969 FXUS66 KMTR 101613 AFDMTR Area Forecast Discussion National Weather Service San Francisco Bay Area 913 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 09:11 AM PDT Wednesday...The ongoing forecast for this morning remains on track and no updates are needed at this time. While patchy drizzle persists along the coast from Santa Cruz County southward down the Big Sur Coast, look for conditions to improve and partial clearing of cloud cover through the remainder of the morning. Please see the previous forecast discussion below for additional information. && .PREV 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 5:00 AM Wednesday...It's a mix of IFR and VFR this morning as a lower level cyclonic circulation currently 30 miles SW of Half Moon Bay draws in post cool frontal air from the W and lifts pre-existing moist air from the now eroded marine layer inversion. Low ceilings are confined to the North Bay and from the Santa Cruz Mountains south along the Central Coast; reports of light rain and drizzle in the Santa Cruz Mountains to Watsonville, Aromas, and now a trace at Salinas Airport. Meso-scale models forecast additional light rain south along the Big Sur Coast today. The Bay Area has the best chance of clearing this morning as the focus of atmospheric lifting mechanisms and residual moist marine air center on the north Central Coast today. Timing ceiling forecasts is low to very low confidence for the period. Vicinity of KSFO...VFR, south wind, tempo MVFR cig forecast 14z-16z today, then a wind shift to onshore is forecast early to mid afternoon. SFO Bridge Approach...Similar to KSFO. Monterey Bay Terminals...IFR ceilings with tempo light drizzle or light rain. Ceilings lifting to MVFR by mid to late morning. Partial clearing this afternoon, low confidence ceilings return this evening. && .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 09:11 AM PDT Wednesday...Split flow with lighter south to southwest winds along the coast and stronger gusty northwest winds over the northern outer waters. 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 && $$ PUBLIC FORECAST: RGass/Dykema AVIATION: Canepa MARINE: DRP 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