968 FXUS66 KMTR 201605 AFDMTR Area Forecast Discussion National Weather Service San Francisco Bay Area 905 AM PDT Sun Jun 20 2021 .SYNOPSIS...Though warmer temperatures can still be expected farther inland, pleasant conditions are forecast for Father's Day 2021 as temperatures continue their cooling trend. As the marine layer has reestablished and lower pressure develops just west of the coast, these cool temperatures will continue for much of the week and possibly drop below average by midweek. The next chance for warmer temperatures looks to be next weekend with inland areas returning to the 90s. && .DISCUSSION...as of 08:43 AM PDT Sunday...Happy Summer Solstice. After several days of heat the return of the marine layer is a welcomed site. Morning satellite shows a decent inland push of clouds up the Salinas Valley, San Francisco Bay, and N Bay Valleys. Despite the inland push the last few images of the satellite imagery show the edges of the cloud deck already starting to thin. Expect this trend to continue through mid-late morning. Therefore, expect inland sunshine today with clouds lingering at the coast. Coastal areas will be closer to normal values for late June, but far interior will still be a few degrees above normal. No update is needed at this time. Satellite and webcams show some overnight activity on the Willow Fire in Monterey county. Transport winds will continue to be SW today so smoke production from the fire will be to the NE. Weather concerns the day shift will focus on for the next week will be: the cool down through mid week, warming temperatures late in the week and next weekend, and potentially some upper level moisture streaming in from the SE next Sunday/Monday. && .PREV DISCUSSION...as of 02:59 AM PDT Sunday...It is good to see a robust stratus deck out over the ocean again on satellite. Onshore winds has helped push the marine layer into the San Francisco Bay as well as through the Monterey Bay and down the Salinas Valley. Ceilings remain fairly low between 300 to 500 ft, but the depth of the marine layer expands to around 1200 to 1500 feet as observed on the Fort Ord and Bodega bay Profilers. In addition to helping overnight temperatures cool, it is helping bring humidity levels back to the region. While locations within the marine layer depth see that relief, highest elevations above it will still see warm overnight temperatures and dry conditions. The forecast also added a slight chance for light drizzle along the coastline. Sadly, the location of the Willow Fire in the Ventana Wilderness is at a spot that is just above and farther inland to be a part of this humidity relief. Southwest winds aloft keep the air mass above 700 mb dry, while winds at the surface will be onshore. With flow out of the west and a retreating upper level ridge to the desert southwest, Sunday is looking to be a fairly pleasant day. Temperatures will be A developing upper level trough 400 to 500 miles west-southwest of San Francisco will strengthen through Sunday and move closer to the California Coastline. As it does so, it will likely act to deepen the marine layer, further helping with nighttime humidity recoveries. It will also keep winds out of the south-southwest and onshore through the week. The NBM is trying to develop scattered showers over over the waters for late Monday into Tuesday as the trough approaches, but with such low confidence, it was scaled back to just a slight chance over the outer waters. Wouldn't be surprised if there was something light out to sea as the trough approaches, but nothing to impact the land area. By Tuesday, the trough will drift slowly to the north toward Eureka where it will likely stall by Thursday. In addition to adding humidity and onshore winds, it will be the driving force to keeping the cooling trend going through the week for the region, returning inland temperatures to the 70s and 80s, with 60s at the coast. This trough will also have additional moisture at higher elevations and could have some weak vertical lift ahead of it. NAM, GFS, and ECMWF models depicting 700 mb to 500 mb layer humidities overnight Monday into Tuesday morning above 50%. Just north of Sonoma and Napa counties show NAM modified 500 mb Total Totals with indices between 30-32, but instabilities in MUCAPE are just not there. While it will be monitored through the week, the chances for a dry thunderstorm to develop is low, or if it does it will likely remain north of our county warning area (CWA). So from a high level, it should be a quiet weather week with cool, possibly even below normal, temperatures. The next big chance to the pattern looks to be next weekend. Ensemble models are showing the ridge over New Mexico try to build back to the California/Nevada border, but it will have to contend with the stationary trough parked over the Central Coast. Still, 850 mb temperatures look to increase back into the mid-20 degree Celsius range. For now, it is forecast for temperatures to warm up, but to what extent and how early will still need to be ironed out. Some models have the warm up beginning on Thursday, but there is low confidence in that. The hottest locations will likely be the traditional ridge tops and far interior East Bay/Southern Monterey County having temperatures return to the 90s. && .AVIATION...As of 4:43 AM PDT Sunday...For the 12z TAFs. The marine layer is near 1,800 feet deep. Coastal stratus, fog and patchy light drizzle /VLIFR-IFR/ until late morning with a return this evening and overnight. Hazy or smoky conditions aloft briefly lowering slant range visibilities over the north Central Coast and mainly the southern Bay Area this morning. Hazy or smoky conditions should be limited per latest HRRR model the Willow wildfire smoke transports toward the Central Valley with lower to mid level coastal troughing resulting in southwest to west winds through the period. Vicinity of KSFO...VFR continuing so far at KSFO with a light west wind, however nocturnal cooling and light northerly advecting winds across the other side of SF Bay to KOAK have boosted stratus and fog development since midnight. Similarly stratus ceilings fairly rapidly formed under nocturnal cooling since midnight to the south along the SF shoreline from KSQL to KNUQ. Forecast for KSFO is VFR with tempo BKN ceiling 12z-16z this morning. VFR later in the morning into the evening, stratus ceiling is possible by early Monday morning. SFO Bridge Approach...To the south of KSFO since midnight stratus ceilings have formed more rapidly under nocturnal cooling. Clearing by late morning, VFR for the remainder of the day into the evening. Wildfire smoke may briefly reduce slant range visibility this morning. Monterey Bay Terminals....VLIFR-IFR in stratus, fog and patchy drizzle until late morning. VFR returning temporarily in the afternoon then VLIFR-IFR redeveloping in stratus, fog and patchy drizzle tonight and Monday morning. && .MARINE...as of 08:43 AM PDT Sunday...A developing weak low pressure system over the outer to offshore waters will allow south to southwest winds to persist through early this week. Locally breezy southwest winds are expected north of the Bay Bridge in the afternoons and evenings the next couple of days. Seas remain northwest at 10 to 11 seconds along with a marginal southerly swell. && .MTR WATCHES/WARNINGS/ADVISORIES... .Tday...SCA...SF Bay from 3 PM && $$ PUBLIC FORECAST: MM/DK AVIATION: Canepa MARINE: AS 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