantly different.
** An analysis is NOT a forecast and is always valid in the past despite what you might see on some flight planning apps.
Most pilots are weatherwise, but some are otherwise™
Weather Systems Engineer
CFI & former NWS research meteorologist