146 FXUS66 KMTR 100955 AFDMTR Area Forecast Discussion National Weather Service San Francisco Bay Area 255 AM PDT Wed Oct 10 2018 .SYNOPSIS...Onshore flow and a deep marine layer will mean cool temperatures today. Weak offshore flow will then likely develop late in the week, resulting in warmer and drier conditions from Friday on through the weekend. Dry weather conditions are expected to persist well into next week. && .DISCUSSION...As of 2:55 AM PDT Wednesday...A shortwave trough approaching from the north triggered a rapid deepening of the marine layer last evening. The deep marine layer, along with light onshore flow, has resulted in widespread low clouds across our region early this morning. In addition, where onshore flow is deepest, from Monterey Bay southward, patchy drizzle has been reported. Expect patchy drizzle to persist through about mid morning for areas near the coast and along the windward side of the coastal mountains, primarily in Monterey and Santa Cruz Counties and perhaps as far north as San Mateo County. Isolated measurable precipitation is possible in these areas. In fact, the Watsonville Airport recently measured 0.01". Farther to the north, light offshore flow aloft will likely suppress precipitation potential. In stark contrast to the cool and damp conditions at the lower elevations early this morning, dry and gusty north winds are currently being observed over the higher hills in the North Bay. Wind gusts of between 30 and 45 mph are being reported at isolated locations above 2000 feet in Sonoma and Napa Counties, along with relative humidity values as low as 30 percent. The latest WRF model indicates these localized gusty winds are at their peak intensity right about now and will taper off by sunrise this morning. The marine layer will likely mix out as the upper level shortwave trough passes across our area around midday. Expect partly cloudy skies by afternoon in most areas. The cooling trend that began yesterday will continue today, especially inland. Highs today are forecast to mostly range from the mid 60s to mid 70s and be about 3-5 degrees cooler than seasonal averages. Today is expected to be the coolest day of the week, although Thursday probably won't be all that much warmer. The models have come into agreement regarding the pattern evolution during the second half of the week. Energy from the longwave upper trough to our east is forecast to shear off from the main flow and eject westward, forming an upper low off the Central California Coast by Thursday night. Meanwhile, an upper ridge over the eastern Pacific is forecast to build inland to our north across Oregon and far northern California. The resulting rex block pattern will result in an extended period of light offshore flow across our area from Friday through the weekend. Skies are expected to clear in all areas by Friday and temperatures will warm into the mid 70s to mid 80s by Friday afternoon. Mostly clear and seasonably warm and dry conditions will then persist for at least 3 or 4 days and potentially well into next week. The upper low that will develop off the Central California Coast later this week is forecast to sink to the south, tap into some remnant moisture from Tropical Storm Sergio, and perhaps bring rain to far southern California this weekend. But this precipitation is expected to remain well to our south. && .AVIATION...as of 10:34 PM Tuesday...For 06z tafs. The marine layer has deepened substantially since this afternoon to over 2000 ft per the Fort Ord Profiler. Satellite imagery shows widespread stratus across the region. Most sites are currently experiencing IFR/MVFR cigs with the exception of KLVK and KSJC. However, given the current state of the marine layer and current satellite imagery it is possible that KLVK and KSJC will see MVFR/IFR cigs at some point tonight. Exact timing is uncertain. Some patchy coastal drizzle is still a possibility overnight. Vicinity of KSFO...Current satellite imagery shows stratus moving southward across the south SF Bay. Concerned this may soon envelope the terminal. Given this and the deepened marine layer, have decided to introduce MVFR cigs overnight. Confidence is low, and taf may be amended should conditions change. Right now, should cigs develop, expecting clearing between 16z-18z. Southerly winds overnight and tomorrow morning before becoming onshore by the afternoon. SFO Bridge Approach...Similar to SFO. Monterey Bay Terminals...Currently MVFR with cigs expected to lower to IFR overnight. Clearing forecast between 16z-19z for now, though some guidance is starting to suggest that the Monterey Peninsula could be under stratus for most of the day. Patchy coastal drizzle possible in the overnight hours. && .BEACHES...Tropical storm Sergio will bring moderate period and height swell to the Central California and Bay Area coastlines beginning Thursday morning. Exposed south facing beaches will be at highest risk for building surf. Some breakers may reach as high as 10 feet with 10 to 20 minute lulls between sets. All beaches will experience an increase in rip current activity during the second half of the week as this swell arrives and transits the area. Therefore, a Beach Hazards Statement has been issued from early Thursday morning through late Friday afternoon. && .MARINE...As of 2:37 AM PDT Wednesday...Gusty northerly winds over the outer coastal waters post cool frontal passage will be situated next to a light southerly flow over the inner coastal waters today. Patchy light drizzle is possible through this morning. Mixed northwesterly and southerly swell will continue through the forecast period, southerly swell is originating from Tropical Storm Sergio. && .MTR WATCHES/WARNINGS/ADVISORIES... .Tday...SCA...Pt Arena to Pigeon Pt 10-60 nm SCA...Pt Arena to Pt Reyes 0-10 nm until 4 AM && $$ PUBLIC FORECAST: Dykema AVIATION: AS MARINE: Canepa 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