FXUS66 KMTR 231129 AFDMTR Area Forecast Discussion National Weather Service San Francisco Bay Area 425 AM PDT Wed Jun 23 2021 .SYNOPSIS...An upper level low continues to provide cooler conditions regionwide with near to below normal daytime highs. There is a slight chance for coastal drizzle this morning before sunny to partly sunny skies this afternoon. As the low moves inland, high pressure builds over the Pacific Northwest bring the next chance for another warming trend this upcoming weekend. && .DISCUSSION...as of 02:59 AM PDT Wednesday...Stratus clouds have built back after clearing out yesterday; a cool night is in store for the Bay Area, with temperatures once again in the upper 50s and low 60s. With a marine layer that is well over 3000 feet in depth, a deep inland push keeps humidity levels on the rebound from this past weekend. As an upper level low sits just off the coast, keeping southerly winds along the coastal waters, there is a chance for some patchy drizzle along the immediate coastline early Wednesday morning. Not enough to be significant, but a bucket tip is possible and the potential for the use of a windshield wipe or two. Thursday may have some patchy coastal drizzle as well, but confidence is low. The short term forecast for Wednesday seems to say, "Let me be that I am and seek not to alter me," as it remains Much Ado About Nothing. With prevailing onshore flow, temperatures this afternoon will range from normal to just below normal for this time of year while the low remains overhead tracking ever so slowly inland. As the low moves eastward, winds over the ocean will switch from the south to northwest, starting as early as Thursday morning. Skies will see clearing in the afternoon today, before the stratus returns Wednesday evening. The pattern starts to change slowly on Thursday as the low moves inland on account of building high pressure over the northeastern Pacific. There is a second weak high that is expected to grow over the eastern Pacific west of Baja California, but will not have much of an impact on our forecast. The main driver is in the PacNW where the previously mentioned high will move inland and strengthen, compressing our low pressure into a trough and pushing it toward the desert southwest. This high pressure centered over Washington, Oregon and Idaho will be the cause for a drier airmass to move in and warm temperatures aloft. The trouble with this high pressure influencing the Bay Area is how far away it is geographically. 850 mb temperatures will certainly warm above 25 degrees Celsius, which will translates to higher elevations warming up, but confidence is decreasing on getting those temperatures to mix down to the surface. ENS and GEFS ensemble temperatures depict inland areas like Livermore only increasing into the upper 90s. The GFS still remains the aggressor in trying to have temperatures into the 100s, so it is still possible, but unfortunately there is still too much uncertainty which is keeping confidence low. For now, this forecaster simply recommends caution as temperatures are expected to warm. A record setting heat wave is not expected at this time. Early next week, ensemble and deterministic models still portray the next trough to move over the Eastern Pacific as the PacNW high drops down to the Utah/Colorado border. Other than increasing onshore flow, models show the trough steering northward away from California having very little impact other than keeping the marine layer in place. But this is also still far out in model-land where a lot can still chance over the course of the week. && .AVIATION...As of 04:26 AM PDT Wednesday...For the 12Z TAFs. Widespread MVFR this morning across all of the terminals. Coastal Profilers continue to signal a deep marine layer 2500-3000 ft. This is evident at LVK where BKN skies at 2500-2900 ft have been observed all morning. This deep stratus deck will gradually mix out across the interior starting around 15-17Z and begin to lift along the coast by 17-19Z. The clearing will make way for a sharp return of onshore, wnw/nw flow along the immediate coast and bays (10-15kts). Have bumped up wind gusts along the bays to account for the strong return-flow as the low continues to sit right off the coast. Rinse and repeat for the evening with the marine layer staying around 3000 ft and stratus quickly filling up the terminals after 5Z this evening. Vicinity of KSFO...MVFR cigs will mix out by 16-19Z at SFO/OAK, allowing for onshore w/nw winds to pick up immediately following the departure of the stratus. Given the stagnant position of the low just off the coast, will once again see 10-15kts winds with gusts up to 20kts. MVFR cigs return tonight after 7Z. SFO Bridge Approach...Similar to KSFO. Monterey Bay Terminals....MVFR/IFR with coastal drizzle along the immediate coast. Cigs will lift after 19Z but will remain FEW/SCT across much of the Monterey Bay and northern Salinas Valley. Onshore winds will pick up in the afternoon (10-15kts) and gradually weaken as marine stratus once again come ashore as early as 2Z. && .MARINE...as of 02:45 AM PDT Wednesday...Mixed seas across the marine interface with a continuation of light south/westsouthwest winds along our coastal waters today. These onshore winds will be the predominant coastal winds as a low continues to spin just outside of our outer waters. Winds will then gradually shift back out of the northwest starting tomorrow as the low falls apart, making way for a high pressure to fill in the void. This high will introduce more north/northwesterly winds along our waters, resulting in a moderate period northwest swell; this will become the predominant swell starting Thursday and continue through the weekend. && .MTR WATCHES/WARNINGS/ADVISORIES... .Tday...None. $$ PUBLIC FORECAST: DK AVIATION: Diaz MARINE: Diaz 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