010 FXUS66 KMTR 100446 AFDMTR Area Forecast Discussion National Weather Service San Francisco Bay Area 946 PM PDT Mon Oct 9 2017 .SYNOPSIS...Seasonable weather on Tuesday, then a dry cool front passes through Tuesday night into Wednesday. This will bring another spike of North winds by Wednesday morning but not nearly as strong as the Sunday night event. After the midweek trough moves through the trend should be for gradual warming and drying as high pressure builds with a thermal trough near the coast producing periods of light offshore winds. && .DISCUSSION...as of 9:46 PM PDT Monday...For the last 25 years or so the Oakland hills fire has been the seminal fire event that was seared into Bay Area residents psyche. For the current generation of North Bay residents todays firestorm will leave an indelible scar and for years to come we will all recall the Columbus Day firestorm. The wind event that roared through the North Bay last night is long behind us as ignitions occurred on the tinder dry fuels, fire literally exploded and raced along the landscape. Fuel analysis just ahead of the winds showed they were at all time record dry levels preceding the front. Recall we had nearly 5 years of extreme drought followed by record rains last winter that produced a bumper crop of grasses and fine fuels on top of drought and disease stressed heavier fuels. The Hawkeye raws station in northern Sonoma peaked to 79 mph during the event with the Santa Rosa raws at only 599 feet reporting an incredible gust to 68 mph that may have been fire induced as the temperature spiked to 91 degrees at 4:30 am. The NWS has meteorologists enroute to support the incoming incident management teams. Looking forward. We have cancelled the Red Flags for the East Bay and Santa Cruz mtns. The Red Flags for the North Bay will stay in place overnight simply for the protection of fire fighting lives and property. There is simply so much active fire on the ground that even the normal onshore winds will be of concern for both fire fighters on the ground and the general public through the overnight hours. For Tuesday the wind flow will turn onshore with a mainly south to southwest direction. Again, its not really a significant weather factor but with miles of open fireline it will represent a change in wind direction for assets on the ground. The transport winds look to be southerly as well so luckily for the greater bay area much of the smoke should be pushed to the north of us and impact other parts of CA. In the fire business, the common refrain is fire goes where wind blows and unfortunately there will be another dry cold frontal passage later Tuesday night into Wednesday morning. To be crystal clear the magnitude of this front will be MUCH weaker than the Sunday night system. However, if there is still a lot of open fireline at this time tomorrow night it will be of concern once again. Following the 00z nam solution expect winds to be southwest late tomorrow afternoon, then shifting to northwest during the evening (as the front approaches) but then quickly be northeast by 12z Weds. The 925 mb winds on the nam are about 35 kt over Napa county on Weds morning (they were 55-60 kt with the Sunday night event). Humidity will be higher and temperatures cooler by Weds morning so that will help but all bets are off when you already have active fire on the ground. Tuesday will be a critical day in the fire fight. By Weds afternoon dry and breezy northerly winds will persist, especially over Napa and interior Sonoma. Looking a little closer at the 00z nam its now keeping those dry northeast winds in the hills Weds night into Thursday morning with humidity values in the 30-40% range. Looking at the Euro for the long range would suggest warming and drying for late in the week and into the weekend with troughs staying well to our east and the Bay Area getting periods of warm offshore wind flow with the thermal trough along the coast. Good weather if you like going to the beaches but not so much if your trying to suppress wildfires. && .AVIATION...as of 5:18 PM PDT Monday...Smoke due to fires in the North Bay continues to reduce visibilities to IFR over the North Bay, smoky and hazy conditions have swept as far southwest as the Farallon Islands with a recent report of 7 miles visibility. Elsewhere weak onshore winds and clear skies have helped keep VFR going at the terminals. Not indicated in 00z tafs, but caveat here is that little change in air mass stability is forecast tonight may trap more smoke and haze overnight degrading slant-wise and horizontal visibilities over more of the Bay Area into Tuesday morning. Visible imagery shows an area of stratus and fog moving rapidly north along the coast reminiscent of a southerly surge, however a nearly flat SMX-SFO gradient does not support a typical wind reversal. As best as can tell from obs and model forecasts this is part of a surface to low level eddy circulation spun up from the recently strong to very strong N-NE Bay Area winds; not a lot of confidence in wind direction forecasts in the models for tonight, but for what it's worth the NAM is not forecasting a southerly pressure gradient. Best guess is that this circulation will continue to spin down with general onshore flow prevailing at the terminals overnight. Marine inversions are quite compressed, down to a few hundred feet, thus any stratus and/or fog should stay confined to the immediate coastline tonight and Tuesday. Vicinity of KSFO...VFR. Recent wind shift to SW per observations will probably stick for the evening, low confidence forecast. Otherwise in general expecting W wind flow near 10 knots tonight and Tuesday. SFO Bridge Approach...Similar to KSFO. Monterey Bay Terminals...Low to moderate confidence VFR. Light onshore winds becoming E-SE overnight should help keep VFR going tonight. Onshore winds redevelop Tuesday afternoon and evening. && .FIRE WEATHER...As of 9:46 PM PDT Monday...Red Flag Warning will continue through 5 am Tuesday for the North Bay. The warnings for the East Bay and Santa Cruz mtns were cancelled. Please see regular afd above for more details. Next period of concern will be early Weds morning with another round of northeast winds for the Napa hills. Not nearly as strong as the Sunday night event but concerning none the less. && .MARINE...as of 09:17 PM PDT Monday...Light winds tonight and Tuesday morning then northwesterly winds will increase from north to south later in the day Tuesday as a cold front sweeps in from the Gulf of Alaska. The cold front is forecast to reach the southern California coastal waters Wednesday morning. High pressure strengthening over the eastern Pacific and over northernmost California will cause an increase in northwesterly winds across the coastal waters through mid to late week. Gusty afternoon and evening westerly winds are forecast across the bays mid to late week. && .MTR WATCHES/WARNINGS/ADVISORIES... .Tngt...NONE. && $$ PUBLIC FORECAST: RWW AVIATION: RGass MARINE: RGass FIRE WEATHER: RWW 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