, Issued at 111 AM PST Sat Dec 13 2025 (Today and tonight)
Today's main impact will be a tale of two fogs: advection and radiation. Let's start with the persistence side of the forecast, or what has been happening. Portions of the region will continue to be affected by tule fog (a type of radiation fog) as it seeps out of the Central Valley through neighboring gaps and passes which will continue to facilitate below normal temperatures. There is also a southerly surge underway which will bring more traditional advection fog to coastal locations. This will not only be a change to the warm, sunny afternoons that have been observed the last week, but also to the cold overnight conditions that have been made possible by clear skies which allow for radiational cooling. We will continue to monitor the evolution of both of these through the morning to see if the Dense Fog Advisory that is in effect through 11AM for North Bay and East Bay Interior Valleys needs to be extended in area, time, or both. Consider delaying unnecessary travel until visibilities improve. If travel is necessary, make sure to use your low-beam headlights, increase following distance between vehicles, and allow extra time and patience. Elsewhere, sensible weather will yield temperatures up to 15 degrees above normal as the region remains under the influence of upper-level longwave ridging. Offshore flow will also continue with high pressure across the Great Basin and a coastal trough off the West Coast.
..issued at 111 AM PST Sat Dec 13 2025 (Sunday through Friday)
The aforementioned upper-level longwave ridge will exit to the east Sunday, giving way to zonal flow for most if not all of the long term forecast. Passing disturbances to the north and an increase in precipitable water and integrated vapor transport values will renew rain chances as early as Monday with no clear cut dry day beyond that. Rainfall in the beginning of the week is expected to be light and beneficial for those who are lucky enough to get any. This will also act to disrupt the pattern that we have been in that has allowed for relentless tule fog. Towards the tail end of the long term forecast period, a relatively more significant system is forecast as a surface low pressure system and its attendant cold front encroach the California Coast. It is too soon to delve into the specifics, but expect a change in the pattern and a return to wet conditions next week. This weekend would be a great time to clean out gutters and trim trees away from your home and power lines.
But wait, there's more! Beyond the long term forecast of seven days, it continues to be advertised that there's a moderate risk of heavy precipitation and high winds Saturday, December 20th through Friday, December 26th. This will not only be important for daily life, but also holiday travel.
(12z TAFS) Issued at 349 AM PST Sat Dec 13 2025
Currently a mixed bag of LIFR-VFR at the terminals with tule fog in the Central Valley spilling into Interior North Bay and Interior East Bay terminals and a southerly surge bringing fog/stratus to coastal terminals. Satellite trends continue to show fog/stratus expanding, so there is still a potential for terminals that are VFR. Coastal terminals will have stratus return early tonight with later times if at all for interior terminals. Offshore winds overnight and onshore winds during the afternoon are expected to be light.
Vicinity of SFO, Currently MVFR with northeasterly flow. An isolated patch of stratus is developing on the western side of the terminal. The TAF may need to be amended if this continues to grow, with the ceiling likely on the cusp of MVFR/IFR. Aside from that, moderate confidence on VFR prevailing through the TAF period with sub-VFR conditions becoming possible towards the end of the TAF period. Northeasterly flow will back to become onshore with the afternoon sea breeze. Winds will largely become variable after the sea breeze diminishes.
SFO Bridge Approach, Similar to SFO.
Monterey Bay Terminals, Currently VFR with drainage flow at both terminals. Low to moderate confidence on the terminals being able to hold out and remain VFR through the afternoon. The things to watch will be the small patch of stratus/fog that is about 10 miles north of the terminals as well as the southerly surge over the Pacific Ocean and if it is able to curl around Monterey Bay. Drainage winds will back to become onshore with the afternoon sea breeze. An early return of stratus (and potentially fog) is expected tonight as the bank is already there and northwesterly flow will feed it right in.
(today through Thursday) Issued at 349 AM PST Sat Dec 13 2025
A southerly surge will bring 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, Dense Fog Advisory until 11 AM PST this morning for CAZ506-510.
PZ, None.
