For a basic understanding of how marine surges in the Bay Area develop you should read the blog below entitled : Anatomy of a marine surge”. The marine surge that swept into the San Francisco Bay Area June 3, 4 and 5 was exceptionally strong. As you can see in the video below Sherman Island in the Delta held winds averaging for over 36 hours. So what made this marine surge so strong and long lived?
Beyond the surface low pressure just north of the Golden Gate mentioned in the preceding blog there are other factors that can jazz up a marine surge. One of these is a Cut-Off Low. These lows, found at around 18,000 feet, are areas of low pressure and upper level winds spiral into them in a counter-clockwise spinning fashion. If there is a Cut-Off Low to the W or SW of the Bay Area this augments the marine surge at the surface in several ways. 1. It tends to deepen the marine layer so the clouds push further into the Bay Area. 2. It induces a weak low pressure at the surface west of the Bay Area which also has counter-clockwise spinning winds and these winds enhance the southerly flow along the coast. 3. It also transfers some momentum to the near surface creating SW winds above the marine layer. This also enhances the southerly winds of the marine layer.
In the video below you can see, at around 18,000 feet, the clouds spinning around the Cut-Off Low. This is a very dry Cut-Off Low so the clouds are sparse making the Cut-Off Low hard to see. But as the wind graph video above shows the impact of the Cut-Off Low on the surface winds is dramatic.
