San Francisco is about to close out summer with one of its hottest stretches of the season and a twist that could bring thunderstorms before fall officially begins. A complex pattern of building high pressure, offshore winds and incoming tropical moisture will drive a three-day warm spell from Monday through Wednesday, with daytime highs climbing into the 80s and a few neighborhoods pushing toward 90 degrees.
September heat in San Francisco isn’t unusual, but it takes a specific atmospheric setup to get there. A combination of building high pressure and downslope winds will hold off the ocean influence just long enough for the city’s eastern half, along with Oakland, San Jose and much of the East Bay, to heat up more like the inland valleys.
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A narrow ridge of high pressure will begin nosing across California on Monday. Its northeast orientation favors the development of offshore winds, the dry, downslope breezes that can briefly silence the marine layer. Those winds look to be at their strongest Monday and Tuesday, even if the high pressure ridge isn’t at its peak strength. That creates the potential for a surprise burst of heat in the city if the sea breeze stays shut down through the afternoon.
On Monday, high temperatures will peak in the low to mid-70s along Ocean Beach and the Sunset and Richmond districts, with the Mission, SoMa and downtown areas reaching the low 80s. A few of the city’s traditionally warmest pockets may brush the mid- to upper 80s. Oakland and the bayside East Bay should land in the upper 70s to mid-80s, while inland valleys including Santa Rosa, Concord and San Jose climb into the upper 80s and 90s.
By Tuesday the ridge strengthens, warming the entire atmospheric column by a few degrees. That will make inland areas notably hotter, with many spots pushing well into the 90s. Walnut Creek and Napa Valley could touch the upper 90s…