Marine Weather and Tide Forecast for Sunnyvale, CA

UPDATE
Issued at 1136 AM PST Mon Dec 8 2025

The morning updating included the lowering of high temperatures in the North and East Bay due to the lingering stratus. Otherwise, the forecast remains on track this morning.

SHORT TERM
Issued at 117 AM PST Mon Dec 8 2025 (Today and tonight)

Tule Fog remains in the Central Valley and there is slightly more low stratus coverage across the Bay Area than this time last night. There is a good chance for these clouds to expand down to the surface as the the wind continues to calm through the morning.
The most likely locations to develop fog are the North Bay Valleys, Interior East Bay Valleys, and the Santa Clara Valley.
There's more uncertainty along the Bay side of San Mateo County.
Typically this area is less prone to fog than the cooler valleys, but is the best set-up for it. Light NNE winds over the Bay don't suffer from the downslope drying of winds with a westerly component. For example, historically the visibility at SFO drops below 3 miles 8% of the time in December between 5AM and 10AM. If we add a NNE wind direction constraint to these variables, the probability increases to 17%. On the other hand, NW winds bring this low visibility only 4% of the time. The otherwise clear skies and long December nights also help with the radiative cooling to generate this fog, and cloud top radiation helps to sustain it. The complicating factor is the relatively warm water in the Bay (55F) compared to the air temp (45F). This induces some instability that will work to lift fog into a low cloud deck if the low level winds don't calm this morning. In the end, I'll give a 30% chance for fog in Eastern San Mateo, 50% for the Santa Clara Valley, and 80% across the North and East Bay Valleys.
Otherwise the weather will be quite similar to the last couple days as the fog and low clouds gradually clear through the day and temperatures under the morning stratus struggle to get into the 60s in the afternoon.

LONG TERM
Issued at 117 AM PST Mon Dec 8 2025 (Tuesday through Sunday)

The long term forecast is mostly boring. There is a hint of rain at the end of the 7-day window, but let's spend a minute discussing this dry spell first. If you've read any of our AFDs in the last week, you know it's caused by a "ridiculously resilient ridge" over California that is keeping the storm track well to our North. It's now been 17 days since San Francisco recorded any rain, and that streak will very likely extend into the mid 20s. How unusual is that? Well it turns out a long dry spell in the middle of the rainy season is actually pretty common. There have been 54 instances of a dry spell lasting 24 days or more between November and March since 1849. In other words, there is a 31% (54/176)
chance of getting a dry spell at least this long any given Winter.
The longest such streak was 60 days from November 17, 1876 to January 15, 1877. We're not expected to get anywhere near that record as it looks like a pattern change is finally on the horizon. More and more ensemble members are agreeing that the ridge will finally break down with more a more active zonal pattern bringing the jet stream and chances for rain back to the Bay Area. 40/50 ECMWF ensemble members bring some rain to the Bay Area next week, with an outside chance for heavier rain the following weekend. We'll wait and see for now, but our official forecast finally has a decent probability of precipitation starting on the 15th.

AVIATION
(18Z TAFS)
Issued at 907 AM PST Mon Dec 8 2025

similar to previous TAF forecast - persistence is the name of the game. Tule fog and marine layer pattern will keep low CATs for N, E, S, and SF Bay terminals today through tomorrow. There is a low chance (20-40%) for some clearing this afternoon for N and E bay, but high confidence for SF and S Bay. Central Coast remains VFR.

Vicinity of SFO...SFO had a briefing clearing earlier, but patch over approach drifted back over terminal. Will keep CIGs through 20Z then thinning thereafter. Cigs return late tonight.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR with diurnal flow.

MARINE
(Today through Saturday)
Issued at 907 AM PST Mon Dec 8 2025

Persistent high pressure off the California coast will maintain moderate north to northwest winds continue for the coastal waters and locally to fresh winds south of Point Sur. Moderate seas with wave heights 6 to 8 feet across the outer waters will subside to around 4 to 6 feet today. Winds increase to a strong breeze Tuesday into mid week across the outer waters. A new, long period northwesterly swell is expected by Wednesday, lasting through the latter half of the week.

MTR WATCHES/WARNINGS/ADVISORIES
CA...None.
PZ...Small Craft Advisory until 9 AM PST Tuesday for Pt Pinos to Pt Piedras Blancas 0-10 nm.

Small Craft Advisory from 3 PM this afternoon to 3 AM PST Wednesday for Pigeon Pt to Pt Pinos 10-60 NM-Pt Arena to Pt Reyes 10-60 NM.