By Mike Godsey

Cut-Off Low moves towards Bay Area

by Mike Godsey,

Today, Saturday August 17 saw the strongest coast NW winds in the Bay Area in many weeks with the Waddell sensor hitting 28 knots and many sites inside the bay seeing winds over 20 knots.NPHgood2weak

The first image to the right shows how the huge dome of high pressure between California and Hawaii known as the North Pacific High.

The curved lines you see on the map are known as isobars or lines of equal pressure. The highest pressure is in the middle of the North Pacific High out in the Pacific today.  The successive lines further from the center of the high pressure are areas of lower pressure.  In the northern hemisphere winds circulate in a clockwise fashion around high pressure. This NW wind over the waters along Northern California.  The closer the lines are together the faster the pressure drop or pressure gradient.

Imagine you were looking at a topographic map of land on a hiking trip. The topographic lines of elevation are far apart in flat areas, steeper in moderate slopes and very close together where there are cliffs.

Likewise when you see isobars stacked close together like you see along the Bay Area coast there is a steep pressure gradient. So wind rushes from areas of higher pressure creating the ocean NW wind.

Now notice how low the pressure is inland over the Central Valley this creates a pressure gradient from the coast to the Central Valley. So now we have 2 pressure gradients: 1. the pressure gradient across the isobars of the North Pacific High which makes the wind out in the pacific  and at the ocean buoys. 2. The pressure gradient from the ocean to the Central Valley. This second pressure gradient creates the thermal winds the Bay Area has even when there is no ocean wind. But this same pressure gradient to the Central Valley sucks the NW wind through gaps in the coast range. The combo of the NW wind and the thermal wind makes for stronger wind than we would otherwise have.

So why am I forecasting weaker NW wind for tomorrow, Aug, 18? Look at the 3rd image at 5AM tomorrow. Notice the low pressure that has developed over the Bay Area and how far apart the North Pacific High’s isobars are apart. So I expect weak wind at dawn.

Now look at the 4th image for 5PM. Notice that the isobars are a bit closer together but the are still further apart that today and they are only along the coast. So I am forecasting weaker winds and only at sites like Waddell, Crissy, Coyote & 3rd.

But is the North Pacific High weaker today at yesterday? To answer this question we have to leave the surface and head up to ≈ 18,000 ft.

Study the video below. First find the southward extending loop in the upper level winds. This is an upper trough. So far it has been too far from California to impact our wind. But towards the end of the brief video notice how the upper trough moves closer to the Northern California coast and pinches off a Cut-Off Low. This small counter-clockwise spinning feature will bring SW to SE winds and clouds above the Bay Area especially Sunday evening. And it is this same feature that is weakening the North Pacific High and its isobars the next few days. Remember that Cut-Off Lows are very unpredictable so I could be wrong on all of this.

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