Local Knowledge and Predicting Wind
December 02, 2015 at 8:00 AM
Have you ever been to an unfamiliar airport and heard some wild weather prediction from a local pilot that turns out to be true? That local knowledge can be invaluable for understanding weather.
In our classes and seminars, I like to use an example that can help predict windy conditions on the Front Range, east of the Continental Divide in Colorado. This was observed by a well-seasoned flight instructor at Rocky Mountain Metropolitan Airport (KBJC).
This instructor noted that the weather and winds could look great at one time then suddenly change to very windy and turbulent conditions. He wanted to see if there was a predictor for this situation.
What he found was that when high pressure built west of the Continental Divide, even as far away as Las Vegas, Nevada, the Divide would act like the lip of a bowl and help contain this pressure. But at some point, the air would spill over and create strong, downsloping winds.
After careful study over time, he determined that if the altimeter setting was 0.20" Hg greater at an airport on the west side of the Divide versus an east side airport, you could expect wind along the Front Range in the next few hours. The greater the differential, the greater the wind. While temperature accounts for some of the pressure differential, this high pressure spillover is largely accounted for by atmospheric pressure.
The day before teaching one of our Stick 'n Rudder classes I found just such an example. Below are METARs for KAPA and KEGE.
KAPA 071553Z 01003KT 10SM FEW120 SCT150 BKN200 13/M07 A3008 RMK AO2 SLP159 ACSL VC W T01281072
KEGE 071550Z 00000KT 10SM BKN150 01/M03 A3039
Nice, calm winds at observation time. But note the altimeter pressure difference -- 0.31" Hg! His rule-of-thumb would predict high winds to occur soon, which was borne out by the METAR at KAPA five hours later.
KAPA 072053Z 32021G29KT 10SM FEW120 SCT150 BKN200 19/M06 A3007 RMK AO2 PK WND 32029/2050 WSHFT 2026 SLP139 VIRGA SE-S ACSL S-W-N T01941061 57007
All heck broke loose, with winds 320 degrees at 21 knots, gusting to 29! Makes for pretty challenging crosswind approach and landing conditions.
If you look at the earlier METAR for KAPA, you'll note ACSL VC W in the remarks section. This is a hint that winds are picking up, as the ACSL stands for Altocumulus Standing Lenticular clouds -- a mountain wave existing west of the airport. In the later METAR, the wave had extended all along the foothills.
We recommend as part of your normal weather briefing you choose an airport on the west side of the Divide in addition to your Front Range airport and look at that pressure differential. Eagle, Aspen, Leadville, Gunnison all will work pretty well. It may influence your go/no go decision.
This article was published in the January 2016 Flight Lines newsletter published by the Colorado Pilots Association and may be downloaded at the link below.
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