The Bay Area will be caught on the edge of a dynamic early season storm system Monday, bringing a mix of dry stretches, spotty showers and even a slim chance of thunderstorms. Forecast confidence remains low, not unusual for September, but the setup could wring out brief downpours in unexpected places.
A deep low-pressure system sitting off the Pacific Northwest is driving Monday’s weather, spinning a cold front toward California that will press into the North Coast by the afternoon. That front will deliver a solid dose of early season rain, anywhere from 1 to 2 inches from Point Arena north to the Oregon line, with even higher totals in the Siskiyous. A second system is expected to develop behind it by midweek, setting up a similar, unsettled environment that could get an extra boost of moisture from the remnants of a former typhoon.
For Northern California and the Bay Area, the story starts before the front arrives. Much of the region sits in the so-called warm sector, where southerly winds pull in moisture, making it feel humid despite a blanket of mid-level clouds keeping temperatures cool. The atmosphere overhead will be primed for scattered convection and that creates a borderline unstable setup where showers and thunderstorms can bubble up almost at random.
As always, areas with rugged and elevated terrain stand the best chance of seeing showers and storms. That includes the Sacramento Valley and Sierra foothills, but also the Bay Area’s own terrain — the Marin Headlands and Santa Cruz Mountains — where short-lived but intense bursts of rain are possible. Forecast models suggest some of these storms may first develop offshore and then drift inland if they can hold together. That makes it difficult to pin down exact locations, but even San Francisco, Oakland or San Jose could be on the receiving end of a sudden downpour…