Rotating storm brings rain, thousands of lightning strikes to Northern California

LATEST Oct. 22, 12:10 p.m. A storm system churning above the Bay Area brought a significant amount of lightning to the region in the past 24 hours. The National Weather Service reported at least 24,000 instances of lightning strikes in the Central Valley, Central Coast and Tahoe region. 

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Only a “small fraction” reached the ground, but the weather service cautioned thunderstorms are still possible Thursday as the storm continues to move through the area. There’s now a 10 to 15% chance of thunderstorms developing in the East Bay, South Bay and Central Coast. Matt Mehle, a meteorologist at the National Weather Service, said most areas have seen beneficial rain, meaning the thousands of strikes are not likely to cause wildfires.

“The storms are relatively slow, so they have what would be classified as wetting rain,” he said, although he cautioned some strikes may have occurred in drier conditions. He also said the relatively low rainfall seen in recent days means another large wildfire is still possible this fall because fuels are dry. 

“This is not a season-ending event in terms of fire,” he said. “We need a lot more rain than we got before we put a fork in fire season.”

Oct. 22, 8:25 a.m.A low-pressure system is pulling a blanket of wet weather over the Bay Area on Thursday, increasing the risk of thunderstorms for the region.

The rain is likely to be heaviest in the morning, potentially affecting the commute for many across the Bay Area, before tapering off this afternoon and evening, according to the National Weather Service.

The unsettled weather is moving in an unusual direction from inland to back out over the ocean. 

“We actually have a low-pressure center … south of Monterey County, that is still spinning,” Roger Gass, a meteorologist at the National Weather Service, told SFGATE. “That rotation around the low is why you’re seeing the clouds moving out over the ocean.”

As that system moves inland, the wet weather will again shift back over the region, resulting in more showers.

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Gass said parts of the Bay Area could see up to a quarter of an inch of rain with a chance of lightning and thunder popping up.

“I wouldn’t rule out a potential isolated thunderstorm here or there,” he told SFGATE. 

The East Bay and South Bay have the highest chance of seeing a thunderstorm, with about a 20% risk of stormy weather developing in those regions; elsewhere in the Bay Area, the risk tops out at 10%, Gass said. Thursday’s rain is part of the Bay Area’s first real storm of the season, which arrived earlier this week. The storm has brought relatively light rainfall so far despite concerns that a historically strong El Niño could bring severe wet weather to the state this winter.

Gass said rainfall in the past 24 hours has reached a quarter of an inch to half an inch in parts of the East Bay, including Danville and Walnut Creek. Elsewhere in the Bay Area, rainfall was far less, with downtown San Francisco seeing 0.02 inch reported.

The risk of heavy rain from this storm causing debris flow in the burn scars of the Timber and Plaskett fires around Big Sur has mostly diminished as of Thursday morning, Gass said, with no movement seen overnight.

The wet weather pattern is expected to shift again starting on Friday. 

“We are expecting drier conditions by tomorrow and then more so into the weekend, as we start to see a gradual warming trend,” Gass said.