280 FXUS66 KMTR 082135 AFDMTR Area Forecast Discussion National Weather Service San Francisco Bay Area 235 PM PDT Fri Oct 8 2021 .SYNOPSIS...Unseasonably cool conditions will prevail this afternoon and evening. The next disturbance will then bring another chance of rain to the region tonight through Saturday morning. Drier conditions and a gradual warming trend will develop for the weekend. An inside slider will dig across the Great Basin late Sunday into Monday, bringing periods of gusty northerly winds to the region from Sunday night through Tuesday, increasing fire weather concerns. && .DISCUSSION...As of 02:35 PM PDT Friday...Water vapor imagery continues to show a deep trough of low pressure over the West this afternoon. At this time, a few fair weather cumulus are lingering over the mountains, but otherwise, skies are predominately sunny. Despite all this sun, temperatures are running similar to or a could of degrees cooler than this time yesterday. The one thing of real note today is with the strong onshore winds. In fact, the westerly winds have been sustained between 10 and 20 kts all day, with gusts up to 30 kts. These breezy conditions should finally taper off after sunset, with conditions cooling off quickly. Water vapor imagery also shows the next disturbance digging down the back side of the trough and approaching Cape Area this afternoon. The composite radar imagery is starting to show some light rain approximately 80 miles off the coast. This upper level disturbance will move inland across Sonoma County later this evening, and move south across the region during the night tonight and early tomorrow morning. At this point in time, the models are hinting that the bulk of the moisture and rainfall with this disturbance will remain off the coast. However, do think that most areas will see some light rain late tonight and early Saturday morning. Rainfall totals look to be less than a tenth of an inch most areas, but between a quarter and a third of an inch could fall along the immediate coast as well as in the coastal Sonoma Mountains between tonight and tomorrow morning. This rainfall will likely cause very slick roadways, so if out and about late tonight, please use caution while driving. The models are hinting at a little elevated instability with this disturbance, but at this point in time, think if any lightning did occur that it would mostly remain offshore. As this trough moves out of the region on Saturday, weak ridging will develop over the region. Therefore, expect dry and warmer conditions to develop for Saturday and Sunday, with temperatures returning to normal ranges by Sunday. The main focus of the forecast will then shift to Sunday night through Tuesday, as a potentially strong inside slider digs across the Great Basin, bringing significantly cooler to the region once again, as 850 mb temperatures fall nearly 9 degrees across the region. However, the bigger impact will be with the offshore winds. North to northeasterly winds will develop across Northern California Sunday night into Monday. At this point in time, the models are already hinting at 925 mb winds ranging between 40 and 50 kts across the North and East Bay Mountains and Hills, with significantly drier air filtering into the region. Therefore, updated the Fire Weather Watch to start at 11 pm local Sunday evening. Believe their will be two separate maxima of the winds, one late Sunday night into early Monday and a second late Monday night into early Tuesday. That said, gusty winds may be possible in the interior valleys as well, so this will be something to watch. High pressure will then build back over the region for the remainder part of the work week, with warmer conditions developing once again. Palmer && .AVIATION...As of 11:35 AM PDT Friday...For the 18z TAFs. VFR this afternoon as fog mixes out and low-level clouds gradually exit the region this morning. Are on track for cigs > 20k feet to linger at all the terminals this afternoon as a weak cold frontal boundary begins its approach from the northwest. This boundary will bring a slight chance for some showers to filter in to our terminals after 6Z Sat and into 12Z Sat, so have added VCSH to all of them with the caveat that some of these showers have the potential to bring isolated stronger northwest wind gusts to our coastal terminals. MVFR cigs are also expected overnight at most of the terminals as the environment attempts to stratify. Winds will remain breezy through the entire TAF period with the exception of wind-locked valleys. Cigs will then lift early tomorrow morning as the boundary exits the region. Vicinity of KSFO... Breezy westerly winds are prevailing this afternoon as a weak frontal boundary slowly moves in. Winds will accelerate along the San Bruno Gap and Golden Gate, building up stronger gusts between 20 to 25 knots this afternoon. This boundary will pass over the terminal Friday night with the possibility of bringing in VCSH near the terminal. Borderline MVFR cigs will return to the vicinity after 7Z Sat. Winds will remain onshore and breezy through the morning before building back up by the end of the TAF period. KSFO Bridge Approach...Similar to KSFO. Monterey Bay...VFR this Friday afternoon with west winds increasing to 10 to 15 knots. Onshore flow from an approaching boundary will help to spread MVFR cigs back inland Friday evening with a chance for weakly scattered showers moving in after 10Z. Winds will gradually calm by morning and will observe high-base clouds in conjunction with the overnight MVFR cigs through the end of the TAF period. && .MARINE...As of 08:45 AM PDT Friday...Gusty northwest winds will persist over our northern waters through the rest of the day, along with breezy winds past the Golden Gate and the Big Sur Coastline as a weak boundary travels north-south over our waters today. This boundary will bring a chance for light showers over our northern waters starting tonight. These light showers will bring with them the possibility for isolated gusty conditions as they move across our waters tonight and into tomorrow morning. Another low pressure system will move in to our waters starting early Sunday evening, bringing in gustier northwest winds with the potential to reach gale force intensity, along with steep seas. The peak of these winds will be late Sunday night into early Monday morning. Gusty winds will linger but gradually subside through early next week as the system exits our region. && .MTR WATCHES/WARNINGS/ADVISORIES... .Tday...SCA...SF Bay SCA...Pt Arena to Pt Reyes 0-10 nm SCA...Pt Reyes to Pigeon Pt 0-10 nm SCA...Pt Arena to Pigeon Pt 10-60 nm SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm && $$ PUBLIC FORECAST: Palmer AVIATION: Diaz MARINE: Diaz 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