FXUS66 KMTR 190532 AFDMTR Area Forecast Discussion National Weather Service San Francisco CA 932 PM PST Thu Dec 18 2025 ...New AVIATION, MARINE... .KEY MESSAGES... Updated at 317 PM PST Thu Dec 18 2025 - Areas of dense fog expected once again tonight - Mostly light, beneficial rainfall late tonight through Saturday - Slight chance of thunderstorms with increasing coverage of rainfall and gusty winds Sunday afternoon and evening - Widespread spread rainfall moderate to heavy at times, gusty winds, increasing chances of flooding and travel impacts Tuesday through Thursday (Christmas) - Pattern appears to remain unsettled beyond the holiday into next weekend && .UPDATE... Issued at 806 PM PST Thu Dec 18 2025 We just switched the radar to precip mode where it will remain for quite some time. Very light rain has moved into the North Bay. This is on the southern periphery of an atmospheric river currently pumping a band of moisture from Hawaii to Oregon. Light and brief isolated showers will continue in the North Bay through the night before a more organized band of more moderate rain pushes through Friday afternoon. While mostly light, this will be the first round of what will become a very wet week. The latest forecast trend has pushed the timing of the heavier rain back about 12 hours, while the total accumulation has trended higher. Specifically the 3rd round of rain from Tuesday through Friday now looks at least on par with the 2nd round from Saturday through Tuesday. Over the next week we expect most of the Bay Area and Central Coast to have received around 5-10" of rain, with more in the coastal mountains. If that holds, it's nearly 25% of the normal precipitation for the entire year and will cause some flooding. There is also a good chance for strong winds all week, but particularly Tuesday night through Wednesday morning as a deep low pressure system develops and approaches the coast. This system could also help trigger some thunderstorms during the same time frame. && .SHORT TERM... Issued at 317 PM PST Thu Dec 18 2025 (This evening through Friday) The remainder of today and tonight across our area will remain mostly dry with the exception of far northern Sonoma County, which may see the onset of light rain this evening. Rainfall is expected to continue to be light throughout the day Friday across the remainder of the North Bay, and may shift as far south as the Bay Area and East Bay with no impacts. Areas where fog and/or drizzle developed last night will likely see similar conditions tonight into Friday morning. && .LONG TERM... Issued at 317 PM PST Thu Dec 18 2025 (Friday night through next Wednesday) Confidence continues to increase that the overall synoptic pattern will remain active through the extended forecast and beyond. If you're a regular reader of our AFD's you won't be surprised to read that the North Bay will see the most rainfall both initially and over the course of the next week. The rainfall we see tomorrow and through the weekend will be mostly on-and-off and beneficial with no impacts anticipated. We are emerging from a fairly dry stretch of weather so the soils will be able to absorb this first round of rainfall fairly easily over the next couple of days. Travel impacts on Sunday will be relegated mostly to the North Bay and points north, with light to moderate rain at times shifting south into the Bay Area, East Bay South Bay and Santa Cruz mountains Sunday afternoon into Sunday night. Sunday afternoon and evening also has a slight chance for occasional thunderstorms that could produce brief downpours and gusty winds up to 50MPH at higher elevations, 40MPH in lower lying areas across the North Bay, Bay Area and East Bay. By late Sunday night a strong area of low pressure in the Gulf of Alaska will continue to deepen and evolve southward offshore toward NorCal over the course of next week, producing numerous trough axis' that will move onshore across the California Pacific coast over that same time period. The deepening area of low pressure will also tap into southern Pacific moisture resulting in several waves of a developing atmospheric river as the sharpening trough axis' interact with the continuous conveyor belt of moisture. There is plenty of uncertainty in the exact timing and location of where the heaviest rain and strongest winds will ultimately impact our area. The strongest portion of the upper level low attm appears to approach the coast at some point on Wednesday bringing very strong winds along the coast, that will spread inland (but not as strong as the coast) and a larger area of moderate to heavy rainfall at times with more widespread impacts. By the middle of next week we can expect the potential of nuisance flooding at the very least, rising streams and rivers, downed trees and powerlines, mud/rock slides and dangerous marine/beach conditions. If you, your friends and relatives plan on traveling within our forecast area over the next week prepare for delays and allow more time to reach your destination and it's imperative to stay in tune with the forecast as we continue to refine and add more details to the widespread impacts and hazards that will develop and compound over time by the end of next week and beyond. && .AVIATION... (06Z TAFS) Issued at 932 PM PST Thu Dec 18 2025 MVFR-IFR stratus developing across the Bay Area underneath a thin layer of high clouds, with the South Bay and Central Coast remaining generally VFR for the time being. Stratus is expected to expand through the night. In the North Bay and the northern SF Bay terminals, lifting stratus will give way to a frontal rain band with MVFR ceilings and impacts to visibility through Friday. At SJC and to the south, a brief period of clearing will ensue early Friday afternoon before the frontal band comes in late afternoon through the early evening hours. By the end of the 24-hour TAF period, showery conditions will extend across the Bay Area and into the North Bay with drizzle possible across the Monterey Bay region. Light winds overnight will become breezy and southerly to southwesterly on Friday, lightening again in the evening hours. Vicinity of SFO... MVFR ceilings through the TAF period. Chances for showers begin overnight, but the bulk of the chances lie to the west of the terminal and the San Mateo mountains. Greater confidence in rainfall in addition to gusty south-southwest winds arriving with the frontal rain band Friday afternoon, with the winds decreasing but the showers continuing through the end of the TAF period. SFO Bridge Approach... Similar to SFO. Monterey Bay Terminals... VFR conditions will continue for a few more hours before IFR stratus rolls in later tonight. Stratus clears out late Friday morning as gentle northwest breezes develop. The MVFR ceilings from the approaching rain band will come to the region Friday evening, with the best chances for scattered showers coming after the end of the TAF period. && .MARINE... (Tonight through next Wednesday) Issued at 932 PM PST Thu Dec 18 2025 Light to moderate onshore breezes prevail with moderate seas through late week. Rain chances increase Friday with moderate to heavy rain over the northern waters Sunday. More substantial rainfall and a strong storm system develops around the middle of next week. && .MTR WATCHES/WARNINGS/ADVISORIES... CA...None. PZ...Small Craft Advisory from 9 AM to 3 PM PST Friday for Pt Arena to Pt Reyes 10-60 NM. && $$ SHORT TERM...JM LONG TERM....JM AVIATION...DialH MARINE...DialH 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