Lightning in Bay Area forecast as monsoonal moisture nears

A heat wave in California was broken on Wednesday as monsoonal moisture from the Gulf of Mexico pushed into Southern California early Wednesday, delivering scattered thunderstorms and lightning across the region.

The National Weather Service reported a line of showers and thunderstorms over Ventura County and Los Angeles County just after 8:30 am “Expect off and on again lightning, small hail, and heavy rain through the day. Plan for extra time on the roads today ,” the weather service’s LA office said on Twitter.

The North American Lightning Detection Network detected 25,000 lightning strikes between Tuesday night and Wednesday morning across California.

The surge was pushing into the San Joaquin Valley and southern Sacramento Valley and will impact the San Francisco Bay Area this afternoon. As the monsoonal moisture moves into the area, clouds are expected to arrive from the east and there’s a slight chance for thunderstorms, hail, showers and dry lightning through Thursday.

A monsoon refers to a seasonal reversal of winds. The summer monsoon is associated with an increase in thunderstorm activity; in North America, winds pull moisture from the Pacific Ocean and the Gulf of Mexico and transport it into the Southwest US

The summer monsoon is a typical weather pattern that is most pronounced in the interior Southwest states of Arizona and New Mexico. Sometimes the moisture pushes northward into California.


In most situations, lightning is more common in the Sierra Nevada than along the coast due to the mountains’ lack of a marine layer and its topography. In the mountains, the air mass is forced to lift, intensifying the thunderstorms.

Drew Peterson, a forecaster with the weather service’s Monterey office, said that as the moisture approaches the Bay Area, a low-pressure system off the coast of Big Sur will drive it in a counterclockwise direction.

As the moisture moves east from the Sacramento Valley into the greater Bay Area, it will lose strength, but there’s still a chance for thunderstorms, bursts of heavy rain and hail and dry lightning, starting at 2 pm or 3 pm with the peak around 4 pm to 8 pm There’s a possibility of another round on Thursday.

“I’d say for the most part, we’re looking at scattered high-based showers with potentially an isolated thunderstorm or two,” Peterson said. “The most likely areas for this are pretty much from Hollister northward to about Benicia, including all of Santa Clara County, Contra Costa County, Alameda County, Solano County and Napa County, with the highest possibility on the eastern side of those counties.”

Peterson said there’s less likelihood for thunderstorms in Marin and San Francisco, but they are still possible.

While fire starts are a concern, Peterson said the weather service didn’t issue a red flag warning for the region.

“The risk is near critical, just below needing to put out a red flag warning, but we’re missing some of the key ingredients such as winds and widespread dry lightning that make the fire risk severe,” Peterson said.

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