FXUS66 KMTR 310957 AFDMTR Area Forecast Discussion National Weather Service San Francisco CA 157 AM PST Wed Dec 31 2025 ...New BEACHES... .KEY MESSAGES... Updated at 1229 AM PST Wed Dec 31 2025 - High astronomical tides combined with storm surge will bring minor to moderate coastal flooding around high tide through Sunday. - A prolonged rainy period starts today. - Strong southerly winds are likely Friday. && .SHORT TERM... Issued at 1229 AM PST Wed Dec 31 2025 (Today and tonight) We switched KMUX to precip mode as rain and/or virga moving in from the south is coming in range. Most of this activity is over the coastal waters, and the lowest radar scan can only see 7,000 feet at this range. Unfortunately there aren't any buoys or ships capable or willing to report if anything is reaching the surface. Based on the 00Z sounding, most of these rain drops will evaporate in the well defined dry layer below 800 mb where the average relative humidity is less than 15%. Of course the evaporation itself raises the humidity and makes it easier for subsequent drops to make it further down. The moistening process will continue through the morning, priming the lower atmosphere for more efficient rain as the day progresses. The overall pattern is driven by a vertically stacked cut-off low roughly 500 miles west of the Channel Islands. The eastern periphery of this feature is pumping a large area of southerly winds and associated moisture from the deep tropics. This low will catch the next ride NE on an the upstream portion of an approaching long wave trough moving across the Eastern Pacific. This upper level pattern will also provide divergence aloft, removing weight from above the surface low, thus decreasing the surface pressure and providing some moderate cyclogenesis as the feature moves towards the Bay Area. We should pick up between 0.5-1.0" of rain over the next 24 hours as a warm up for the rest of the week. Will it rain for NYE celebrations? Probably, yes. In the Bay Area a rain jacket or umbrella should do just fine in the steady light rain and gentle wind expected up till midnight. If you're celebrating on the Central Coast, it may be a little heavier in the last hour or two of the year. && .LONG TERM... Issued at 1229 AM PST Wed Dec 31 2025 (Thursday through Tuesday) The warm, moist southerly winds with an approaching, deepening low pressure system to our west means we should get a somewhat uncommon warm frontal passage Thursday. Antecedent warm moist air with increasing kinematics support as the front approaches should support some stronger convection and possibly trigger some thunderstorms as the front passes. This threat will be greatest if the triple point (where the occlusion, cold front, and warm front meet) moves over land. The Central Coast has the best chance for thunderstorms. The thermodynamics don't look great until midday on Thursday, but if the timing lines up with frontal passage, it could get interesting. A NAM point sounding at Salinas shows the FROPA timing will be very close to the instability window opening around noon. Otherwise the afternoon of New Years Day should just bring scattered showers as opposed to the more widespread rain on NYE. After the low makes landfall Thursday, a more typical atmospheric river set-up will takes shape for Friday. A deep trough over the Eastern Pacific will bring strong SW flow to the California, with a ribbon of moisture pointed somewhere between the North Bay and Southern California. The high moisture anomaly near Baja never really clears out, and will push north again, reinforcing this round of rain. This should be the wettest and windiest day of the week, with bigger impacts bleeding into Saturday morning. Southerly winds will likely gust into the 50 mph range along the coast and in higher terrain just ahead of the cold frontal passage. Conditions will improve behind the front, but scattered showers will continue through the weekend, with 0.5-1.0" expected each day from Saturday through Monday. A new low pressure system and associated cold front is possible Monday or Tuesday. The ensemble clusters have differing opinions on the strength and timing of this feature, but they all show a trough off the coast, which is enough to keep rain chances around through the entire 7-day forecast period. KMUX will stay in precip mode for the foreseeable future. && .AVIATION... (06Z TAFS) Issued at 929 PM PST Tue Dec 30 2025 LIFR/IFR visibility is expected to prevail at KSTS through the overnight into mid-morning, with LIFR/IFR cigs expected for KAPC and lower confidence on visibility there. Area cameras attm show patch areas of limited visibility but not below half-mile. VFR is for the remaining terminals through the overnight and much of the daytime hours Wednesday. Vicinity of SFO...VFR expected through the overnight and daytime hours Wednesday. Scattered showers moving up the Pacific coastline Wednesday evening through the remainder of the TAF period have the potential for MVFR ceilings and slight chance of an isolated thunderstorm. Confidence for isolated thunderstorm is currently too low for mentioning in the TAF but could limit visibility along with strong erratic wind gusts. SFO Bridge Approach...Similar to SFO. Monterey Bay Terminals...VFR is expected to prevail overnight. There is a chance for MVFR cigs by late morning if as a coastal storm moves north just offshore. Once MVFR/IFR cigs are established, they're expected to persist through the remainder of the TAF period. && .MARINE... (Tonight through next Monday) Issued at 929 PM PST Tue Dec 30 2025 Moderate seas with moderate to fresh northeast winds will persist through Wednesday, with southerly breezes Thursday, ramping up quickly overnight Thursday into Friday when widespread gale force gusts spread across our outer and inner waters. There is also a slight chance of isolated thunderstorms Wednesday and Thursday, with higher confidence on Friday. Winds gradually ease overnight into Saturday becoming southwesterly. && .BEACHES... Issued at 157 AM PST Wed Dec 31 2025 Perigean spring tides (King Tides) return Wednesday - Sunday. This is due to the combination of the full moon on 1/3, lunar perigee on 1/1, and perihelion on 1/3. In other words all three planetary bodies are lined up and close to each other. In addition to the high astronomical tide, the incoming low pressure system will bring strong southerly winds, generating some storm surge. The combination of these factors will bring minor to moderate coastal flooding to the low-lying areas near shorelines and tidal waterways during high tide Wednesday through Sunday. At the San Francisco tidal gauge, high tide is expected to be 1.5 ft above normal at 7:51 AM Wednesday, 2.0 ft above normal at 8:41 AM Thursday, 2.2 ft above normal at 9:34 AM Friday, 2.2 ft above normal at 10:26 AM Saturday, and 1.8 ft above normal at 11:18 AM Sunday. These predictions include up to 1 foot of storm surge that will enhance the astronomical tide and flooding threat. High tide varies up to 90 minutes earlier or later along the Pacific Coast and through the San Francisco Bay, respectively. && .MTR WATCHES/WARNINGS/ADVISORIES... CA...Coastal Flood Advisory from 5 AM early this morning to 2 PM PST Sunday for CAZ006-505-506-508-509-529-530. PZ...Small Craft Advisory from 9 PM Thursday to 3 AM PST Friday for Pt Arena to Pt Reyes 0-10 nm-Pt Arena to Pt Reyes 10-60 NM. Gale Warning from 3 AM to 9 AM PST Friday for 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. Small Craft Advisory from 3 AM to 3 PM PST Thursday for Pigeon Pt to Pt Pinos 0-10 nm. Gale Warning from 9 PM this evening to 3 PM PST Thursday for Pt Pinos to Pt Piedras Blancas 0-10 nm. Small Craft Advisory from 9 PM this evening to 3 PM PST Thursday for Pigeon Pt to Pt Pinos 10-60 NM. Small Craft Advisory from 3 AM to 9 AM PST Friday for Pigeon Pt to Pt Pinos 10-60 NM. && $$ SHORT TERM...Flynn LONG TERM....Flynn AVIATION...JM MARINE...JM 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