Issued at 918 AM PST Sat Dec 20 2025
Issued a Dense Fog Advisory until Noon PST today for the North Bay and Marin Hills with traffic cameras indicating poor visibility. There are pockets of limited visibility in the Bay Area and East Bay as well, but not widespread enough for an advisory attm. Beyond the fog, the forecast remains on track for the most part, with scattered light rain expected for much of our area throughout the day with the exception of the interior Central Coast. By tonight, the plume of moisture associated with the ongoing atmospheric river oscillates a little farther north than HiRes guidance indicated at this time yesterday, which would be a sliver of good news wrt to impacts across the North Bay in the short term.
, Issued at 238 AM PST Sat Dec 20 2025 (Today through Monday)
Some light rain showers are streaming in from the west along a stationary front with a subtropical moisture tap. These will increase in coverage and intensity through the morning and into the afternoon, relatively speaking. Rainfall during this timeframe is expected to be beneficial; however, it will act to prime the pump for the succession of systems that are slated to impact our region through the forecast.
A Flood Watch is in effect from 4 PM today through 4 PM Monday for the North Bay as flooding caused by excessive rainfall is possible. Impacts include but are not limited to: excessive runoff may result in flooding of rivers, creeks, streams, and other low-lying and flood-prone locations, creeks and streams may rise out of their banks, low water crossings may become flooded, and standing water is likely in urban areas. People living or travelling through the North Bay should monitor later forecasts and be alert for possible Flood Warnings. Those living in areas prone to flooding should be prepared to take action should flooding develop. Confidence is high that even though the mainstem rivers (namely the Napa and Russian) will rise, they will not have any problems during this timeframe with ensemble forecasts showing a 0% probability of them reaching action/monitor stage. This is not to say that their flashy creeks/streams will not have problems, like Mark West Creek. If you encounter flooded roads, do not drive around barricades and turn around, don't drown!
Moderate to heavy rainfall arrives this evening for the North Bay as IVT rebounds above 500 kg/ms for 24 hours and gets picked up by a developing surface low off the California Coast. The firehose will be focused on the North Bay through Sunday morning before it slowly begins to slide south Sunday afternoon to include the rest of the Bay Area and Northern Monterey Bay. As it does so, moderate to heavy rainfall will spread across the aforementioned areas. Southern Monterey Bay will remain on the periphery, so periods of moderate to heavy rainfall can be expected there as well. While the Central Coast and the rest of the Bay Area are not in the Flood Watch, I would still expect ponding on roadways and at least some flooding in low-lying, poor drainage, and urban areas including connectors, off/onramps, and underpasses of freeways. Strong southerly winds will also arrive tonight and persist through Sunday as the attendant cold front approaches the area. Gusts up to 40 mph can be expected along the coast, across ridgelines, and through gaps and passes. The cold front will slowly move in Sunday into Monday, allowing moderate to heavy rainfall to persist across the same areas. Likely after we get through this period will we issue a region wide Flood Watch beginning sometime in the Monday evening/Tuesday timeframe.
..issued at 238 AM PST Sat Dec 20 2025 (Tuesday through Friday)
By Tuesday, a developing gale force low off the California Coast will tap into that same stream of subtropical moisture. Unfortunately, there still remains a lot of uncertainty between the deterministic ECMWF and GFS as well as their ensembles, most notably when it comes to moisture content and strength. These differences also get propagated downstream into location and timing. Nonetheless, this system is expected to bring the most impacts up to this point. Why? Well, one of the reasons is that it is forecast to track parallel to our coastline, not only allowing for more impacts based on proximity but also geography. An efficient wind field will develop along coastal jet regions and through northwest/southeast oriented valleys. The other reason is that the region will have seen three days of rain at this point with at least some minor/nuisance flooding expected by this point. This is where the differences matter. The ECMWF is offering a bomb cyclone as a solution while the GFS is not as strong. I understand bomb cyclone can be a scary word for those not in the atmospheric sciences world. All it means is that an extratropical cyclone decreases in central minimum pressure by 24 mb in 24 hours using a reference latitude of 60 degrees North. Without boring you with the math, since we are lower in latitude (36 to 38 degrees North), we don't need the full 24 mb. Similar to atmospheric river, the term bomb cyclone does not matter, but its impacts do. Continuing along with the theme that the ECMWF is stronger than the GFS, the former is nosing a 70 knot low-level jet into the region. Some meteorological rule of thumb is that in a perfect atmosphere these speeds (80 mph) can be realized at higher elevations and potentially along the coast. More tree impacts may be possible due to the opposite of normal wind directions and loose, saturated soils increasing the risk for downed trees and thus downed power lines. Thunderstorm chances increase to become slight (15%) during this timeframe as well, but once again this will be heavily dependent on the track of the low. While there is a lot of uncertainty, it is certain that there will be more rain and wind Tuesday into Wednesday as a surface low and its attendant cold front make landfall somewhere in Northern California. This is also the timeframe where we can start to see probabilities of exceedance starting to creep up on ensemble forecasts for mainstem rivers.
Global ensemble clusters are in agreement in an anomalously deep upper-level trough digging off the West Coast into Thursday (Christmas Day). There is discrepancy on whether it will be an open wave or a closed low as well as its tilt. Nonetheless, it will pick up yet another surface low from the Eastern Pacific Ocean and bring it into California Thursday into Friday. Thus, more rain and wind can be expected. This will also bring the return of hazardous beach conditions to all Pacific Coast beaches. Locals and visitors alike should remain away from the Pacific Ocean during this time.
Over the next seven days expect impacts to worsen as soils become saturated. Any wind impacts will be exacerbated by the rain and vice versa. The combination of wind and moist soils will increase the risk for downed trees and thus downed power lines. Additionally, any leaves that are left on trees will pose the risk of blocking/clogging storm drains if/when they fall, potentially leading to more flooding. When all is said and done, locations can generally expect a December's worth of rain or a quarter of their annual averages. If you are travelling by car for the Christmas holiday or otherwise, I urge you to take the weather into account. If you would like a localized forecast, like many of you do, visit weather.gov/forecastpoints for a zip code specific forecast.
(12z TAFS) Issued at 418 AM PST Sat Dec 20 2025
An approaching low pressure system is bringing reduced visibilities, light rain, and fluctuating ceilings to airports across the region. Confidence is highest in MVFR conditions persisting but temporary drops to IFR and LIFR CIGs are possible through late this morning. Guidance favors IFR CIGs but CIGs have been consistently coming in as MVFR or above 3000 ft. Current thinking is that CIGs will stay MVFR or higher due to the approaching system and incoming showers. Similarly, guidance is showing more significant drops in visibility while obs show it remaining largely stable so far. Thinking is that visibility will initially remain more stable but decrease as more persistent moderate showers arrive this afternoon/tonight. Patchy fog has developed in the North Bay - impacting STS - but this is not expected to be widespread. Winds strengthen towards the end of the TAF period with gusts in excess of 20 to 25 knots possible along the coast, North Bay, and SF Bay Shoreline.
Vicinity of SFO, Light showers are moving through the Bay Area with ceiling heights fluctuating between MVFR and VFR. Moderate confidence that VFR CIGs will generally prevail through the afternoon with a tempo for MVFR CIGs through 15Z. Leaned on LAMP guidance for the return of MVFR CIGs and brought in MVFR conditions around 02Z when chances increased and more widespread light to moderate rain returns to SFO. Winds strengthen by mid to late tomorrow morning with gusts between 25 to 30 knots likely.
SFO Bridge Approach, Similar to SFO.
Monterey Bay Terminals, CIGs remain on the IFR-MVFR border with light offshore winds continuing. Showers are developing in the vicinity of MRY and SNS over the Monterey Bay but they are not expected to cause any major impacts this morning. Rain showers become more widespread tonight into tomorrow morning with light to moderate rain reaching both terminals by 08/09Z tomorrow. Generally expecting overcast conditions to persist for much of today but may
(today through Thursday) Issued at 238 AM PST Sat Dec 20 2025
Light to moderate rain continues through Tuesday with shower coverage increasing today. Rain intensity increases over the northern waters by Sunday. Winds remain light and southerly through tonight when winds strengthen to fresh to locally strong ahead of cold frontal passage Sunday into Monday. Winds diminish early Monday before restrengthening late Tuesday as a more substantial, stronger system arrives Tuesday into Wednesday. This system will bring widespread gale force to near gale force winds and the potential for isolated hurricane force gusts across the interior coastal waters from Point Sur to Point Pinos. Seas build through the second half of next week, becoming very hazardous for mariners with the potential for wave heights in excess of 20 feet.
(Today through Thursday) Issued at 918 AM PST Sat Dec 20 2025
Wet and unsettled weather to prevail through Tuesday, with increasing coverage in the showers activity today. Rain intensity increases over the northern waters by Sunday. Winds remain light and southerly through tonight when winds strengthen to fresh to locally strong ahead of cold frontal passage Sunday into Monday. Winds diminish early Monday before restrengthening late Tuesday as a more substantial, stronger system arrives Tuesday into Wednesday. This system will bring widespread gale force to near gale force winds and the potential for isolated hurricane force gusts across the interior coastal waters from Point Sur to Point Pinos. Seas build through the second half of next week, becoming very hazardous for mariners with the potential for wave heights in excess of 20 feet.
Ca, Dense Fog Advisory until noon PST today for CAZ502-506.
Flood Watch from 4 PM PST this afternoon through Monday afternoon for CAZ502>506.
PZ, Small Craft Advisory from 9 PM this evening to 9 PM PST Sunday for Pt Arena to Pt Reyes 0-10 nm-Pt Arena to Pt Reyes 10-60 NM-Pt Reyes to Pigeon Pt 0-10 nm-SF Bay N of Bay Bridge.
Small Craft Advisory from 9 AM to 9 PM PST Sunday for Mry Bay-Pt Pinos to Pt Piedras Blancas 0-10 nm.
Small Craft Advisory from 3 AM to 9 PM PST Sunday for Pigeon Pt to Pt Pinos 0-10 nm-Pigeon Pt to Pt Pinos 10-60 NM.
