563 FXUS66 KMTR 091750 AFDMTR Area Forecast Discussion National Weather Service San Francisco Bay Area 1050 AM PDT Tue Oct 9 2018 .SYNOPSIS...A return of onshore flow will mean cooler temperatures today through Wednesday. 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 08:40 AM PDT Tuesday...Morning visible satellite imagery shows coastal low clouds from the central Sonoma coast down through Monterey County. Clouds have some inland reach in Monterey and Santa Cruz County, as well as southern Santa Clara and Northern San Benito Counties. As daytime warming takes hold, clouds should burn off nicely. Temps for the most part will be cooler today than yesterday, but around normal. Overall the forecast remains on track and no significant changes have been made this morning. Of focus for the afternoon package will be possible gusty northeast winds tonight over the North Bay mountains. We'll see how the 12Z models handle this pattern. Have a great day! && .PREV DISCUSSION...As of 3:45 AM PDT Tuesday...Low clouds and patchy fog rapidly developed along the coast from the Golden Gate southward overnight as light onshore flow returned. Low clouds have also moved locally inland from Monterey Bay. The marine layer is relatively shallow with a depth of about 1000 feet per the Fort Ord Profiler and onshore surface pressure gradients are weak. Thus, low clouds are not expected to develop much farther inland prior to daybreak. A return of coastal low clouds and light onshore flow will mean considerably cooler temperatures near the ocean, around the bays, and in the coastal valleys today -- anywhere from 7 to 15 degrees cooler compared to Monday. Due to the relatively shallow nature of the marine layer and light onshore flow, the inland valleys and hills will experience less cooling. But strengthening onshore flow during the afternoon hours is expected to result in at least slight cooling even in the valleys farthest inland. An upper trough currently located over British Columbia is forecast to drop to the SSE over the northern Rockies and eastern Great Basin by tonight. This will result in modest height falls across California and thus a likely increase in the depth of the marine layer over our region. Thus, expect more widespread low clouds by late tonight and into Wednesday morning. However, the models indicate localized offshore flow developing in the North Bay tonight as surface high pressure builds across Oregon and northern Nevada. In fact, the latest WRF forecasts sustained north winds of up to 25 knots in the hills of eastern and northern Napa County, and far northeastern Sonoma County, late tonight and into early Wednesday morning which will increase fire weather concerns for these localized areas (see Fire Weather Discussion below for details). Aside from brief offshore winds in the hills of the North Bay late tonight, the overall trend through Wednesday is for increasing onshore flow and continued cooling. Wednesday is forecast to be the coolest day of the week with high temperatures forecast to be at least slightly cooler than normal, ranging from the mid 60s to mid 70s in most areas. The synoptic pattern evolution during the second half of the week is rather complex. The NAM and the GFS both indicate that the trough to our east will elongate westward across California and form an upper low off the central California coast by Thursday night. Meanwhile an upper ridge over the eastern Pacific will build inland across Oregon and far northern California, with the end result being a rex block along the West Coast. The ECMWF does not develop the Rex Block as quickly as the NCEP models. In any case, this pattern evolution will result in light offshore flow late in the week and a warming and drying trend from Friday on into the weekend. Earlier model runs indicated the potential for gusty offshore winds over the North Bay Thursday night and Friday morning, but latest models indicate lighter winds in these areas and thus less concern for fire danger. In the longer range, although the models don't agree at that well on the specifics, the general idea is for a blocking pattern to persist, maintaining relatively high heights, dry weather, and warmer than normal temps well into next week. && .AVIATION...as of 10:50 AM Tuesday... Marine stratus surged up the coast last night and into this morning, making slight inroads inland through this morning. VFR prevailing across the inland regions except for a few patches of stratus over the Monterey Bay incl KMRY. Light to locally breezy onshore winds are forecast for most terminals today. For tonight, the marine stratus returns and surges slightly deeper inland, bringing local LIFR to MVFR cigs to most coastally influenced terminals while KLVK and KSJC are left mostly clear. Some offshore winds aloft will be possible overnight at KLVK KAPC and especially KOAK, with NE winds 15-25kt possible at around 1500-2500 ft AGL and calm winds at the sfc. Drizzle is possible along the coast overnight. Vicinity of KSFO...VFR. Light wind this morning shifts to become breezy and onshore by the afternoon and into the evening. Low to med confidence for tonight for patchy stratus cigs around 1000 ft, with highest chance of BKN-OVC cigs from 08-14Z WED. SFO Bridge Approach...Similar to SFO. Monterey Bay Terminals...Patches of stratus linger along the Monterey Bay late this morning. Expect to see KMRY clear out within the next 30 to 45 minutes. Light to occasionally, locally breezy onshore winds into the early afternoon. LIFR to IFR cigs return as early as 00Z but definitely by 02Z for KMRY and KSNS. && .FIRE WEATHER...As of 3:30 AM 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 possible localized dry and gusty north winds in the North Bay Mountains late tonight and Wednesday morning. 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 35 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. Will headline local gusty offshore winds in the Fire Weather Forecast. 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. && .MARINE...as of 10:44 AM PDT Tuesday...Generally split flow across the waters with stronger northwest winds in the northern and outer waters and southerly winds along the coast. Mixed swell with small to locally moderate northwest and southerly swells. The southerly swells increase through mid week due to the presence of a tropical system west of Southern Baja. && .MTR WATCHES/WARNINGS/ADVISORIES... .Tday...SCA...Pt Arena to Pt Reyes 0-10 nm SCA...Pt Arena to Pigeon Pt 10-60 nm SCA...SF Bay from 1 PM && $$ PUBLIC FORECAST: BFG/Dykema AVIATION: DRP MARINE: DRP FIRE WEATHER: Dykema 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