Cut-Off Low at ≈ 18,000 ft. deepens Southern California marine layer

by Mike Godsey, mike@iwindsurf.com

The Southern California coast wind picture does not look very promising until at least Tuesday and then improves Wednesday. Why? Because the marine layer clouds will be deep and lingering while the inland valleys will be relatively cool so the pressure gradient from the ocean to the valleys will be weak. So why does this happen every week or two to Southern California winds?

Well the video at the bottom shows what is happening but since there is a lot moving about in the video let’s look at a still image first.

At the start of the video first find the Cut-Off Low due west of Southern California. Note how the green arrows show the counter-clockwise spinning around the Cut-Off Low. These winds are a bit hard to see since the air in the area is very dry and this satellite image shows water vapor.

This Cut-Off Low creates SW flow aloft which brings warm moist surface air towards the cooler waters of the Southern California bight. Hence marine layer clouds thicken and push inland each night. This SW onshore flow creates strong wind in the inland areas but keeps the coast wind weak.

Over the next 24 hours this Cut-Off Low fades away as it joins the upper trough marked by the pink arrow. At the same time this upper trough drops southward towards Southern California. This also creates SW onshore flow keeping the marine layer deep and the pressure gradient weak Monday.

By Tuesday things get tricky. Notice the upper level high pressure east of Southern California as defined by the red arrows. These clockwise spinning upper level winds circulate around a mass of descending warming air. The models have this upper high moving towards Southern California Tuesday. If this happens the inland valleys heat up and the pressure gradient goes up while the marine layer clouds are crushed thinner and the wind ramps up. But all of this depends upon the fate of the upper trough. If it departs then we see  improved winds Tuesday and at least upper teens winds in the Leo to Sunset corridor Wednesday.

Now let’s look at all of this in a video animation. Look at the wind flow of the Cut-Off Low and the upper trough and notice the SW flow directed at Southern California.

Then note the flow around the upper level high pressure. See all the monsoonal clouds swept up in the high pressure. The green areas may contain showers. The orange and magenta areas are T storms. If those clouds come over Southern California all wind bets are off. If they stay away then mid week will see useful coast winds.