by Mike Godsey
Cut-Off Lows form when a counter-clockwise spinning low pressure pinches off from passing upper troughs at around 18K feet. The winds at about 18K feet on the average are important in moving weather from west to east around the world. Once you have a Cut-Off Low to your west the weather does not change much until the Cut-Off Low finally gets sucked up by a passing upper trough. Since the Cut-Off Low itself is detached from the stream of weather going from west to east it is is extremely difficult for the models to forecast. But so far all the models have this Cut-Off Low only wobbling a bit west of the Pacific Northwest.
Cut-Off Lows anywhere west of California are bad news for Southern California since they tend to push the marine layer far inland cooling the inland valleys and weakening the pressure gradient. This leaves all the strong wind focused in the inland valleys and passes.
For the Bay Area a Cut-Off Low just west of the Bay Area pushes the North Pacific High and it’s Nw winds to the south killing the coast wind and makes the Peninsula unreliable while it enhances the winds in the East Bay and Sherman Island.
But this Cut-Off Low is west of Pacific Northwest. In that position the North Pacific High’s surface NW winds do not full retreat so we have some NW wind along the coast and Peninsula. The marine layer does push way inland in the AM but the NW wind scours it out of the bay except near the Golden Gate. And the SW flow enhances the East Bay and Sherman Island winds in the PM.
This video shows what our current Cut-Off Low looks over the last 12 hours. Remember that all the clouds you see moving are above 10K feet. Far below all these clouds are the marine layer clouds and the North Pacific High’s surface NW winds.