FXUS66 KMTR 180533 AFDMTR Area Forecast Discussion National Weather Service San Francisco CA 933 PM PST Tue Dec 17 2024 ...New UPDATE... .SYNOPSIS... Issued at 127 PM PST Tue Dec 17 2024 Dry conditions continue through early Friday with possible fog and low stratus tonight into Wednesday morning. Active weather pattern begins this upcoming weekend with multiple rounds of impactful rain and wind possible. && .UPDATE... Issued at 913 PM PST Tue Dec 17 2024 No changes to the forecast this evening. Tule fog is observed in the San Joaquin Valley with it bumping up against our counties. The forecast for fog development in our region remains tricky as we have high cloud passing over and lighter winds being observed at most points. Should the clouds thin and perhaps winds ease, there could be more opportunity for fog development. && .SHORT TERM... (Today and tonight) Issued at 127 PM PST Tue Dec 17 2024 Low stratus clouds are slowly continuing to mix out into the afternoon, with some patches of cloud cover, including eastern San Jose and areas near Livermore, persisting through the day. Light winds will prevail through the day with high temperatures ranging from the lower to middle 60s in the inland valleys, into the middle 50s to the lower 60s along the coast and Bayshore. Tonight sees chilly temperatures, with lows reaching the upper 30s to the lower 40s in the inland valleys and the middle 40s to near 50s near the coast, with the potential for a return of widespread fog and low stratus, especially across the North Bay and the northern Bay Area. There's not enough confidence to issue a Dense Fog Advisory just yet, but one may be issued later today as updated model guidance comes in. && .LONG TERM... (Wednesday through Monday) Issued at 127 PM PST Tue Dec 17 2024 With an upper level ridge sticking around through the early part of Friday, dry conditions remain across the region through the next couple of days with the inland valleys reaching highs in the upper 60s to lower 70s on Wednesday and Thursday. Building long period swell will result in an increased risk for sneaker waves late Wednesday into Thursday (see the BEACHES section for further details). Later on Friday, the pattern shifts to bring more active weather into the region. Ensemble model guidance is beginning to coalesce on multiple rounds of impactful rain and wind beginning this weekend and potentially extending past Christmas Day. The initial round of rain late Friday into Saturday will bring around 0.5-1.5" of rain across the North Bay and less than 0.3" everywhere else (although some ensemble members show rainfall totals significantly higher), with totals decreasing the further south you go. The Weather Prediction Center has went ahead with a Marginal Risk of Excessive Rainfall (at least a 5% chance of exceeding flash flood guidance) across Sonoma County and nearby portions of northern Marin and western Napa counties. In addition to the immediate impacts of the rainfall, this system will set up antecedent conditions for further systems to continually increase the flooding risk as soils saturate and creeks fill. By Christmas Eve, the Climate Prediction Center is already showing a moderate risk (40-60% probability) of heavy precipitation and high winds. The risk for high winds extends into the days after Christmas, while the CPC outlook suggests two distinct danger periods for heavy precipitation, one on the 24th and another for the days after Christmas. In addition, high surf will lead to localized coastal flooding impacts (see the BEACHES section for further details). This pattern shift is something that the operations team will watch closely for the rest of this week, with particular emphasis on any migration of the heaviest impacts south of the North Bay and potential for heavily impactful rainfall totals across the higher elevations. As of today, the North Bay mountains see a 30-40% probability of 24-hour rainfall totals of 4 inches or more on Christmas Eve, and the Santa Cruz Mountains see around a 10% percent chance of the same. Enjoy this nice weather while you can, but also take some time to prepare for potential flooding and heavy wind concerns. && .AVIATION... (00Z TAFS) Issued at 425 PM PST Tue Dec 17 2024 VFR for the TAF sites continues into the night with some pockets of fog lingering in the North Bay and in portions of the SF Bay. The fog spreads overnight, becoming dense and filling over the North Bay terminals and LVK further into the night. Dense fog looks to affect SFO and OAK into Wednesday morning. Expect fog and mist to linger into late Tuesday morning and the early afternoon, with widespread VFR expected after. Winds stay light to moderate through the TAF period. Vicinity of SFO...VRF lasts into the late night before mist and fog begin to affect the SF Bay. Moments of passing dense fog will affect SFO in the early morning. Fog and mist look to erode late Tuesday morning. Moderate winds linger into the night tonight before becoming light and variable through the rest of the TAF period. SFO Bridge Approach...Similar to SFO. Monterey Bay Terminals...VFR with light to moderate winds through the TAF period. && .MARINE... (Tonight through next Monday) Issued at 425 PM PST Tue Dec 17 2024 High pressure returns with dry weather and mainly light offshore winds tonight through Thursday night. Moderate to long period northwesterly to westerly swell will arrive midweek causing an increase in shoaling on harbor entrances and elevated sneaker wave risk along the coastline. Another low pressure system arrives Friday into the weekend which will result in elevated wave heights and create hazardous conditions for small crafts. && .BEACHES... Issued at 145 PM PST Tue Dec 17 2024 A Beach Hazards Statement was issued for 6 pm Wednesday to 6 pm Thursday from Sonoma County to Monterey County. Moderate to long period northwesterly swell will result in elevated sneaker wave development with sneaker waves likely to break between 10 and 13 feet. This along with rip currents both pose a risk to beachgoers who are caught unaware. Visitors to the coast midweek are advised to take extra care and never turn their backs to the ocean as sneaker waves may sweep unaware beachgoers into the ocean. A second period of dangerous, elevated surf near 15 feet will occur Saturday into Sunday as another system approaches the West Coast. Then, a very large and powerful, long period west swell train will likely arrive along our entire coastline Monday into Tuesday next week. Current global wave models are forecasting swell heights to 20+ feet with very energetic swell periods of 20+ seconds, this will lead to much higher breakers including localized coastal flooding impacts. Stay tuned for further updates. && .MTR WATCHES/WARNINGS/ADVISORIES... CA...Beach Hazards Statement from Wednesday evening through Thursday afternoon for CAZ006-505-509-529-530. PZ...Small Craft Advisory until 3 AM PST Wednesday for Pigeon Pt to Pt Pinos 10-60 NM-Pt Pinos to Pt Piedras Blancas 0-10 nm. && $$ SHORT TERM...DialH LONG TERM....DialH AVIATION...Canepa MARINE...Murdock Visit us at www.weather.gov/sanfrancisco Follow us on Facebook, Twitter, and YouTube at: www.facebook.com/nwsbayarea www.twitter.com/nwsbayarea www.youtube.com/nwsbayarea