FXUS66 KMTR 162327 AFDMTR Area Forecast Discussion National Weather Service San Francisco CA 327 PM PST Tue Dec 16 2025 ...New AVIATION... .KEY MESSAGES... Updated at 251 PM PST Tue Dec 16 2025 - Light rain will spread into the North Bay and portions of the rest of the Bay Area tonight into tomorrow. - A second system arrives late Thursday with beneficial rain continuing through Saturday. - Confidence continues to increase that a stronger, impactful system will arrive late weekend and continue into next week. && .SHORT TERM... Issued at 251 PM PST Tue Dec 16 2025 (This evening through Wednesday) A weak system is currently bringing light rain to the North Bay. Showers have been fairly isolated so only a few locations in northern Sonoma and Napa counties reporting totals of a few hundredths of an inch. Scattered showers will continue through this afternoon/evening with the bulk of the precipitation to our north over Mendocino and Humboldt counties. This precipitation will shift southwards overnight, but, it won't be as widespread with showers largely diminishing/falling apart as they move into the North Bay. Not much change to rain totals for this event, totals continue to be highest in the North Bay with up to 0.1" in the valleys and 0.2"- 0.3" in the coastal mountain ranges. Showers will move south of the Golden Gate Bridge by mid to late tomorrow morning with the potential for up to 0.1" along the Bay Shoreline and San Francisco/San Mateo peninsula. Overall, this is a light and beneficial rain system and leaves the storm door open behind it for additional systems to enter California. Residents of the North and East Bays may notice it is less foggy today than it has been for the past few days. Tule Fog has been the talk of the last few weeks as it persisted over the Central Valley and portions of the Bay Area. If you're tired of the gloom of fog, you're in luck. This system reduces fog chances across the Bay Area but you may not see the sun just yet as generally overcast conditions persist while this system rolls through. Given morning fog and overcast skies, lowered the forecast high temperatures for the North Bay into the mid 50s from the upper 50s. This should be the last day requiring major adjustments to the temperature given the series of incoming systems (Long Term) and more zonal upper level flow compressing high pressure/diminishing fog potential. && .LONG TERM... Issued at 251 PM PST Tue Dec 16 2025 (Wednesday night through next Monday) Things remain active in the long term with a second system expected late week and a third system expected next week. Rain from the first system (Tuesday - Wednesday) will diminish by Wednesday afternoon with dry weather and upper level ridging briefly rebounding. The sun may even briefly come out Wednesday afternoon in this break between systems. By early Thursday morning, the next system will arrive and bring with it renewed, more widespread rain chances across the entire Bay Area and portions of the Central Coast. This system is associated with a surface low pressure system and slight upper level trough moving into the Pacific Northwest. This system will help to compress surface high pressure and replace upper level ridging with more zonal upper level flow. PWAT values associated with this system peak around 1.5" to 1.6" with IVT values ranging from 250 to 500 kg/ms. The PWAT plume initially stays to our north before moving into our CWA Friday this plume then lingers over the Bay Area and Central Coast for much of the weekend. QPF totals have gone up for this system with between 2.0"-2.5" across the North Bay Coastal Mountains, 1"-1.5" for interior Marin and the North Bay Valleys, 0.5- 1.0" south of the Golden Gate, and up to 0.25" across the Central Coast. These values may be further refined as we get closer to Friday with NBM percentile guidance suggesting the current forecast is on the lower end. While this system is not expected to be the strongest of the three, preparations for wet weather should be completed by Thursday as the second and third systems do not have a defined break between them. The third system is expected to be the most impactful, but, there is still some forecast uncertainty as it is a more prolonged system and is not entirely within our 7-day forecast period. Beginning Sunday, a stronger, deep upper level trough will approach the West Coast and bring a renewed source of tropical moisture into the region. This storm will bring potentially moderate to heavy rain with the highest rain totals in the North Bay. Given that totals continue to fluctuate, it is still too early to give exact totals for this event. The key messages are as follows: confidence is increasing that a potentially impactful atmospheric river will arrive Sunday and continue for much of next week. This system will bring periods of moderate to heavy rain and potentially impact holiday travel. If you are traveling next week, make sure to check road conditions, flight delays, and the most up to date weather forecast for both here and the location that you are traveling to (especially if it is within California). Winds look to strengthen next week with this system but, in agreement with the previous forecaster, there is not a clear signal that winds will meet wind advisory criteria. All in all, this system bears keeping an eye on especially as the forecast surrounding it becomes clearer. Make sure to stay up to date with the forecast so you can best prepare for any upcoming holiday travel. High temperatures rise slightly (particularly for regions that have been foggy) with more seasonal high temperatures in the 50s to low 60s across the region. Morning low temperatures stay in the 40s to low 50s with some patchy cooler temperatures in the interior mountain ranges. && .AVIATION... (00Z TAFS) Issued at 327 PM PST Tue Dec 16 2025 Generally VFR with varying levels of mid- to high level cloud cover across the region with lingering stratus near STS and the far interior eastern Bay east of LVK. Onshore winds are expected in the coastal regions with a lingering offshore flow inland turning onshore tonight. A weak frontal boundary moves into the Bay Area overnight, leading to light rain across the North Bay terminals and showers across the rest of the Bay Area, in addition to lower ceilings and/or visibilities as the boundary moves through. The boundary will dissipate as it reaches the Monterey Bay region. Stratus coverage on Wednesday morning is a little uncertain, especially for terminals close to the SF Bay, but expect a gradual clearing through the late morning and early afternoon hours. Vicinity of SFO... VFR with mid- to high level clouds at present, with MVFR conditions expected to return within the next few hours. Light rain showers are expected in the early hours of Wednesday morning, with IFR conditions possible as the showers move through. Conditions will improve through Wednesday morning and afternoon, but some high resolution models are showing lingering MVFR stratus in the SF Bay which could impact the terminal. Considered keeping BKN conditions through the end of the TAF period, but opted to add a SCT line to show the trend towards improving conditions. Light winds through the rest of the day and overnight, before gentle northwest winds return Wednesday. SFO Bridge Approach... Low to moderate confidence that lingering stratus over the SF Bay impacts the approach path on Wednesday. Otherwise, similar to SFO. Monterey Bay Terminals... VFR conditions prevail through the evening hours with a gentle northwest breeze. MVFR-IFR conditions develop tonight with a low confidence for drizzle at MRY Wednesday morning. Light drainage winds develop overnight before northwest flow resumes Wednesday morning as the skies begin to clear at SNS. For MRY, low confidence for a brief window of clearing in the afternoon, but stratus lingering at the immediate coast of the Monterey Bay could continue to impact the terminal. && .MARINE... (Tonight through next Sunday) Issued at 251 PM PST Tue Dec 16 2025 Light rain is forecast over the northern waters tonight through Wednesday afternoon as a plume of moisture arrives from the north. Light to moderate west to northwesterly breezes will also prevail through Wednesday afternoon. Rain chances return by Thursday night and increase as we head into the upcoming weekend. Moderate seas will prevail through the remainder of the week. && .MTR WATCHES/WARNINGS/ADVISORIES... CA...None. PZ...Small Craft Advisory until 9 PM PST this evening for Pt Pinos to Pt Piedras Blancas 0-10 nm. && $$ SHORT TERM...Kennedy LONG TERM....Kennedy AVIATION...DialH MARINE...RGass 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