FXUS66 KMTR 211712 AFDMTR Area Forecast Discussion National Weather Service San Francisco CA 912 AM PST Sun Dec 21 2025 ...New UPDATE... .KEY MESSAGES... Updated at 151 AM PST Sun Dec 21 2025 - Flood Watch in effect now through Monday afternoon for the North Bay; extension in time and coverage likely - Moderate to heavy rain reaches North Bay this morning; Bay Area and Central Coast this afternoon - Moderate to heavy rain continues with increasing flooding potential through the extended - Impactful and likely very hazardous winds Tuesday through early Friday along our entire coastline and higher elevations inland - Dangerous beach conditions likely late Tuesday through Friday for all Pacific Coast beaches && .UPDATE... Issued at 911 AM PST Sun Dec 21 2025 Forecast remains on track with only a slight change to PoPs to lower chances out side the atmospheric plume that may see only brief scattered light rain to no rain as the atmospheric river continues its oscillation up and down our coast line between now an early Tuesday morning. The Flood Watch for the North Bay remains in effect through Monday afternoon with high confidence that there will be several waves of moderate to heavy rain within the watch area today and tonight. Fortunately, the pulses of increased rainfall rates are fairly progressive which should help mitigate the potential for more impactful widespread nuisance flooding and/or flash flooding as we move through the day. The latest update for the Mark West Creek near Mirabel Heights is expected to approach 'action stage' early this evening around 7pm PST. It's an area prone to flooding in these types of environments. Do not ignore road closures or signs and never drive through standing or moving water, TURN AROUND, DONT DROWN! Give yourself more time this week if traveling within our forecast area, especially across the North Bay. && .SHORT TERM... Issued at 151 AM PST Sun Dec 21 2025 (Today and tonight) Moderate to heavy rain is entering the North Bay with flooding concerns to increase over the course of the day. Currently, the heaviest rain is still located to our north in the Eureka WFO's county warning area. This band is expected to shift southwards overnight with the heaviest rain reaching Sonoma, Napa, and Marin Counties by late morning into the early afternoon hours. The main rainband will shift further south into the rest of the Bay Area and northern Monterey Bay by late tonight into early Monday morning. The highest rain totals are still expected to be across the North Bay where the higher elevations will see between 5-6" of rain, 2-3" across the valleys, and 1-3" across the rest of the Bay Area. The Santa Clara Valley will be rainshadowed by the Santa Cruz Mountains with totals ranging from 0.5-1.0" expected. Up to half an inch is expected for the Monterey Bay region with between 1-2" across the Big Sur/Santa Lucia Range. The surface low pressure associated with this system is expected to move inland to our north along the California/Oregon border. While we will still see gustier winds, particularly along the coast, the strongest winds will be located closer to the low pressure system across far northern California. Gusts will peak between 30 to 40 mph along the coastline and higher elevations with gusts between 25 to 35 mph possible within the interior valleys. All outdoor preparations for both precipitation and wind concerns should be completed by this time. There is a 15-20% chance of thunder across the coastal waters and a 10-15% chance of thunder across the Bay Area this afternoon and evening as the main rain band shifts south. High resolution guidance suggests stratiform rain will continue through the remainder of today with MUCAPE values increasing to around 100 J/kg this evening/overnight. The highest chances for thunderstorms would be within the Monterey Bay region tonight into tomorrow. Not going to rule out an isolated thunderstorm or two tonight for the Monterey Bay region but confidence in thunderstorm development at this time is low. The initial Flood Watch remains in effect for the North Bay through 4PM Monday and is expected to be followed by a more widespread Flood Watch across our CWA. Initial flooding concerns tonight remain low but will start to increase by late today as light rain gives way to more moderate to at times heavy rain. For residents of the North Bay, nuisance flooding is likely to start increasing in frequency by this evening with sharp rises in flashy (i.e. smaller) creeks and streams expected. The best example of this would be the Mark West Creek (forecast to reach action stage Sunday evening) in Sonoma County which can rise quickly during periods of heavy rain. If you encounter a flooded roadway, Turn Around, Don't Drown! Over the last few years, particularly at Mark West Creek, there have been several fatalities with regards to people driving into and getting swept away by flood waters. It is not worth it to try and drive through a flooded roadway, plan alternate routes to reach your destination in the event that you encounter a flooded roadway. Flooding concerns will ramp up as we head into next week and the long duration atmospheric river continues. && .LONG TERM... Issued at 151 AM PST Sun Dec 21 2025 (Monday through Saturday) Not too much change to the overall precipitation totals in the Long Term forecast. Monday looks to be a break between incoming rounds of moderate to heavy rain as forecast IVT values weaken and generally light to moderate rain continues for the Bay Area and northern Monterey Bay. Orthographic lift may sustain locally higher rain rates/totals across the Santa Cruz Mountains which could see up to 1" of rain on Monday. Tuesday into Wednesday, widespread moderate to heavy rain returns to the Bay Area and Central Coast as a low pressure system moves northwards parallel to the California coastline. A weaker low pressure system will move into northern CA Thursday into Friday and bring another round of moderate to heavy rain to the region. Light rain then continues into the weekend but precipitation totals look to be on the light. Flooding impacts are expected to increase moving through the week as soils become saturated from successive rounds of rain. This will result in increased ponding/flooding on roadways and increased surface runoff into creeks/streams resulting in sharp rises and potential for rivers in the North Bay (Russian and Napa) to reach at least action to minor flood stage. More flood prone creeks such as the San Lorenzo in Santa Cruz and the Mark West Creek in Sonoma are likely see rises as heavy rain continues. If you live along the Russian/Napa Rivers it is highly recommended you stay up to date on the latest river forecasts in the event that river flooding potential increases. As mentioned in the short term, the initial Flood Watch remains in effect through 4PM Monday. This Flood Watch is expected to be followed by a more widespread Flood Watch that will last for much of the upcoming week. In terms of forecast confidence, there continues to be decent uncertainty as to the system arriving Tuesday-Wednesday with large differences between the ECMWF and GFS models continuing. The ECMWF suggests the low will deepen to around 977 hPa as it moves up the CA coastline before moving inland along the CA/OR border. The GFS keeps the low less well defined and only has it dropping to around 1000 hPa. Under a situation described by the ECMWF, impacts would be greater and more widespread across our CWA whereas under the GFS impacts would be not as impactful. The current forecast falls more in line with the scenario provided by the ECMWF where stronger impacts are expected across our CWA. While rain and flooding are two of the most predominant hazards, strong, hazardous winds are expected to develop Tuesday-Thursday. The 925 mb ECMWF winds shows a strong low level jet (peaking between 70 to 80 knots) moving parallel up the Bay Area and Central Coast coastline. This will result in the potential for widespread gusts in excess of 40-50 mph across the CWA and gusts in excess of 60 mph directly along the coastline, the Salinas Valley, and over the marine environment. At this time, there is strong consideration of wind hazard products for the Tuesday-Thursday time period but the exact locations are still being narrowed in on. As these strong winds will come after several days of rain, soils are likely to be saturated (or close to saturated) which may result in trees being toppled more easily and potentially result in power outages. Conditions along the coastline will be extremely dangerous for beachgoers (and over the marine environment for mariners) as stronger winds move in Tuesday to Thursday. The highest end WRF guidance suggests that isolated hurricane force gusts (winds in excess of 64 knots/74 mph) may occur along the Big Sur coastline as the jet moves northwards. Winds strengthen Tuesday into early Wednesday, briefly diminish Wednesday morning, then strengthen again Wednesday into Thursday before diminishing by Friday. Another factor is the potential for thunderstorms. There is a 15-25% chance of thunderstorms across the South Bay and Central Coast Tuesday night into Wednesday as this system moves inland. ECMWF guidance shows several hundred joules of CAPE moving in with this system so confidence is slightly higher that thunderstorm activity may develop as this system moves up the coastline. The next 24 hours will be important to finalize what the impacts from the Tuesday-Wednesday system will be as that timeframe starts to get in the range of high resolution models. Bottom line: moderate to heavy rain and hazardous winds are expected for much of next week. Residents should be prepared for impacts to holiday travel (driving, flight delays, etc) and should pay careful attention to the weather forecast, adjusting holiday travel plans as needed to avoid the most impactful weather. If the forecast of hazardous, storm force to isolated hurricane force winds holds true; the beach and marine environment will be very dangerous this week. && .AVIATION... (12Z TAFS) Issued at 503 AM PST Sun Dec 21 2025 Low clouds are spreading across the region as the next rainmaker builds in the North Bay. Light prefrontal rains continue to spread south ahead of the front through the day. Strong southerly winds and gusts along with moderate to heavy rains build into the afternoon in the North Bay as the main rain band arrives. This band of rain will slowly push south and east through the day. The band slows as it passes into the South Bay and Monterey Bay, leading to chances for prolonged strong winds into the night. The system moves northward into early into early Monday, but winds and rain rates look to reduce. Mid-level and high level clouds persist through the forecast, but low cloud cover becomes spottier into Monday. Vicinity of SFO...Light showers and low CIGs continue continue to move through the area into the early evening. Expect southerly winds to increase into the late afternoon and early evening as the main rain band approaches. The added mixing from the increased winds could offer some moments of lifted CIGS, but these look to be short lived. Expect visibilities to suffer as the rain rates increase through the evening. Shower chances thin into the late night as the rain band move farther south, but reform as the front shifts northward into late Monday morning. SFO Bridge Approach...Similar to SFO. Monterey Bay Terminals...Low clouds are building with light rain chances through the morning. Southerly winds increase into the afternoon, helping lower clouds and showers scatter. Winds increase into the late night as the main rain band approaches. Expect reduced visibilities from rainfall as the band focuses over the area through the end of the TAF period. && .MARINE... (Today through Friday) Issued at 503 AM PST Sun Dec 21 2025 Rainfall intensity increases through Sunday across the northern waters with a strong cold front sagging southward to near Pigeon Point by Sunday evening. Gusty southerly winds increase along the front through Sunday before weakening into Monday. A more substantial period of unsettled weather enters the picture Tuesday with a gale force low forecasted to enter the coastal waters. This system appears likely to bring near storm force and isolated hurricane force gusts to the waters south of Pigeon Point and near Point Sur Tuesday night into Wednesday morning. Seas build through the week and will be very hazardous to mariners with the potential for wave heights in excess of 20 feet. && .MTR WATCHES/WARNINGS/ADVISORIES... CA...Flood Watch through Monday afternoon for CAZ502>506. PZ...Gale Warning until 9 PM PST this evening for Mry Bay-Pigeon Pt to Pt Pinos 0-10 nm-Pt Arena to Pt Reyes 0-10 nm-Pt Arena to Pt Reyes 10-60 NM-Pt Pinos to Pt Piedras Blancas 0-10 nm- Pt Reyes to Pigeon Pt 0-10 nm-SF Bay N of Bay Bridge. Small Craft Advisory until 3 AM PST Monday for Pigeon Pt to Pt Pinos 10-60 NM. && $$ SHORT TERM...Kennedy LONG TERM....Kennedy AVIATION...Murdock MARINE...Murdock Visit us at www.weather.gov/sanfrancisco Follow us on Facebook, X, and YouTube at: www.facebook.com/nwsbayarea x.com/nwsbayarea www.youtube.com/nwsbayarea