First rain of season coming to Bay Area, flash flood warning on coast

Days after hot and dry weather led to red flag warnings in the North Bay, the Bay Area is forecast to get the first rain of the fall season this week. 

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A changing weather pattern is expected to bring cooler temperatures and wet weather to the region starting Tuesday, according to the National Weather Service. The shifting weather is due to a cut-off low-pressure system moving in from the Pacific Ocean and settling in over the area.

Light showers are expected to arrive on the Central Coast on Tuesday morning before making their way up to the Bay Area. The wet weather should last on and off through at least Thursday morning, according to the weather service forecast discussion.

Matt Mehle, a meteorologist at the National Weather Service, said the entire Bay Area could see between 0.05 to 0.15 inch of rain with the Santa Cruz Mountains and parts of the East Bay getting up to 0.25 inch of rain.

“Wednesday morning, people in San Francisco could wake up to some light rain moving through,” he said. “We have rain in the forecast all the way into Thursday.”

While the rainfall will be light, it’s the first rainstorm to arrive in the Bay Area in months, Mehle noted. 

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“This will wash the roads,” Mehle said of the light rain forecast. “We do urge people to have some caution on roadways because oil that has accumulated could cause issues.”

Further south, the Central Coast is likely to see heavier rain in contrast to the Bay Area, reaching up to 0.75 inch. The changing weather pattern is also forecast to bring unsettled weather to that region, leading to an up to 20% risk of thunderstorms in the area. The weather service warned this heavier rain could put Big Sur and nearby areas affected by the Timber and Plaskett wildfires at risk for flash flooding.

“If rainfall rates exceed their thresholds, the water will slip right off the surface, resulting in burn scar debris flows, which may lead to flash flooding. If your home is downslope of these burn scars, now is the time to prepare for the possibility of an emergency,” the forecast discussion said.