Issued at 920 AM PST Tue Dec 16 2025
Light rain is moving into Sonoma County as the first of two weak systems arrives this week. Rain totals over the last six hour show a trace of rain up to 0.04" across the North Bay. Precipitation totals for this system are expected to be on the lower side with up to 0.25" across the North Bay and up to 0.10" along the Bay Shoreline and along the San Francisco/San Mateo peninsula coastline. We are continuing to monitor the potential for a stronger system to arrive next week which may be impactful for holiday travel. Confidence has increased slightly that this system will be impactful but there is still time for the forecast to change as the storm is not fully within our forecast period.
Kennedy
, Issued at 253 AM PST Tue Dec 16 2025 (Today and Wednesday)
Tule fog in the Central Valley has seeped into the Delta and subsequently into the North Bay Interior Valleys and East Bay Interior Valleys. Given the slower arrival of the beneficial, light rain and its accompanying cloud shield, expect conditions to deteriorate through the night, although they will not be as encompassing as last night. If driving, slow down, use your low-beam headlights, and leave plenty of distance ahead of you. If commuting, allow extra drive time to reach your destination safely. Now onto the rain. As mentioned it is arriving behind schedule with no measurable precipitation falling yet. In fact, you will have to look upstream at Eureka's radar (KBHX) to see anything at all. There's also been a small cluster of lightning activity about 200 miles off the Mendocino County Coast just to the north of our area. There's a slight (up to 15% chance) for thunderstorms today for far northern areas of the North Bay and adjacent coastal waters. The critical ingredients for thunderstorms of lift, instability, and moisture are there, but barely. Frontolysis of the cold front and conditionally unstable lapse rates of 6 degrees C/km producing low CAPE of 50 J/kg are not ideal for the first two ingredients. Moisture is how we even have a slight chance. The 00Z OAK sounding observed a PWAT value of 0.84 inches which is near the 90th percentile (0.89 inches) for the date and time. The moisture tap goes back to the subtropics and is essentially on a conveyor belt between a storm force low in the Gulf of Alaska and surface high pressure off the California/Mexico border. PWATs and IVT values will continue to increase through the day. Still, only light and beneficial rain is expected as our region remains on the periphery of the main event to the north. More beneficial light rain is expected into Wednesday as a second cold front wrings out the leftover moisture from the past two days. Rainfall totals through Wednesday: Coastal North Bay up to 0.50", rest of Bay Area less than 0.10", Central Coast coastal/higher terrain drizzle if not dry.
..issued at 253 AM PST Tue Dec 16 2025 (Thursday through Monday)
Some more beneficial, light rain is possible Thursday as the cold front becomes a stationary front across the state of California. By Thursday, global ensemble clusters are in agreement that upper-level high pressure near the California/Mexico border will begin to weaken and exit east thanks to an anomalously deep longwave trough developing over the Northeast Pacific Ocean. This switch to meridional flow from zonal flow will allow the storm door to open. The next system is a little complicated with the surface feature separated from the moisture source. We'll start with the gale force low that is currently north of Hawaii. This will move very slowly to the northeast, ultimately becoming stationary. Its moisture will get pulled into a gale force low in British Columbia. This trajectory will allow for the firehose of moisture to get directed at the Pacific Northwest, at least initially. The plume of moisture will slink down the West Coast into California on Friday with a cold front following suit on Saturday. This will bring widespread rainfall from north to south Friday into Saturday. It is important to note that everything as of now looks beneficial up until this point and rainfall totals have trended down. Yes, it may rain every day between now and next Tuesday, but we have been dry for the last three weeks or so. The aforementioned plume of moisture that is tied to the stationary front north of Hawaii is expected to gain steam with the development of a new low and linger into at least Monday based on ECMWF and GFS IVT forecasts. GEFS and ECMWF ensemble IVT forecasts generally show the ensemble means and +/- 1 standard deviation generally in the 250-500 kg/ms range. Rainfall totals Thursday through Monday: Coastal North Bay up to 5.00", rest of North Bay: 1.50"-4.00", rest of Bay Area 0.50"-1.50", Central Coast: 0.10"-1.50". Another potential impact could be wind Sunday into next week. It is important to know that the details of the long term forecast will be dependent on what happens between now and then as it will be a case of cumulative impacts. It is also important to note that any details beyond seven days are beyond the reaches of the official forecast and are incredibly difficult to predict with any kind of accuracy. It can be continued to be said though that we are expecting the wet pattern to continue at least through Christmas Day (60% confidence) with the potential (20-40% confidence) for it to linger through December 29th. Whether you are travelling or staying home for the holiday, now is the time to begin preparing by cleaning out your gutters and ensuring that loose outdoor items are secured (like your Christmas inflatables). If you are travelling out of the area by car, please take the weather into account and check the forecast of the National Weather Service for that area and the areas in between here and there.
(18z TAFS) Issued at 1009 AM PST Tue Dec 16 2025
Conditions vary from LIFR to VFR this morning before ceilings lift briefly this afternoon (generally MVFR to VFR). It's currently raining in the far northern portions of the North Bay this morning, with that plume of moisture spreading southward throughout this evening and into Wednesday morning as a weak frontal boundary approaches the Bay Area. As such, we are expecting lowering ceilings and/or visibilities later this afternoon across the North Bay and into early Wednesday morning over the Bay Area as the boundary advances southward. The boundary will begin to dissipate as it reaches the Monterey Bay terminals.
Vicinity of SFO, MVFR currently, with periods of VFR early this afternoon. Ceilings and visibilities are forecast to gradually lower this evening and into Wednesday morning (IFR-MVFR and potentially LIFR) with the greatest probability for light rain to be early to midmorning on Wednesday.
SFO Bridge Approach, Greater probability for VFR through this afternoon across the approach. Otherwise, similar to SFO.
Monterey Bay Terminals, VFR. Ceilings and visibilities are forecast to lower to IFR-MVFR late this evening with pockets of LIFR early Wednesday morning.
(today through Sunday) Issued at 920 AM PST Tue Dec 16 2025
A plume of moisture from the Pacific will arrive through tonight ahead of a cool front moving southeast across the waters and bays Wednesday. The interaction between the plume moisture, chilly air entrenched across the bays and cool front arriving Wednesday will result in periodic light rain, mainly over the northern waters and bays later today into Wednesday. Moderate seas will prevail during the week.
Ca, None. PZ, Small Craft Advisory until 9 PM PST this evening for Pt Pinos to Pt Piedras Blancas 0-10 nm.
