🕒 Last updated on September 23, 2025
A heat advisory is in effect for the Bay Area as the official start of fall brings summer-like conditions instead of cooler weather. At 11:19 a.m. Monday, the calendar will welcome autumn, but the atmosphere has other plans. Instead of crisp air, the region is bracing for a surge of heat that could push temperatures to some of their highest levels of the year.
A heat advisory is in effect as temperatures continue to rise
The heat advisory covers Monday and Tuesday, with Tuesday expected to bring the most extreme heat. A small ridge of high pressure has formed north of an offshore low-pressure system, locking it into place. This pattern allows hot, dry air to spread across California and fuels an offshore wind setup that drives warm air straight to the coast.
This means the marine layer—the natural cooling system for the Bay Area—will flatten and fail to rebuild overnight. With that buffer gone, daytime highs will soar. Weather models show temperatures three to six degrees above normal for late September.
San Francisco could climb to 90 degrees east of Twin Peaks, while the East Bay’s interior valleys may spike into the upper 90s and possibly the low 100s. Tuesday could be the hottest day of the year so far. The heat advisory warns that this unusual early-fall warmth will bring increased health risks, especially for sensitive groups such as older adults, children, and those without access to cooling.
Adding to the concern, a stretch of dry offshore winds is forecast from Monday into Tuesday, lasting 24 to 48 hours. This will raise the fire danger, as hot, dry, and windy conditions combine. The heat advisory urges residents to take precautions by staying hydrated, avoiding outdoor activity during peak heat, and checking in on vulnerable neighbors.
Heat gives way to showers and storms
Relief will come, but not before another weather twist. By Tuesday afternoon, the offshore low-pressure system will begin sliding northeast. This shift carries tropical moisture and a pocket of colder air aloft, creating instability that often leads to showers and thunderstorms.
Forecasters say the most likely time for rainfall is Wednesday morning into the afternoon. The North Bay could continue to see storms into the evening. But the exact timing is uncertain because cutoff lows are famously difficult to track.
If the system moves in quickly, showers could start in the morning with limited storm activity. If it lingers longer, the region could see heavier thunderstorms later in the day. Either way, enough moisture is in place to bring downpours, lightning, and thunder across parts of the Bay Area.
The Storm Prediction Center has placed the Bay Area and Central Coast under a slight risk of severe thunderstorms on Wednesday. This designation means the chance exists for scattered storms, heavy rain, and localized flooding. The heat advisory remains the most immediate concern, but residents are also being told to stay alert for sudden weather changes midweek.
Local Monday forecast under the heat advisory
San Francisco will start Monday with shallow morning clouds before the sun takes over. East of Sutro Tower, temperatures could reach the mid-70s to around 80, while the west side holds in the upper 60s to low 70s. The city will stay mostly clear at night, with lows in the upper 50s to low 60s.
The North Bay will heat quickly under sunny skies. Cities like Petaluma, Napa, and Santa Rosa are forecast to reach the mid- to upper 80s, while Fairfield and Vacaville may push into the low 90s. Even coastal towns such as Point Reyes and Bodega Bay will feel mild warmth in the mid-70s. Overnight lows will range from the upper 50s to low 60s.
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The East Bay will see its hottest conditions in the interior valleys. Concord, Walnut Creek, and Livermore are expected to hit the mid-90s. Closer to the bay, Oakland, Berkeley, and Richmond will be somewhat cooler but still warm, topping out in the upper 70s to mid-80s. Winds will pick up in the afternoon with gusts up to 15 mph.
Along the peninsula and Pacific Coast, sunshine will dominate. Pacifica, Half Moon Bay, and Daly City should see highs in the low to mid-70s, while San Mateo, Redwood City, and South San Francisco warm into the upper 70s to mid-80s. Overnight will stay mild, with lows in the low 60s.
In the South Bay, San Jose, Cupertino, and Milpitas are forecast to hit the mid-80s to low 90s. Santa Cruz will start with patchy fog and low clouds before skies clear, allowing highs to reach the upper 70s to near 80. Evening temperatures will dip into the upper 50s to low 60s.