The Bay Area is expected to receive less rain than areas farther south, but Wednesday could be the wettest day since May in San Francisco, Oakland and San Jose. The storm’s most serious local risks include possible debris flows in wildfire burn areas near Big Sur.

The National Weather Service has issued a slight risk of flash flooding for parts of the Central and Southern California coast, including areas from Monterey and Big Sur toward Santa Barbara and Los Angeles. A lower, marginal flood risk extends across a broader area, including coastal Monterey County.

Forecasters are particularly concerned about heavy bursts of rain on the Big Sur coast, where the Timber and Plaskett wildfire burn scars could be vulnerable. Weather service forecasters warned of potential debris flows. The U.S. Geological Survey says a quarter-inch of rain in 15 minutes could be enough to send mud, rocks and other debris across Highway 1.

“Late Wednesday morning through late Thursday morning will probably be the peak of it,” said Bryan Lewis, a meteorologist with the National Weather Service in Los Angeles, noting that forecast rain totals have been increasing.

Most Southern California coastal and valley locations are forecast to receive half an inch to 1.5 inches of rain, with as much as 3 inches possible in higher terrain. Big Sur and the Santa Lucia Range could receive 1 to 1.5 inches, while San Luis Obispo is forecast to get about an inch.

Thunderstorms could develop from around Big Sur to Southern California on Wednesday. The weather service has placed the coast from Monterey to Orange County under a marginal severe-weather risk, citing lightning, wind gusts of up to 60 mph and a low chance of a brief, weak tornado or waterspout.

The Bay Area’s rainfall is expected to be lighter, though still enough to mark a notable shift after the dry summer. The storm arrives as a historically strong El Niño is in place; warmer-than-normal ocean temperatures may also add heat and moisture to coastal storms, according to the forecast.

In the Sierra, a winter weather advisory covers the central and southern range, where more than 6 inches of snow could fall above 8,000 feet, including upper elevations of Yosemite, Kings Canyon and Sequoia national parks. Tioga Pass is scheduled to close temporarily at 5 p.m. Wednesday ahead of the storm.

Up to 10 inches of snow is possible at high elevations in Mono County, including Mammoth Mountain. Lake Tahoe is expected to see rain, while nearby mountain peaks could receive an inch or two of snow. Thursday is forecast to be the snowiest day in the Sierra.