Upper level ridge brings perfect fall weather and slack winds.
by Mike Godsey, email@example.com
Take a look at the first image and find California and the Bay Area. Now notice that northward loop in the upper level winds. The area within that loop is called an upper ridge. It is a mass of warm descending air at ≈ 18,000 ft. that blankets the entire west coast. The heat from the upper ridge is warming the Central Valley and the Great Basin creating low pressure in those zones.
Normally we think of low pressure in the Central Valley as creating Bay Area thermal winds. However when the Central Valley thermal low gets too big for the confines of the valley it spills over the coast range and out into the ocean. You can see that in the 2nd image where is says thermal low pressure. With low pressure to the east and west of us there is almost no pressure gradient most of today.
If you look at the small pressure graphs you can see that the pressure gradient hovers near zero until about 5PM.
Ok, so we have lost one half of the Bay Area wind machine but where is the second half of the engine the North Pacific High? Going back to the 2nd image find the North Pacific High. Yesterday the North Pacific High pushed an extension or surface ridge into Pacific Northwest and Canada. Overnight this ridge pinched off and you can see it as a separate high pressure over those areas. If you look carefully you can see that when the NPH is in this location the isobars are at an angle to the California coast and they are far from shore near the Bay Area due to the Central Valley thermal low which has expanded into these waters.
Looking at the 3rd. image you can see that the angle of the isobars creates NNW winds out over the ocean far from shore. Wind at this NNW angle has a hard time reaching the gaps of the coast range and curving into the bay. Late this afternoon the Central Valley thermal low retracts to near Treasure Island and the isobars of the NPH line up a bit more parallel to the coast. So we see a slight ramp up in the wind but barely to the upper teens.