Video: nagging upper trough @ ≈ 18,000 ft. brings unending SW flow.
For almost 2 weeks now a deep marine layer and SW flow has dominated the Bay Area wind. While the North Pacific High’s surface NW wind has seldom made an appearance.
This pattern favors Sherman Island and the East Bay but it has mostly killed the coast winds and often made Crissy, Coyote & 3rd. weak and unreliable especially inside.
Part of this SW flow has been caused by a tiny counter-clockwise spinning about 20 miles across that has often formed just west of the Golden Gate.
But the major cause of the SW flow and of this tiny eddy itself is caused by by a southward extending loop in the upper level winds know as an upper trough.
The satellite imagery in top image from yesterday clearly shows this loop in the upper level winds at around ≈ 18,000 ft.. This image actually shows water vapor with the dry areas being black to orange and the white ares showing higher moisture and clouds.
The green arrows show the wind around the upper trough. Notice how the upper trough is sending SW flow over the Bay Area. When an upper trough is near the Bay Area the marine layer deepens and the clouds are driven deeper into the bay by the onshore flow.
Typically an upper trough moves towards us from the west then passes over California and departs to the east. That is why we normally see a period of SW flow and a deep marine layer for a couple of days then sparser clouds and NW surface wind.
But this upper trough has be unable move eastward since there is a Rex Block anchored in the middle of the USA . Until this blocking combo of an upper low and upper high breaks down the upper trough will linger to our west.
However thing look like they could get worse before improving. (of course if you are a Sherman Island kiter or windsurfer you have the opposite perspective)
Let’s look at the 2nd image from today August 5. Notice how the upper trough is beginning to form a counter-clockwise spinning mass at ≈ 18,000 ft.
If this process continues over the next 24 hours the upper trough will disappear and we will have a upper low pressure that has lost its connection with the west to east movement of weather. At this point we have what is called a Cut-Off Low.
Once a upper low pressure is Cut-Off Low from the rest of the upper level winds it can meander in an almost unpredictable fashion to our west. And if you are visualizing this it would mean the southerly flow aloft could continue for days. Which would be great for Sherman Island and the East Bay.