What’s Causing our Heat, Thunderstorms?

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If you’re wondering where our unusual weather is coming from, you’re not alone. Even long-time forecasters at the National Weather Service office in Monterey say these conditions of extreme heat along with severe thunderstorms have rarely, if ever, been seen in the Bay Area.

To begin with, we are on the west side of a huge and persistent dome of high pressure. It measures 600dm at a atmospheric height of 500 millibars. Without going into too much detail let’s just say that strong a center of high pressure is rarely seen, and it’s controlling the weather throughout the Western U.S.

Through heating caused (mainly) by downward compression of the air column, the air under the high is being superheated, and it’s currently locked between low pressure troughs to the west and east.

On the western side of this giant high, clockwise circulation is driving tropical air north. That air is full of moisture being ejected by tropical systems, like former tropical cyclone Fausto.

Fausto is falling apart off the coast of Mexico and sending its moisture steam our way. A conveyor belt of water vapor headed north! And, almost all of it pointed straight at the Bay Area and North Bay.

Here’s a look from the National Weather Service.

Post Tropical Cyclone Fausto is streaming moisture across the #BayArea, seen here in 🛰️. This is triggering #thunderstorms in our area.#cawx pic.twitter.com/No4FHPgYgm

— NWS Bay Area (@NWSBayArea) August 17, 2020

How long will the pattern last?

Most models predict the high will weaken and shift to the east after Wednesday, and temperatures will slowly fall. Here’s a look at Thursday. The weakening high pressure center now over Arizona.

That change will cause the winds to shift, cutting off the stream of tropical moisture into Northern Califonia.

To be clear, temperatures will remain above normal for some time after Wednesday. Just not record-breaking.

And, as unusual as this stormy pattern has been, it shows signs of continuing in the long range forecast.

Models now show the chance of more moisture, this time ejected by Hurricane Genevieve possibly bringing thundershowers to the area as early as this Sunday, but more likely next Monday or Tuesday.