258 FXUS66 KMTR 100534 AFDMTR Area Forecast Discussion National Weather Service San Francisco Bay Area 1034 PM PDT Tue Oct 9 2018 .SYNOPSIS...Onshore flow will continue through Wednesday with cooler temperatures. 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 will persist through at least the middle of next week. && .DISCUSSION...as of 9:00 PM PDT Tuesday...Southerly surge has allowed coastal stratus to penetrate inland through coastal gaps early this evening. The effects of the southerly surge is apparent by comparing today's temperatures with yesterday's. Temperatures cooled 10 to 15 degrees at places like Santa Cruz...the San Francisco-Oakland corridor...and the North Bay Valleys. The marine layer has deepened to around 1500-2000 feet and has brought general cooling to the district. However weather conditions are quite different above the marine layer. An upper level disturbance moving into the Great Basin this evening will cause winds to switch to northeast over the North and East Bay Hills late tonight and Wednesday morning with winds forecast to switch to north to northeast. Strongest winds will be over the North Bay Mountains where gusts to 40 mph are possible. There will also be offshore winds in the East Bay Hills but they are expected to be light. The airmass is a little more moist so RH values are not expected to get super low. Nevertheless the magnitude of the offshore winds heightens fire weather conditions. Offshore winds are expected to diminish Wednesday and switch back to northwest Wednesday night. Cooler temperatures of 3-5 degrees are expected Wednesday as the marine layer deepens and marine air spreads further inland. Things will start to warm up again Thursday through Saturday as an upper level high builds off the west coast. Meanwhile another inside slider is expected with an upper trough sliding through the Great Basin. Medium range models now forecast this trough will form into a closed low over southern California this weekend. If this verifies it would bring a northeast flow aloft over our CWA which could mean another potential offshore event for the North and East Bay Hills. At this time the wind speeds will not be as strong as last weekend but temperatures will be warmer. && .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. && .FIRE WEATHER...As of 9:00 PM PDT Tuesday...The general trend through Wednesday is for increasing onshore flow and a deeper marine layer across the district, and thus cooling temperatures and better nighttime humidity recoveries in the hills. However, the exception is in the North Bay Mountains late tonight and Wednesday morning where dry and gusty north winds are expected. Strongest winds are expected to be across the higher hills of eastern and northern Napa County, as well as extreme northeast Sonoma County. In these areas wind gusts of up to 40 mph are possible late tonight through Wednesday morning. Because this is expected to be a short-duration and localized offshore wind event Red Flag Warnings are not currently anticipated. More widespread light offshore flow is forecast to develop late in the week which will result in warm and dry conditions across most of the district from Friday into the weekend. && .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 08:28 PM PDT Tuesday...Split flow will continue over the next few days across the waters with stronger northwest winds in the northern and outer waters and southerly winds along the coast. Occasional to infrequent gale- force gusts will be possible in the northern outer waters through this evening. Mixed northwest and southerly swells will continue through the forecast period. Southerly swell will increase late this week due to Tropical Storm Sergio. && .MTR WATCHES/WARNINGS/ADVISORIES... .Tngt...SCA...Pt Arena to Pigeon Pt 10-60 nm SCA...Pt Arena to Pt Reyes 0-10 nm until 4 AM SCA...SF Bay until 11 PM && $$ PUBLIC FORECAST: W Pi AVIATION: AS MARINE: AS FIRE WEATHER: W Pi BEACHES: BFG 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