Marine Weather and Tide Forecast for Sunnyvale, CA

UPDATE
Issued at 844 PM PST Fri Dec 12 2025

Tule Fog and radiational fog have begun to develop and spread across larger areas of the North Bay interior valleys and East Bay interior valleys. A Dense Fog Advisory has been issued until 11 AM Saturday morning. Please use caution if traveling in these areas tonight and Saturday morning. Don't use your high beams, give yourself extra time and extra distance between vehicles. Most locations that saw fog last night and this morning will see similar conditions tonight and tomorrow morning. The Beach Hazards Statement will be allowed to expire at 10 PM tonight with diminishing seas through the weekend.

SHORT TERM
Issued at 1248 PM PST Fri Dec 12 2025 (This evening through Saturday)

Persistent stratus and fog continues across the valleys of the North Bay and interior East Bay, with the clouds continuing to mix out throughout the afternoon and into the early evening hours, when the Tule Fog outgrowth returns to the Bay Area. To the south, a southerly surge is developing to the west of San Luis Obispo County, and will travel northwards along the Big Sur coast, flow into the Monterey Bay region, and progress along the San Mateo coast through the evening and overnight hours. Continuing to opt for a more persistent forecast for high temperatures today, with highs in the Central Coast valleys reaching the lower to middle 70s, the Bay Area valleys reaching the upper 50s to lower 60s barring the northern and eastern parts of the interior East Bay, where highs reach the middle 40s to near 50, and highs across the higher elevations into the lower to middle 70s. Overnight, expect lows in the upper 30s to middle 40s for the valley regions, and the middle 40s to the lower 50s in the higher elevations. Dense Fog Advisories may need to be issued early Saturday morning for the inland valleys, but also the immediate coast depending on the evolution of the Tule Fog and southerly surge through the next day.

By Saturday, the ridge that has brought us the last few days of stable conditions will begin move off to the east, with the coastal stratus retreating to the immediate coastal region. How the Tule Fog will react to the change isn't all too clear. The current forecast shows a similar clearing pattern to today's. High temperatures are currently expected to remain nearly the same as today's in the lower elevations, perhaps a couple of degrees cooler in the higher elevations.

LONG TERM
Issued at 1248 PM PST Fri Dec 12 2025 (Saturday night through next Thursday)

As we head into the early part of next week, the ridge continues to move to the east and a pattern change 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 1.2 inches, brings a chance of light rain to the region Tuesday and Wednesday, 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.

Beyond 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. The CPC outlooks are leaning towards precipitation totals above the seasonal average in their latest 6-10 day outlooks (from December 18th to the 22nd, 55-70% probability), giving the region a moderate chance for heavy rain and high winds (40-60% probability of total rainfall and sustained winds reaching the 85th percentile of the seasonal distribution) starting on the 20th. For context, when taking the long-term averages, December is the wettest month for downtown San Francisco with monthly average precipitation of 4.76 inches. We have not seen a drop of rain since the 20th of November, and for the 6-10 day outlook, the expected rainfall total lies somewhere around three quarters of an inch of rain. The chances for rain don't stop that weekend: CPC outlooks maintain the lean towards precipitation totals above seasonal averages in the 8-14 day outlook (December 20th-26th, seasonal average of over an inch of rainfall at downtown San Francisco), the moderate chances for heavy rain and high winds mentioned earlier in this paragraph extend through the 26th, and the ensemble mean products suggest a lingering trough in the eastern Pacific through this period, a pretty notable signal especially this far out into the forecast.

AVIATION
(06Z TAFS)
Issued at 949 PM PST Fri Dec 12 2025

Fog returns across the Bay Area and stratus returns to the Central Coast. For the Bay Area, followed a persistence forecast with fog arrival and dissipation timing for STS, APC, LVK, and SJC with fog not reaching SFO or OAK. Current obs show dense fog developing across the the Bay Area so have made the TAFs more pessimistic with more widespread 1/4SM visibilities. A southerly surge of stratus is bringing stratus south to north across the southern coastal waters with stratus to reach HAF, MRY, and SNS overnight. There is some potential for fog to develop, at least temporarily, at MRY and SNS overnight. Another round of stratus and fog is likely again tomorrow night.

Vicinity of SFO...VFR. Currently not anticipating stratus or fog to reach SFO tonight. Confidence is increasing that stratus will redevelop along the coastline by tomorrow night and will likely reach SFO early Sunday morning. Winds remain light and generally offshore through the TAF period but may become more variable at times.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...IFR-LIFR overnight. Kept the TAFs slightly more pessimistic with dense fog expected at least temporarily early tomorrow morning. The NBM is most supportive of dense fog developing and seems to be doing a good job with the arrival of the southerly surge. Winds generally stay light and offshore through the TAF period but may shift more onshore during the afternoon/evening hours.



MARINE
(Tonight through next Thursday)
Issued at 844 PM PST Fri Dec 12 2025

Dense fog is possible across the southern coastal waters tonight as a southerly surge of marine stratus moves northward. Gentle northeasterly winds will gradually shift southerly on Saturday morning before moderate northerly winds return again on Monday.
Seas continue to subside with slight waves continuing through the weekend. Unsettled weather returns by the middle of next week with seas to build and light rain possible across the coastal waters.

MTR WATCHES/WARNINGS/ADVISORIES
CA...Beach Hazards Statement until 10 PM PST this evening for CAZ006- 505-509-529-530.

Dense Fog Advisory until 11 AM PST Saturday for CAZ506-510.

PZ...None.