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The Southwest monsoon has ramped up tremendously this summer, and as a result, California is experiencing relatively strong (for us) surges of monsoonal moisture from the desert Southwest.

Beginning Sunday but more importantly Monday, another push of subtropical moisture at the mid level of the atmosphere will arrive in Northern California. Will it be anything like August of 2020? Definitely not. But it does have the potential to spread scattered convective storms over Northern California. At this point it looks like most activity will be out to sea, and north of Sonoma County. However this far out, the models don’t agree.

Here is how the monsoonal moisture looks right now (Saturday) on satellite (Image 1). Lots of moisture in the atmosphere in the Southwest beginning to enter Southern California. Northern California into Oregon is covered in wildfire smoke.

Now, here’s where the moisture plume is today (Saturday) (Image 2), and where it is forecast to be by early Monday (Image 3) and early Tuesday (Image 4)

What forecasters are most concerned about is late Monday into early Tuesday. Without getting too far into technical details, as this moisture rotates into our area around the western edge of high pressure, it will be accompanied by a ripple in the atmospheric column which will arrive in the Monday-Tuesday time frame. This could set off more storms if things line up. However, it is too far out to be certain.

Most models show any activity may stay out to sea, or to our north. However the NWS office in Eureka sees enough potential for dry lightning that they have issued a fire weather watch for the Monday afternoon through Tuesday evening time frame. Here’s what they say:


.An anomalous plume of monsoon moisture is forecast to spread
north across the region Monday and Tuesday. Isolated to scattered
thunderstorm development will be possible as a result. Lightning
occurring across very dry fuels will support a threat for fire

Upper Smith-Lower Middle Klamath-Hoopa-Van Duzen/Mad River-
Lake County Portion of Lake/Napa/Sonoma RU-Interior Mendocino-
W Mendocino NF/E Mendocino Unit-Trinity-

The watch applies to Lake County north. This is because most models, at this point, show anything that might occur will head that direction. Here is the North American Model for early Tuesday, (Image 5) predicting a few showers rotating into NorCal from the ocean.

This corresponds with the height of the moisture plume reaching the area. (Image 6) In fact, almost 2 inches of precipitable moisture in the atmosphere is almost unheard of in the summer across California. Of course, any lightning can be disastrous so we will be watching this closely and following up.

The bottom line however is that the chances of storms in the Bay Area and North Bay remain low, but the most likely time frame if they do develop would be late Monday into Tuesday afternoon, and the most likely location is Lake County and north.

More updates to come.

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