Upper level Cut-Off Low north of Hawaii targets California

by Mike Godsey, mike@iwindsurf.com

The last week or so the Bay Area has basked under a upper level ridge at about 18,000 ft near the 500 mb level. The heat created by this high pressure has generally crushed the marine layer clouds and caused the Central Valley thermal low to expand over the coast. This has resulted in limp pressure gradients and wind but beautiful weather for the entire west coast.

However far out in the Pacific a Cut-Off Low is gathering force and will, if the models are correct, bring a damp end to the warm dry fall weather. Looking at the video find California on the right and note the sparse clouds aloft over the state. This water vapor imagery does not show clouds below 10K feet so the patchy marine layer clouds at dawn today, Oct. 20 are not visible.

Now look to the far left and find the counter-clockwise spinning Cut-Off Low. The whirl of clouds is roughly at the 500 mb level. At the surface below the Cut-Off Low a surface storm is slowly forming. This weather system should reach the California just south of the Bay Area about Saturday Oct 26. So watch for increasing clouds and a chance of sprinkles or even rain south of the Bay Area.

As the old saying goes:  Cut-Off Lows are a weatherman’s woe” Why? Because they are cut off from the much more predictable flow of upper level winds that flow mostly from west to east around the earth.