An upper-level low-pressure system will reach peak strength Wednesday as its center drifts over the Bay Area, triggering a broader round of storms across Northern California. The Sacramento Valley and Sierra foothills east of Highway 99 face the greatest risk for widespread showers and thunderstorms, while strong cells are also likely to flare over the Mendocino National Forest. Some of that activity could slip south into Sonoma and Napa counties, bringing brief downpours, gusty winds and lightning to the Bay Area’s northern edge.
Storms will spark more easily on Wednesday for two reasons: the low itself will be stronger, and skies will feature more breaks in the clouds, allowing surface temperatures to climb higher than Tuesday. That extra daytime heating, combined with a pocket of cold air aloft, will steepen lapse rates — the kind of vertical temperature drop that primes the atmosphere for convection. With the system’s center parked over the Sacramento Valley by afternoon, ingredients will finally line up for more widespread and robust thunderstorm development.
Elevation will also matter. Thunderstorms need strong upward motion to get going, so hills and mountains will provide a natural lift. That’s why the Mendocino National Forest is likely to see the first afternoon storms, with radar lighting up after about noon. Weak upper-level winds mean these cells will move slowly, so any spot could pick up an inch or more of rain in just a few hours, raising minor flood concerns in low-lying areas. Some cells may pack small hail and lightning.
By mid- to late afternoon, storm activity will spread farther. After about 3 p.m., thunderstorms are expected across the Sacramento Valley near Redding and Chico and along the Sierra Nevada north of Lake Tahoe. At the same time, a few storms that originate in Mendocino County may drift into Sonoma and Napa counties, reaching places like Healdsburg, Santa Rosa and St. Helena. These cells will carry the same risks — heavy bursts of rain, small hail and lightning — and could make for a disruptive few hours in wine country and the North Bay hills.
Showers and storms are unlikely to push much farther south than the Golden Gate on Wednesday afternoon and evening, though a stray cell can’t be ruled out anywhere given the overall instability. If cloud cover breaks more than expected, the odds of an isolated downpour increase. Activity should diminish after sunset as daytime heating fades. By Thursday, the upper-level low will drift farther inland, reducing storm chances for the Bay Area while keeping the Central Valley and Sierra in line for another round of unsettled weather.
Wednesday breakdown
San Francisco and Pacific Coast: The day will start out mostly cloudy across the city and shoreline, with some clearing by the afternoon and a few scattered showers possible. Highs will range from the mid-60s in the Sunset, Richmond and coastal towns like Daly City and Pacifica to the upper 60s and low 70s downtown, SoMa and the Mission. West winds of 5 mph to 15 mph will build through the afternoon, funneled strongest through the Golden Gate and Twin Peaks. Skies remain cloudy into the evening with lows in the 60s…