San Francisco posing as japanese art work

Upper level high pressure crushes the marine layer clouds

by Mike Godsey,

marine layernewThe San Francisco Bay Area and The Gorge are about to transition into to a hot weather pattern with a contraction of the wind zone to sites near the coast. Let’s take a look at what causes this by focusing on the Bay Area but basically the same thing will happen in the Gorge. But the Gorge is stretched out so unlike the Bay Area one cam shot can not capture the whole thing.

You must have wondered why that last storm took so long to depart and why there were not powerful NW clearing winds when the storm departed. Normally storm systems, the surface rainy low pressure and the upper trough that steers and reinforces the storm arrive from the west and depart to the east along with the upper level winds. But the last several days a huge ridge of upper level high pressure has been forming over the middle of the USA. This has blocked the eastward progress of weather systems and is why the main body of the North Pacific High is NW of Hawaii.

Today that upper high pressure is retrograding, geek speak for going the opposite direction (east) than the normal flow of weather. So by today the edge of the upper high pressure is nosing over the Central Valley and the East Bay hills. Upper level high pressure areas are zones of descending air which 1, compresses creating heat 2. dries the air and 3. crushes the marine layer clouds into the thin pancake you see below. As the upper level high pressure moves over the Central Valley the temperatures will skyrocket.  In response the resident surface thermal low that normally drives our thermal winds will expand towards the coast. If the low pressure just reaches the East Bay hills it will spoil the pressure gradient for sites east of Treasure Island but sites to the west will see upper teens to low 20’s winds. If the low pressure expands all the way to the coast then the winds will be weaker and later than my Bay Area forecast.

In the Gorge this process starts one day later as the upper high pressure slide first over the Columbian Basin and then all the way past Portland making for a weak pressure gradient until late in the day. Can you figure out why Jones Beach has the most promising wind forecast? Mike G.

Posted in Columbia River Gorge, San Francisco