396 FXUS66 KMTR 200605 AFDMTR Area Forecast Discussion National Weather Service San Francisco Bay Area 1005 PM PST Tue Jan 19 2021 .SYNOPSIS...Strong and gusty offshore winds earlier today have now largely subsided. An upper level ridge of high pressure is building in and will prevail through Thursday, along with light offshore flow and an entrenched very dry air mass. The net result will be clear skies and pleasant midwinter conditions. A cooling trend is expected to begin Thursday and continue Friday as an upper level trough approaches from the Pacific Northwest. Shower chances develop on Friday, then Saturday and Sunday morning look to be dry before another cold front moves in later Sunday into early Monday with a better chance of rain. The longer range pattern looks cool and unsettled. && .DISCUSSION...as of 9:15 PM PST Tuesday...Clear skies prevail across the district and even over all of our waters, as a very dry air mass now blankets the region. All Fire Weather and Wind related warnings and advisories were allowed to expire during the afternoon hours, with stronger wind gusts now confined to just a small number of locations high up in the North and East Bay hills. Winds reached pretty impressive levels early in the day, with peak gusts to 86 mph near the summit of Mt St Helena, 84 mph near the summit of Mt Diablo and 81 mph near the summit of Mt Umunhum. Even at SFO, wind gusts reached to 54 mph around sunrise this morning. Offshore pressure gradients are now continuing to diminish, as well as coupling with the very strong NNE winds aloft wrapping around the upper level low which has now moved southward to well off the northern Baja coast. Present forecasts appear on track and no updates currently anticipated. Will note that just received new 00Z GFS deterministic output continues to show a significantly wetter and cooler system moving through Sunday and Sunday night, with trailing showers on Monday. .PREV DISCUSSION...as of 1:56 PM PST Tuesday...Offshore wind event is slowly subsiding with Wind Advisories in effect through 6 pm this evening. The large scale SFO-WMC gradient is still a robust 12 mb offshore. As of 1 pm still seeing wind gusts in excess of 50 mph across the hills with widespread wind gusts from 30-40 mph at lower elevations. Given this will keep the wind advisory in place through sunset. Red Flag Warning for the Central coast through 3 pm today. We've had lots of reported wind impacts today with fires, downed trees, limbs and powerlines leading to power outages, etc. The large scale pattern features a developing cut-off low now over Southern California in a highly amplified pattern. The flow aloft remains northeast with cold advection and still robust surface pressure gradients. These gradients will ease overnight. By Wednesday the circulation around the low will bring dry but mainly light offshore winds to the region, especially the Central Coast where temps may warm into the 70s once again. Across the Bay Area large scale high pressure will briefly try to nose in over the region. Upshot will be dry and tranquil midweek weather for Weds and Thursday with seasonably mild readings across the Bay Area though nights will be much cooler given the lighter winds and very dry airmass. The ongoing small fires throughout Santa Cruz county will benefit from the lighter winds though humidity values will remain dry. Long and cold night will be favorable with some uptick in fuel moistures though overall fire-fighting conditions will remain somewhat unfavorable for mid January given the low humidity and light offshore winds. After another quiet weather day on Thursday the weather will be changing once again. A fairly potent shortwave trough will approach late Thursday night into Friday. The main impact regionwide will be much cooler temps for Friday with highs in the mid 50s as the airmass cools with increasing clouds. Rain chances look meager with the moisture starved system. Expecting mainly scattered rain showers on Friday with qpf numbers looking like a tenth or less where it does rain. Most shower activity should be done by sunrise Saturday. But in the wake expect cool and blustery weather conditions for the start of the weekend. The next front will quickly move in on Sunday with increasing clouds leading to a chance of rain Sunday afternoon and evening. Trends on this front look better in terms of rainfall regionwide. Early estimates look to be in the 0.25-0.75 range on average with 1 inch totals for the coastal hills. Nothing excessive but at this point we'll take what we can get. The gfs and canadian models look wetter by the middle of next week keeping the longwave trough over the West Coast while the deterministic ecmwf is drier. However the PNA remains negative while ensemble and cluster tools keep a general trough over the West Coast for the last week of January into early Feb. We're way behind on rainfall numbers but at least there is hope to put some dent into those by months end. && .AVIATION...as of 10:05 PM PST Tuesday...For the 06Z TAFs. VFR. Surface winds have diminished across terminals to mostly under 10 kt, with KHAF the exception at 17 kt. LLWS remains in TAF for KSTS only, with stronger winds aloft. Very dry airmass in place should keep skies clear and VFR conditions throughout the area for the whole TAF period. Light offshore winds will prevail tomorrow. Vicinity of KSFO...Winds have diminished to around 5 kt. Light and variable winds should continue overnight. Offshore winds in the morning and afternoon, perhaps turning onshore late in the day. Clear skies and VFR conditions will prevail through the taf period. SFO Bridge Approach...Similar to KSFO. Monterey Bay Terminals...VFR. Winds have diminished to under 10 kt throughout area terminals. Light winds continue overnight with perhaps breezy SE winds in the Salinas Valley into the morning. VFR through the taf period with clear skies. && .MARINE...as of 08:43 PM PST Tuesday...A moderate period northwest swell will prevail over the seas through the week. Breezy winds remain out of the north-northeast at this hour, but have weakened considerably other than an occasion stronger gust. Winds will shift to the northwest Wednesday morning and are expected to be light to locally moderate through mid-week. The next weather system then arrives at the end of this week allowing northwest winds to strengthen. && .MTR WATCHES/WARNINGS/ADVISORIES... .Tngt...None. $$ PUBLIC FORECAST: Blier/RWW AVIATION: Lorber MARINE: DK 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