National Weather Service — craft rewrite

Voice: Black 101 (craft) Original: National Weather Service

The National Weather Service San Francisco Bay Area discussion, issued mid-morning Wednesday 16 September 2026, opens on a simple two-part message: cooler weather holds through Thursday, then a warming trend arrives Friday and into the weekend.

An update at dawn reported the marine layer already near 2,400 feet with widespread stratus. Northwest flow was producing downslope drying over the Santa Cruz Mountains, leaving Silicon Valley clear while morning cloud reached Healdsburg, Dublin and Pacheco Pass. No measurable drizzle had fallen. Clouds were expected to recede toward the coast after sunrise.

The short-term picture, valid through Wednesday night, is set by an upper-level low digging along the Oregon–California border. Water-vapor imagery showed the longwave pattern; a band of high cloud that had crossed the Bay Area the previous two days had been shunted south. Surface stratus, visible on the fog product, covered the coast and pushed into inland valleys, with observed depths earlier in the 1,100–1,500-foot range. Patchy drizzle along the immediate coast could not be ruled out, yet coverage and confidence were judged too low to put drizzle in the official morning forecast.

Colder 850-mb temperatures accompanying the dropping low support Wednesday highs in the 60s at the coast and 70s, with a few low 80s, inland—five to ten degrees below mid-September normals. The same airmass and onshore pressure gradients produce gusty flow this afternoon along the coast, inland valleys and gaps, notably Altamont and Pacheco Passes, where the windiest locations could gust 40–45 mph. Winds ease late in the day and overnight as the gradient relaxes.

Wednesday night, lower 500-mb heights allow the marine layer to deepen beyond 2,000 feet and toward 2,500–3,000 feet by Thursday morning. Combined with onshore flow and enhanced orographic lift, that raises confidence in widespread coastal drizzle. The setup is not one of nocturnal drying, a point noted as favorable from a fire-weather standpoint.

Thursday through Tuesday begins cloudy, cool and damp while the upper low lingers over northern California. By Friday the cutoff low exits northeast and a weak short-wave ridge follows for Saturday; the marine layer compresses slightly and temperatures rise. Confidence falls farther out as another upper low moves through California on Monday, producing a brief temperature dip. The strength of the subsequent ridge remains uncertain, though the forecast still shows warming through the middle of next week.

Aviation guidance (18Z TAFs) describes an interesting stratus pattern. STS and APC should clear within the hour; whether OAK and SFO clear at all is in question. Models allow patchy reductions at OAK, SFO, MRY and SNS, yet prevailing conditions are expected to remain MVFR. TAFs were kept slightly pessimistic, with possible brief improvements. Onshore winds this afternoon and evening run 10–15 knots with gusts near 25 knots at OAK and SFO. Tonight’s marine layer near 3,000 feet may place clouds on the MVFR/VFR border. Vicinity of SFO: stratus has returned after a brief morning clearance; a slightly optimistic TAF still brings VFR this afternoon before MVFR returns late tonight through Thursday morning, with gusts around 25 knots. Monterey Bay terminals remain MVFR through the day, with 10–15-knot westerlies this afternoon and lighter winds overnight.

Marine conditions through Monday feature moderate northwest-to-north winds over the outer waters, with occasional fresh-to-strong gusts diminishing today. West-to-southwest moderate breezes over the bays, gusting to 25 knots, create hazardous small-craft conditions from late morning through this evening. By Friday, widespread northwest-to-north breezes increase to fresh with occasional strong gusts. Wind-driven seas of 9–10 feet subside to moderate by evening and stay slight to moderate into the weekend.

Fire weather notes a passing trough expanding the marine layer to an estimated 2,000–2,500 feet around the Timber and Plaskett Fires. Above the inversion, relative humidity remains low, with values as low as 15 percent observed. Deeper marine layers are expected to continue into Friday, raising humidities at higher elevations. Some nocturnal drying is possible on the highest peaks, but coverage is expected to be minimal.

No California watches, warnings or advisories are in effect. Small-craft advisories cover San Francisco Bay north of the Bay Bridge until 9 p.m. PDT this evening, Monterey Bay from 3 p.m. to 9 p.m., and the 10–60 nm waters from Point Arena to Point Reyes until 3 p.m. this afternoon.