The warm-up continues in the Gulf region. When does it reach its climax?

After nearly five days under the influence of an early-season storm system, California is experiencing a major shift in its weather pattern.

Thursday’s big warm-up is a friendly reminder that this is the climatically warmest month for San Francisco and much of the coast. That’s thanks to flow from high pressure systems causing our offshore winds to ripple, including some Diablo winds.

These hot, dry winds often blow across the North Bay and East Bay countryside, warming the surrounding air as it approaches the San Francisco Peninsula.

Offshore winds, including a few weak Diablo winds, are set to bring gusts of 20 to 25 mph to some peaks in North Bay this weekend, while gusts of less than 10 to 15 mph are possible in Some tops of the Diablo series.


Some of those mechanisms will be operational this weekend as the high-pressure system ramps up over California’s Texas, blowing some hot, dry winds into the North Bay Mountains.

The good news is that the fire risk is low.

At first glance, it looks like we’ve got all the ingredients for a perilous weekend under the fire: a big warm-up across the state and some dry sea wind. So why aren’t there any red flags?

This is where some of the rain that occurred earlier this week comes in.

The Sierra range's 100-hour fuel moisture ratios, levels currently at 20.4%, or nearly double the average for this time of year.
The Sierra range’s 100-hour fuel moisture ratios, levels currently at 20.4%, or nearly double the average for this time of year.

Northern California Geographical Coordination Center

This week’s rain and snow accumulations across Northern California have been impressive for September.

This rain brought nearly three times the average monthly rainfall to downtown San Francisco — three tenths of an inch, compared to the average one tenth of an inch — and many other weather stations across the state have seen similar jumps.

And while these last showers were not spread evenly across the state, or the Bay Area for that matter, what they did do was saturate some of the most fire-prone plants in the Santa Cruz Mountains, the Southern Cascades, and the Sierra Nevada. Humidity levels are currently about double the average for this time of year and help protect the bushes from drying out and fire carrying

So, despite the 10 to 15 degree warm-up coming into the Bay Area, it looks like the non-human side of the fire risk will be low this weekend in our region and much of Northern California.

Friday and weekend

• San Francisco: Some scattered clouds will still linger on the west side before noon Friday, followed by sunny skies for the entire city. Enjoy the warm, dry weather this weekend, with afternoons peaking in the low 70s in the Sunset and Richmond areas. Saturday appears to be the warmest day of the weekend, with barely a cloud in the sky as daytime temperatures reach a range of 73 to 75 degrees.

Heading east of the Sutro Tower, look for daytime temperatures this weekend to reach about 5 degrees higher in downtown San Francisco. Friday afternoon temporary employees will quickly reach their mid-70s. Saturday is also set to be the warmest day on this side of the city, with downtown and Castro likely approaching 80 degrees. Fortunately, those temperatures drop back into the mid-70s by Sunday.

• Pacific coast and peninsula: Some of the faint morning clouds off the coast from Pacifica to Half Moon Bay on Friday morning will be replaced by sunny skies and a bit of a warm-up over the weekend. Look for lows of the 70s for highs on Friday, peaking in the mid-70s by Saturday before dropping to 70 by Sunday.

Warmer weather has also begun for residents of San Gregorio and Pescadero, where Saturday can record temperatures between 74 and 76 degrees. Otherwise, look for the lows of the 70s on Friday and Sunday.

Heading east of Interstate 280, the rest of San Mateo County can expect a marked warming trend, with less maritime influence allowing temperatures to soar into the upper 70s in southern San Francisco and Brisbane and into the upper 80s in Redwood City. Fortunately, those temperatures will drop by two to three degrees on Sunday as the bay breeze blows into the San Bruno Gap.

• North Bay: The warming trend comes in Napa, Sonoma, and Petaluma, where temperatures will rise 5-7 degrees on Friday, reaching 90 degrees in the afternoon. Saturday looks much warmer, with wide-range temperatures of 90 to 93 degrees expected for everyone in the Sonoma and Napa Valleys. By Sunday, the breeze off San Pablo Bay will weather the hot, dry air and bring the ’80s back to the area.

Looking to beat the heat? Sea breezes will shield Bodega Bay and the rest of the Sonoma Coast, keeping temperatures in the low 70s throughout the weekend.

Heading south of Novato, cities in Marin County like San Rafael and Tiburon will see hot air in the front lands this weekend. Expect a wave of warm temperatures starting in the low 80s on Friday, followed by the high 80s on Saturday and back into the low 80s on Sunday.

A similar wave of weekend temperatures awaits the rest of San Pablo Bay as it heads east on Highway 37 toward Vallejo this weekend.

• East Bay: The I-880 corridor from Oakland to Fremont could look for another 3 to 5 degrees of warmer Friday, with broadly low 80s combined with some scattering mid-80s temperatures in the Oakland and Berkeley hills. Temperatures will peak on Saturday as the trail and adjacent hills reach their upper 80s. But by Sunday, look for the bay breeze to take the area back to the low ’80s.

Residents east of I-680 are in for a 5-10 degree warm-up starting Friday. The 1990s are expected to spread to Antioch, Brentwood and Knott Creek. The mid-’90s will be possible in the San Ramon and Livermore Valleys on Saturday. Then, Sunday’s delta breeze will reduce some of the hot dry air, bringing temperatures down to 90.

• South Bay and Santa Cruz: Early Friday morning, clouds in South Bay cities like San Jose, Los Gatos and Sunnyvale will be replaced by sunny skies and dry air. Weather models lean toward another 3 to 5 degrees of warmer on Friday and Saturday. We expect the high ’80s to broadly shift on Friday to the low ’90s on Saturday before Gulf breezes cool us down to the mid-80s on Sunday.

Heading south along Highway 17, cities including Scotts Valley and Santa Cruz will remain on the cooler side, thanks to the breeze coming from Monterey Bay. Look for mid-70s and light US 1 winds from Davenport to Capitola, while gaps along the Santa Cruz Mountains will be bathed in cool breezes keeping temperatures in the mid-80s across most of the range.

Jerry Diaz (he/them) is a meteorologist for the San Francisco Chronicle newsroom. Email: [email protected] Twitter @geravitywave

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