, Issued at 1216 PM PST Sat Dec 13 2025 (This evening through Sunday)
The upper level ridge that brought us the days of stable weather is beginning to move off to the east, and an shortwave trough has set up across the State. Satellite imagery reveals that stratus persists across the valleys over the North Bay and interior East Bay valleys in addition to the southeastern part of San Mateo County, outgrowths of the Tule Fog across the Central Valley, and the immediate coastal regions near Point Reyes, San Mateo County, and south of Point Sur, as a southerly surge takes hold just off the coast. There is low to very low confidence in any clearing across the coastal regions, the Sonoma County valleys, the 101 corridor of Marin County, and the northern and eastern parts of Contra Costa County. Everywhere else, some clearing is possible through the rest of the afternoon.
High temperatures will be broadly similar to those yesterday. The coldest regions will be the eastern part of Marin County and the northern and eastern regions of Contra Costa County, where highs will remain in the middle to upper 40s. At the other extreme, the interior Central Coast valleys, shielded from both the coastal stratus and the Tule Fog, reach the middle to upper 70s. In general, the Bay Area valleys see highs in the middle 50s to lower 60s, although lingering stratus in the Sonoma County valleys makes the high temperatures a little uncertain, with the potential for the region to remain in the 40s if the stratus persists all day.
On Sunday, the shortwave trough crawls across the state and ridging redevelops on its backside, with conditions expected to be broadly similar to today's.
..issued at 1216 PM PST Sat Dec 13 2025 (Sunday night through next Friday)
As we head into the early part of upcoming work week, the ridge will continue to flatten, a pattern change that brings more zonal flow to the region by Tuesday. This new pattern should help scour out the Tule Fog and, when combined with a plume of moisture with PWAT values up to 1.25 inches, brings a chance of light rain to the region Tuesday and Wednesday, with rainfall totals focused on the North Bay. A weak ridge will move through the state on Thursday and Friday, but the overall flow should remain generally zonal and lingering showers and drizzle are possible from remnant moisture in the region.
Towards and beyond the end of the 7-day outlook, the forecast points towards another round of wet weather across the region for the weekend of the 20th and 21st as a deep trough in the eastern Pacific coincides with another plume of moisture moving through the West Coast. With a system this far out, details are still very much in flux. The CPC maintains a strong lean (60-80% probability) towards rainfall totals above the seasonal average in the 6-10 day window (December 19-23). Beyond that, the CPC maintains a moderate risk (40- 60% probability) of heavy rainfall and high winds into the Christmas holiday period with a 60-70% probability of rainfall totals above the seasonal average through the 8-14 day period (December 21-27), which would impact any holiday travel plans.
(18z TAFS) Issued at 1035 AM PST Sat Dec 13 2025
It's going to be slow to very slow going today regarding fog and low stratus /VLIFR-IFR/ mixing out in the North Bay and the East Bay. With offshore pressure gradients 8.0 mb WMC-SFO, 1.5 mb SAC- SFO and weak surface low pressure located over the coastal waters, the surface to near surface pattern will continue to strongly favor tapping the Central Valley source of cold air, fog and low stratus today. With a weak 500 mb trough overhead and a weakening of the lower level temperature inversion, it's up to the Sun to do what it can to try to break through the solid pool of incoming fog and low stratus; pilot reports indicate ~ 1,000 feet thickness from bases to tops. Peak surface heating this time of year is ~ 3 pm, only 5 hours from now. Away from the current fog and stratus, the sky is mainly VFR with a few lingering cold pockets of air around the Bay Shoreline and the south San Francisco Bay. Maritime fog and stratus /LIFR-IFR/ continues to brush along the coastline; local, limited inland intrusions remain possible with this area of fog and stratus through late morning and afternoon.
Below normal water vapor distribution mainly in the mid to upper levels of the troposphere supports nocturnal radiative cooling tonight to early Sunday, this will reinforce the ongoing Central Valley tule fog/low stratus. Neither the GFS nor the ECMWF are forecasting cirrus tonight and Sunday morning. Light offshore winds will quite likely transport fog and low stratus to areas that have nightly/daily fog and low stratus.
Vicinity of SFO, A patch of stratus lingers just north of KSFO nudged southward and westward with an influx of colder air arriving on light NE-E surface winds. This patch should continue to mix out, favoring moderate to high confidence VFR for the remainder of the morning and afternoon. Surface wind may shift back around to light northwest during the afternoon to early evening, then back to light northeast tonight and Sunday morning. Low confidence VFR forecast tonight and Sunday morning primarily because of the meso-scale models having difficulty forecasting this ongoing pattern.
SFO Bridge Approach, Low stratus clouds are getting nudged southward from influx of NE-E winds arriving at KSFO terminal. Cold surface air continues to wobble around the SF Bay, and this may result in redevelopment of low stratus patches back in the approach zone from late morning to early afternoon. Otherwise higher probability of VFR for the afternoon to early evening. Slantwise visibility may be reduced to moderate to poor at sunset and sunrise due to hazy conditions.
Monterey Bay Terminals, LIFR due to stratus and mist rotated inland from the northwest reaching KMRY, this will need another ~ 1/2 hour to mix out to VFR at KMRY. Otherwise moderate to high confidence VFR for the afternoon. Low to moderate confidence IFR due to stratus developing tonight and Sunday morning.
(today through Thursday) Issued at 859 AM PST Sat Dec 13 2025
A weak surface low pressure circulation over the outer coastal waters will result in gentle to moderate southerly breezes and the potential for dense fog today. Winds will veer to become northwesterly tomorrow night. Seas will remain moderate through Monday, building to become rough in the outer waters and northern inner waters by Tuesday. Several disturbances passing to the north next week will bring renewed rain chances and wind shifts.
Ca, None. PZ, None.
