Video:

Upper Level low and haze trim pressure gradient

by Mike Godsey, mike@iwindsurf.com

Glad I was not forecasting today. It is one of those frustrating days when the wind at almost every site is weaker than forecast. Worse at many sites the winds some sites were only 3-4 knots lower than forecast. The that 3-4 knots spelled the difference between planing and slogging. Backcasts are a hell of a lot easier to do than forecasts so I think the following backcast is pretty accurate.

See image #1.The ocean has nice southerly flow which is promising sites favored by southerly flow like Sherman Island, Berkeley, Pt. Isabel and Larkspur.

See image #2. And the modeled pressure gradients , see pink  lines on pressure graphs, are modeled to reach around .10 to .14 which should have Sherman Island and Treasure Island in the mid 20’s and Berkeley and Pt. Isabel at least upper teens to 20. But notice how the 3PM ACTUAL pressure gradient, Red squares, is significantly weaker than the modeled pressure gradient. What happened?

See image #4.Looking at the brightly colored model output find the YELLOW circle over S. Nevada. That is a tiny upper level low pressure at ≈ 18,000 ft.. Notice its counter-clockwise spinning winds.

See image #3.Now look at the satellite imagery showing water vapor and clouds above 10,000 feet. See how the counter-clockwise spinning flow way aloft caught up some monsoonal moisture from Mexico and swept it over California’s Central Valley. Most of that was just haze but there were some clouds which you could see looking east from the Bay Area this afternoon. That haze was enough to limit the heating in the Central Valley so the Central Valley thermal low is was not as deep as expected nor was the resulting pressure gradient.

You can see all of this happening in the video below. 

Really hard to forecast something like that in advance.