textproduct: San Francisco Bay Area

KEY MESSAGES

Updated at 1210 AM PDT Mon May 4 2026

- Cooler temperatures through Tuesday with a chance for drizzle and light rain, mainly along the coastline and the Bay Area

- Warmer and drier conditions return mid to late week

SHORT TERM

Issued at 151 AM PDT Mon May 4 2026 (Today and tonight)

The 500 mb low center we've been monitoring is currently drifting and wobbling around approx 100 miles southwest of Monterey. It's temporarily quasi-stationary with the polar jet stream winds on the back side of the low while supported by subtropical jet stream winds at the base of the low. The geopotential height of the low center is ~ 559 decameters, for reference that's near the 10th percentile compared to Oakland upper air long term climatology for early May. Higher surface pressures extend underneath the upper low corresponding to chilly to cold lower level temperatures (925 mb and 850 mb temps). The cold air intrusion lifted the temperature inversion and ultimately did cause deeper vertical mixing to partially clear the sky for sunny breaks Sunday thanks to the May sun angle (heat energy into the system i.e. the Earth-atmosphere system).

And it's chilly across the area with temperatures in the 50s, 40s and a few upper 30s at higher elevations. Partial cloudiness continues through this morning.

With the low moving slowly today and tonight, warm air advection to the north of the low center will have a chance to interact with the colder air aloft resulting in light to moderate stratiform rain beginning later today over the North Bay then spreading across much of the Bay Area tonight. Rainfall forecasts have increased. Several hundredths to two to three tenths of an inch of rain (possibly slightly higher amounts may occur) are forecast across much of the Bay Area.

On first glance, one may think the counter-clockwise wind flow to the north of low center (east to northeast winds) from the Sierra Nevada would produce downsloping, a drier atmosphere and near zero or zero rain chances. And it is true it most certainly can do this. However, this low is in a different synoptic pattern. The low is in a temporary quasi-balance long enough for the warm air advection to arrive while most likely in near balance because of near surface higher pressures (surface pressures are lower over the Great Basin, meaning no offshore winds) and the net forces due to the upper level winds around the low. This is a great example as to how complex the atmosphere is, with numerous modes of circulation, etc.

LONG TERM

Issued at 151 AM PDT Mon May 4 2026 (Tuesday through Sunday)

The subtropical jet stream at the base of the 500 mb low will move the low eastward away from California by mid-week. After this, high pressure will move in from the west and northwest bringing with it dry conditions and a warming trend. In general because of the strengthening long wave troughing on each side of the high pressure ridge, meridional (north-south) displacements will also amplify/strengthen the ridge, with greater compressional warming and surface warming resulting in above to well above normal high temperatures in our forecast area by late week and weekend. Far inland locations especially by next weekend will likely reach the 80s and 90s.

AVIATION

(12Z TAFS) Issued at 438 AM PDT Mon May 4 2026

Scattered showers are expected to move into the Bay Area today with the highest confidence in showers impacting the North Bay. Given the more mixed out marine layer ceilings are higher with VFR conditions across the board. There have been temporary dips into MVFR territory overnight but these have been short lived with CIGs quickly rising. Moderate confidence that VFR conditions will last for much of the TAF period with MVFR conditions returning first to the North Bay this evening and spreading into the rest of the Bay Area overnight. Rain chances increase around 00Z for STS and APC with chances increasing closer to 09-12Z for the rest of the Bay Area. Not currently expecting showers to reach the Central Coast.

Vicinity of SFO...VFR with MVFR conditions becoming more likely overnight as showers reach SFO. Periods of MVFR CIGs are possible this morning but they are expected to be short lived with ceilings predominantly >3000 ft. Breezy onshore winds are expected this afternoon with gusts to around 18 knots. Showers are likely to reach SFO around 09Z but could arrive as early as 06Z. CIGs are expected to lower as showers return with MVFR CIGs returning tonight into tomorrow morning.

SFO Bridge Approach....Similar to SFO.

Monterey Bay Terminals...VFR with increasing chances for MVFR to return towards the end of the TAF period. Periods of MVFR CIGs are possible this morning but they are expected to be temporary. Breezy onshore winds return during the afternoon/evening before winds ease overnight. Isolated showers may impact MRY/SNS today into tonight but confidence is low that widespread showers will reach the Central Coast.

MARINE

(Today through Saturday) Issued at 438 AM PDT Mon May 4 2026

A light to gentle breeze continues over the waters with winds to become more moderate by Tuesday. A weak upper level disturbance lingers over the region through midweek. This disturbance will bring light, scattered showers and a non-zero chance of thunderstorms Monday into Tuesday. Northerly winds and seas strengthen Thursday into next weekend as high pressure rebuilds.

MTR WATCHES/WARNINGS/ADVISORIES

CA...None. PZ...None.