FXUS66 KMTR 221928 AFDMTR Area Forecast Discussion National Weather Service San Francisco CA 1128 AM PST Mon Dec 22 2025 ...New AVIATION... .KEY MESSAGES... Updated at 336 AM PST Mon Dec 22 2025 - Flood Watch remains in effect for the North Bay through Friday evening; Flood Watch for the rest of Bay Area and Central Coast begins Tuesday morning - Light to moderate rain continues through Monday with minor nuisance flooding and rises in rivers/streams across North Bay possible. Turn Around, Don't Drown! - Increasing chances for urban flooding and extensive travel delays late Tuesday into early Friday across entire Bay Area and Central Coast as widespread heavy rain returns - Very hazardous, impactful winds Tuesday through early Friday along our entire coastline and higher elevations inland with power outages likely - Dangerous beach conditions anticipated late Tuesday through Friday for all Pacific Coast Beaches && .SHORT TERM... Issued at 242 AM PST Mon Dec 22 2025 (Today and tonight) A Flood Watch remains in effect through 4 PM PST for the East Bay with the overall plan of expanding the Flood Watch in areal coverage and time on track. The Russian River at Geyserville reached minor flood stage at 11:30 PM PST 12/21 (39.74 ft) with a Flood Advisory in effect through 7:15 AM this morning. Minor flooding is likely to continue in the vicinity of the Russian River between Geyserville and Jimtown in Sonoma and in the vicinity of Mark West Creek. If you are driving through the North Bay, be prepared in case you encounter flooded roadways. Roadway flooding may be hard to see in the dark so stay aware of your surroundings and if you encounter flooding - Turn Around, Don't Drown. Flooding impacts will be exacerbated by the additional rounds of moderate to heavy rain expected Tuesday through Friday. Wind impacts will also increase Tuesday with a High Wind Watch in effect from 7PM PST Tuesday to 4AM PST Wednesday. Today is a transition day between the first big round of rain yesterday and the next big round of rain coming in tomorrow. Light rain will continue across the Bay Area through late tonight as stationary front lingers over the Bay Area. Rainfall amounts are expected to decrease with the IVT plume weakening from around 500 kg/ms this morning to around 250 kg/ms by this evening. The main rain band will remain locked in over the Bay Area today, with minor oscillations towards the North and South Bays, rain will be more stratiform within the main rain band with scattered showers on the periphery of it. This will result in an additional 0.5" of rain across the interior today. For the coastal mountain ranges (Santa Cruz Mountains, Mt. Tamalpais), the orientation of this moisture plume is perpendicular with the coastline which supports increased orographic uplift. The coastal mountains will see an additional 1- 1.5" today (primarily in the early morning hours when the IVT plume is the strongest) with locally higher totals possible depending on where the most orographic uplift is able to occur. Locally breezy winds peaking around 30 mph are will continue through the late morning along the coastline, higher elevations, and SF Bay Shoreline through late morning. Winds will then generally decrease throughout the afternoon and evening as the surface low progresses eastward. If you did not finish your outdoor preparations over the weekend or need to make additional preparations, today is your last chance to do so before two stronger systems arrive Tuesday into Wednesday and Wednesday into Thursday. Thunderstorm chances are low today (5-10%). The ECMWF and HRRR models show around 100 J/kg of MUCAPE across the South Bay this morning but wind shear in the lowest levels does not look too supportive of thunderstorm development. && .LONG TERM... Issued at 242 AM PST Mon Dec 22 2025 (Tuesday through Sunday) To best address the upcoming weather and associated hazards, the Long Term discussion will be broken into three parts: Flooding, High Winds, and Thunderstorms. Flooding: Tuesday morning, light rain will reintensify to heavy rain first over the North Bay before extending southwards into the rest of the Bay Area and Central Coast Tuesday afternoon into Tuesday evening. The ECMWF continues to show a rapidly intensifying cyclone meeting bomb cyclone criteria (a drop of 24 millibars or more in 24 hours with some adjustments required depending on the latitude of the cyclone) moving parallel to the California coastline on Tuesday. This low pressure system will deepen from around 1000 hPa early Tuesday morning to 974-975 hPa as it moves up the coastline by early Wednesday morning. The GFS and Canadian models are starting to catch up to the ECMWF and are also showing a deepening, strong low pressure system off the CA coast. At the same time, this system will bring a renewed transport of moisture into California with IVT values in excess of 700-800 kg/ms likely. Between Tuesday and early Wednesday, an additional 1.5-3.5" are expected across the North Bay, 1-3" across the Santa Cruz Mountains, 2-3" across the Santa Lucia Range and 1-1.5" across the interior Bay Area, Monterey Bay region, and interior Central Coast. Late Wednesday into Thursday, a second low pressure system will start to deepen and move northwards parallel to the CA coast. This system does not deepen as much as the first system so it is not considered a bomb cyclone. It will bring another round of widespread moderate to heavy rain and widespread totals of 2-4" (locally up to 5" in the mountains) between late Wednesday through Friday. After Friday, rain amounts start to diminish with light rain to persist into the weekend. In terms of flooding impacts, flooding impacts will get worse throughout the week as multiple rounds of moderate to heavy rain occur and soils saturate. This will result in increased nuisance flooding across urban areas and see sharp rises in minor streams and creeks. Minor flooding is likely across at least a few of the mainstream rivers (such as the Russian River and Napa River in the North Bay) by late in the week. Anyone who lives in the vicinity of a river or a small creek known to rise rapidly should pay careful attention to the forecast and timing of heaviest precipitation. Flooding can often be hard to see at night so exercise extreme caution when driving at night. If you see roadway flooding, it cannot be expressed enough that it is absolutely not safe to drive through. Turn Around, Don't Drown is the most important motto for you to remember this week. High Winds: In addition to heavy rain and flooding concerns, the two strong systems Tuesday-Wednesday and late Wednesday-Thursday will result in dangerously strong winds. As the first low pressure system moves parallel to the coast Tuesday, it will bring a strong low level coastal jet directly against the Bay Area and Central Coast coastlines. High resolution models suggests 925 mb winds could be in excess of 90 knots (103 mph) along our coastline. At the surface, this will result in gusts to at least 70 mph (potentially higher) as the main cold front moves through. Winds will be strongest over the marine environment where storm force to isolated hurricane force winds are possible. A High Wind Watch, to be upgraded to a High Wind Warning, is in effect from 7PM PST Tuesday to 4AM PST Wednesday. The good news is these winds will diminish fairly quickly Wednesday morning. The bad news is this reprieve will be short lived as winds strengthen Wednesday evening as the second low pressure system approaches. Winds looks to be slightly weaker late Wednesday- Thursday with gusts peaking between 40-60 mph. Right now, everything is leaning towards a Wind Advisory being necessary for the late Wednesday-Thursday storm over a second High Wind Warning. The combination of heavy rain and strong winds will result in trees falling down and widespread power outages. It will additionally result in incredibly dangerous conditions to any mariners or anyone who goes to the beach while high winds are occurring. Thunderstorms: The final piece of the puzzle is the possibility of thunderstorms, potentially severe, this week. The potential for thunderstorms Tuesday into Wednesday and again late Wednesday into Thursday (all day) has increased from last nights forecast. Probability of thunderstorms is still between 15-25% but models are starting to support a more favorable environment for thunderstorm development with these systems. The most likely scenario would be for thunderstorms to be embedded within the main rain bands and then the potential for scattered thunderstorms in the post cold frontal environment. The SPC has issued a general mention of thunderstorms for our entire CWA Tuesday into Wednesday. The SPC additionally issued a marginal risk of severe thunderstorms for the coastal Central Coast (including the city of Monterey and Big Sur) and a general mention of thunderstorms for the rest of the CWA Wednesday into Thursday (Christmas Day). While not in the range of high resolution models yet, the ECMWF and GFS are both suggesting good instability (several hundred J/kg of MUCAPE) associated with both systems, good low level wind shear thanks to the strong winds, and the cold front acting as a source of lift. Both systems have the potential for thunderstorms and severe weather but the Wednesday into Thursday system initially appears to be the most favorable. Have multiple ways to receive warnings this week in the event that we see strong thunderstorm development with either system. && .AVIATION... (18Z TAFS) Issued at 1128 AM PST Mon Dec 22 2025 Frontal boundary is hanging over the area this morning bifurcating the Bay Area at the Golden Gate. This boundary will slowly lift to the north through the day. This will allow for some clearing with scattered showers south of the boundary while widespread rain will ride the boundary north. Winds south of the boundary will generally be out of the south while north of the boundary will generally be out of the north. Eventually the boundary will lift out of the Bay Area allowing for scattered showers across the region. By late Tuesday morning, expect strong gusty winds from the south to develop, accompanied by widely scattered showers. Vicinity of SFO...Showers will be the main story at SFO today. Winds will remain out of the south, but on the lighter side of the coin. CIGS will bounce a bit with showers in the area, but generally should remain VFR or MVFR. The only concern I have over the next 24 hours would be light winds overnight, wet grounds, and radiational cooling, leading to some localized fog development. This remains a spot of contemplation, but needs to be noted. The big story is on Tuesday as a potent system approaches from the southwest and brings rapidly increasing winds from the south. Expect winds to peak late in the day and overnight with gusts exceeding 40 knots. SFO Bridge Approach...Similar to SFO. Monterey Bay Terminals...Light to moderate winds out of the SE with an occasional passing shower. Winds ramp up Tuesday afternoon. && .MARINE... (Today through Saturday) Issued at 500 AM PST Mon Dec 22 2025 Winds and rain rates subside slightly as the front moves northward. A significant change takes place Tuesday as a strong surface low traverses the waters from the southwest towards the Northern California coast. This surface low will undergo rapid intensification as it approaches the coast Tuesday leading to nearly widespread gales, with storm-force winds expected along coastal jets near Point Pigeon and along the Big Sur Coast. There is a chance for isolated thunderstorms across the waters through Wednesday before another strong surface low enters the waters late Wednesday night. 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 from Tuesday morning through Friday evening for CAZ006-508>510-512>518-528>530. High Wind Watch from Tuesday evening through late Tuesday night for CAZ006-502>506-508>510-512>518-528>530. Flood Watch through Friday evening for CAZ502>506. PZ...Gale Warning from 9 AM Tuesday to 3 AM PST Wednesday for Mry Bay- Pt Arena to Pt Reyes 10-60 NM-SF Bay N of Bay Bridge. Gale Warning from 3 AM Tuesday to 3 AM PST Wednesday for Pigeon Pt to Pt Pinos 10-60 NM-Pt Arena to Pt Reyes 0-10 nm-Pt Reyes to Pigeon Pt 0-10 nm. Small Craft Advisory until 3 PM PST this afternoon for Pigeon Pt to Pt Pinos 0-10 nm-Pigeon Pt to Pt Pinos 10-60 NM-Pt Pinos to Pt Piedras Blancas 0-10 nm-Pt Reyes to Pigeon Pt 0-10 nm. Gale Warning from 3 AM to 9 AM PST Tuesday for Pigeon Pt to Pt Pinos 0-10 nm-Pt Pinos to Pt Piedras Blancas 0-10 nm. Storm Warning from 9 AM Tuesday to 3 AM PST Wednesday for Pigeon Pt to Pt Pinos 0-10 nm-Pt Pinos to Pt Piedras Blancas 0-10 nm. && $$ SHORT TERM...Kennedy LONG TERM....Kennedy AVIATION...BFG 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