, Issued at 1204 PM PDT Mon May 4 2026 (This evening through Tuesday)
The satellite image shows some lingering low and mid-level clouds throughout the Bay Area and Central Coast, accompanied by a east-to- west band of high clouds entering the North and East Bays. Zooming out to the synoptic level pattern, an upper level low is crawling into the state, centered near the coast of Monterey County. This is bringing cool conditions throughout the region and rain across the SF Bay Area through Tuesday morning, as warm air advection driven by the low pressure system interacts with the cooler air aloft. As noted by the previous forecaster, lower surface pressures over the Great Basin are practically canceling the offshore flow effects that a east-to-west flow over the Sierra Nevada would typically produce. In fact, the SFO-WMC pressure gradient is strongly positive (that is, much higher pressure at the San Francisco Airport compared to Winnemucca in north-central Nevada -- 9.8 mb higher as of 11 AM) and will remain so through the overnight period. The rain band will come into Sonoma and Napa Counties this afternoon into the evening commute, steadily moving into the SF Bay Area this evening and overnight before dissipating Tuesday morning. Rain totals have continued to trend upwards, and the latest forecast now has totals of around half an inch for the interior mountains of the North Bay, around a quarter to half an inch for the North Bay in general, and a tenth to a quarter of an inch south of the Golden Gate through the southern reaches of San Mateo and Alameda counties. Localized rain totals near one inch, as suggested by some high-resolution model output, are very possible if the rain band overproduces. In addition, there remains a 5 to 10% chance for thunderstorms through the early part of Tuesday morning, with the chance for enhanced rainfall intensity where storms set up.
Temperatures remain rather cool today with highs in the middle to upper 60s across the inland valleys, and the upper 50s to lower 60s along the coast. There may be a slight warm up in the most interior valleys on Tuesday as highs go into the lower 70s, but otherwise similar conditions are expected on Tuesday.
..issued at 1204 PM PDT Mon May 4 2026 (Tuesday night through next Sunday)
By Wednesday, the upper level low moves into the Great Basin and southern Rockies with ridging building back behind it, leading to a warming and drying trend across the region. Highs in the inland valleys will steadily climb from the middle 70s to lower 80s on Wednesday up to the middle 80s to the lower 90s for the upcoming weekend, while locations near the Bays will warm from the middle 60s to lower 70s on Wednesday, to the lower 70s to the lower 80s for the weekend. These rising temperatures could bring Moderate HeatRisk for the inland valleys on Sunday, corresponding to a moderate chance for heat-related illnesses in sensitive populations (including children, the elderly, pregnant women, and people who work or live outdoors). Towards the latter part of the work week, a decaying front will sweep through the far northern parts of California, which may bring gusty conditions for the coast, ridgelines, and favorably oriented valleys and passes Thursday through Saturday.
The CPC extended outlook shows that temperatures above the seasonal averages are likely to persist through the middle of May, while precipitation totals lean below the seasonal average. For context, the seasonal average high in downtown San Francisco is around 64 degrees, while the seasonal average precipitation total for the May 11 to 17 period, as covered in the CPC 8-14 day outlook, falls a shade under two tenths of an inch for the same station.
(18z TAFS) Issued at 1130 AM PDT Mon May 4 2026
An upper low west of Monterey is allowing for showers to rotate off the Sierra and towards the Bay Area. Isolated showers and t-storms possible this afternoon but best chance of stratiform precip will occur between 05z-12z Tuesday over the the North Bay and SF Bay terminals. Cigs mainly VFR through 05z this evening then lowering at times. Morning overcast Tuesday should clear by 17-18z as the low moves eastward and high pressure builds.
Vicinity of SFO, Cigs mainly above 3000 feet this afternoon due to instability of nearby low. Best chance of showers occurs between 07-14z Tuesday with lowering of cigs. Clearing trend by 18z Tuesday as upper low ejects and drier NW flow returns with building high pressure.
SFO Bridge Approach, Similar to SFO under this synoptic pattern with clouds developing over the SF Peninsula but blowing over the bridge approach this afternoon/evening.
Monterey Bay Terminals, Cig heights mainly above 3000 feet the rest of this afternoon and evening.
(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.
Ca, None. PZ, None.
