FXUS66 KMTR 290431 AFDMTR Area Forecast Discussion National Weather Service San Francisco CA 931 PM PDT Wed Aug 28 2024 ...New MARINE... .SYNOPSIS... Issued at 1240 PM PDT Wed Aug 28 2024 Slight cooling trend begins Thursday as a trough develops, with warmer temperatures early next week. && .UPDATE... Issued at 834 PM PDT Wed Aug 28 2024 Temperatures came down nicely today, especially in the coastal and near coastal areas. For example, Redwood City reached 94 yesterday, and only 83 today. This was caused by a disturbance that weakened the ridge. The 00Z balloon today reached 500 mb at 5890 m while 24 hours ago it found the same pressure level at 5910 m. This lower pressure allowed the marine layer to deepen from around 500 ft to over 1,000 ft and climbing. Moving forward, this cooling trend is expected to continue through the next few days and we are already seeing a good influx of coastal stratus signaling the healthy marine layer. In the longer range outlook, there is some indication that offshore winds may accompany the warmer weather next week in the North Bay. It's too early to say for sure, but given the time of year we will keep a close eye on this development and its impact on fire weather. && .SHORT TERM... (This evening through Thursday) Issued at 1240 PM PDT Wed Aug 28 2024 Satellite imagery shows an offshore stratus deck with impacts limited to the immediate coast. Warm temperatures continue today, especially in the interior, with highs in the inland valleys reaching the 90s to near 100 in the warmest locations. The bayshore sees highs in the mid 70s to upper 80s while highs in the 60s continue along the Pacific coast. Breezy and gusty onshore flow will develop each afternoon and evening with gusts up to 20 to 25 miles per hour in the gaps, passes, and Salinas Valley. A slight cooling trend begins on Thursday, as the upper level ridge flattens and a cut off low approaches the coast. The cooling will be gradual, with temperatures in the inland valleys in the mid 80s to lower 90s, with little cooling in the most interior parts of the East Bay and southern Monterey County. The Bayshore cools to around the mid 70s to mid 80s, while the coastal areas stay in the 70s. && .LONG TERM... (Thursday night through next Tuesday) Issued at 1240 PM PDT Wed Aug 28 2024 The cooling trend continues through the weekend as the cut off low approaches and impacts the state. By Sunday, temperatures should be close to or slightly below seasonal averages, with the inland valleys seeing highs in the low to mid 80s, while the Bayshore should see highs in the upper 60s to the mid 70s. By the early part of next week, ensemble means and clusters are converging on a ridge developing over the western United States, bringing warmer temperatures back to the region. Exactly how warm temperatures will get is an question for future me and my future colleagues to clarify. CPC outlooks also suggest that temperatures above seasonal averages are likely to continue into the second week of September. && .AVIATION... (00Z TAFS) Issued at 534 PM PDT Wed Aug 28 2024 A coastally trapped southerly wind reversal with stratus and fog /LIFR-IFR/ continues, the SMX-SFO and SBA-SFO pressure gradients are a robust 2.5 mb and 1.6 mb respectively; model forecasts continue to struggle with surface pressure gradients, e.g. the 22z HRRR is forecasting a slightly north to near flat gradient between SFO-SMX. Visible imagery shows coastal stratus and fog holding together well during the maximum diurnal mixing part of the day, thus there's a strong foothold in place for tonight's radiative cooling and further stratus and fog development. The marine layer depth is near 1000 feet along the coast. The STS-UKI pressure gradient is currently strong at 2.1 mb, this should help usher stratus and fog /LIFR-IFR/ conditions earlier this evening in the North Bay. Currently HREF IFR probability is on the increase forecasting IFR between 04z-05z (after sunset); an intrusion of stratus and fog into the Bay Area may be greater than current models are forecasting. Highest confidence VFR holds through mid evening away from the immediate coast, while LIFR-IFR prevails on the coastline with lowering confidence in the areal coverage of stratus and fog all locations tonight and Thursday morning. Will monitor satellite and observations and amend 00z TAFs as needed. Vicinity of SFO...VFR continues this evening, then by 08z IFR is forecast continuing to 17z Thursday. Evening west to northwest wind gusts to near 25 knots. IFR ceiling is forecast 08z-17z Thursday, along with decreasing wind forecast tonight though caveat, it's a low to very low confidence forecast since recent model forecasts are not resolving recent surface pressure gradients. SFO Bridge Approach...Similar to SFO. Monterey Bay Terminals...Visible satellite imagery shows stratus curling around the Monterey Bay with the southerly wind reversal. Stratus /MVFR-IFR/ is on the doorstep of SNS Airport while it'll take a while this evening for the southwesterly downsloping wind at MRY to ease allowing stratus /IFR/ to fill in. The marine layer depth is near 1000 feet. The southwesterly wind is advecting the stratus to Jacks Peak which is subsequently downsloping, warming and drying on the other side of the terrain. With nocturnal cooling, winds will gradually ease allowing stratus and fog /LIFR-IFR/ to fill in tonight and Thursday morning. Clearing /VFR/ is forecast to return by late Thursday morning. Onshore winds 10 to 20 knots easing to light onshore winds tonight, then onshore 10 to 20 knots Thursday afternoon and evening. && .MARINE... (Tonight through next Tuesday) Issued at 931 PM PDT Wed Aug 28 2024 Breezy northwesterly wind continues this evening over the northern outer waters, while the rest of the waters remain relatively calm. Northwesterly winds will redevelop over the coastal waters later in the week and weekend. && .MTR WATCHES/WARNINGS/ADVISORIES... CA...None. PZ...None. && $$ SHORT TERM...DialH LONG TERM....DialH AVIATION...Canepa MARINE...Canepa 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