Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA
Issued by NWS San Francisco Bay Area, CA
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724 FXUS66 KMTR 170358 AFDMTR Area Forecast Discussion National Weather Service San Francisco CA 858 PM PDT Mon Sep 16 2024 ...New UPDATE, MARINE... .SYNOPSIS... Issued at 1203 PM PDT Mon Sep 16 2024 Rain showers will taper off from north to south through this evening with temperatures remaining below seasonal averages through late week. Another system enters the picture Wednesday with additional rain chances, however rainfall amounts will be light. Warming into the weekend. && .UPDATE... Issued at 852 PM PDT Mon Sep 16 2024 Showers have tapered off this evening. We generally received a few hundredths of an inch in the East Bay and most populated valleys, around 1/10" in the Santa Cruz Mountains, and around 1/4" in the Santa Lucia mountains. Skies have cleared in the North Bay, and that could lead to some fog formation overnight. We would have more confidence if the North Bay got any rain earlier, but 100% humidity, light winds, and clear skies could bring some shallow fog. && .SHORT TERM... (This evening through Tuesday) Issued at 1203 PM PDT Mon Sep 16 2024 Isolated rain showers continue over the region early this afternoon and are expected to diminish in coverage while shifting southward through this evening. Tonight, coastal drizzle will be possible as moist conditions persist and troughing lingers over the region. Temperatures overnight will be in the upper 40`s (in the coldest interior spots) to mid 50`s for much of the region. Tomorrow, temperatures will remain 5-15 degrees below average. Outside coastal drizzle in the morning, tomorrow will be dry with partial clearing of sky conditions through the afternoon. && .LONG TERM... (Tuesday through Sunday) Issued at 154 AM PDT Mon Sep 16 2024 The next system set to come through Wednesday looks to be very similar, but with slightly less of a wind threat. Still looking at breezy winds in the same areas early Wednesday morning, but more on the order of 25-35 mph as opposed to the 45-50 mph we saw over the last 24 hours. A bit of a better chance for thunder with the Wednesday system as well. Still a low chance (~15%), but with a bit more cooling aloft and more lower level moisture, there is more widespread instability late Tuesday night going into Wednesday morning. Rain amounts still look to be on the lower side, if any, similar to today. Beyond Wednesday there really isn`t much noteworthy to write about. A casual warm up is on tap going into the weekend that will bring our temperatures to around seasonal normals. By the end of the weekend, there is some discrepancy in ensemble guidance regarding the strength of a possible ridge pattern. More on that in the coming days. && .AVIATION... (00Z TAFS) Issued at 532 PM PDT Mon Sep 16 2024 It`s VFR-MVFR at the terminals with the greatest areal low cloud coverage over the southern half of the forecast area this afternoon. Where it is a mainly clear to clear start this evening and tonight, radiative cooling will likely result in patches of valley fog /VLIFR-IFR/ for late tonight and Tuesday morning. Otherwise where cloud cover persists with VFR-MVFR, these conditions are likely to prevail tonight. The forecast gets more challenging Tuesday morning because the 18z NAM is forecasting a lower level cool front arriving along the coast from the SF Peninsula to Monterey County coastline. The front and cooler air intrusion are bounded below by the sea surface and land and above by a thermal ridge (capping stability). The HREF output shows a higher probability of IFR becoming focused in this area along the coast Tuesday. Caveat, it may take a while for ceilings and surface visibilities to improve Tuesday, lower level cooling resulting in patchy light coastal drizzle. Vicinity of SFO...VFR-MVFR, however the HREF is showing an increasing probability of IFR 08z-18z Tuesday and the RAP model shows IFR 09z through 16z Tuesday. Part of this may be due to nocturnal radiative cooling including the arrival of the above described lower level cool front Tuesday morning; may need to amend the 00z TAF for IFR. Onshore wind near 15 knots decreasing tonight to 5 to 10 knots then increasing to 15 to 20 knots Tuesday afternoon and early evening. SFO Bridge Approach...Similar to SFO. Monterey Bay Terminals...VFR-MVFR, similar to vicinity SFO the HREF is showing an increasing probability of IFR beginning (earlier) 06z Tuesday (this evening) through approx 15z Tuesday when conditions may begin to improve over the southern Monterey Bay Area due to increasing lower level cool air advection. Caveat, downward low cloud ceiling development due to cool air advection, gentle upsloping along the terrain and lowering visibilities in either mist or drizzle may still take place during the mid morning hours. It may take until late morning to early afternoon for conditions to improve to MVFR-VFR. HREF indicates post 00z TAF early return of low clouds along the Monterey Bay shoreline and the Salinas Valley. && .MARINE... (Tonight through next Sunday) Issued at 852 PM PDT Mon Sep 16 2024 Northwest winds continue to gradually decrease overnight in the wake of yesterday`s cold front. Small craft warnings remain in effect in the NW waters mainly for wind gusts more than 45 NM from the coast. Rough seas are gradually abating in the NW waters and will continue through the next 24 hours. There is a bit of a break on Tuesday before a second cold front brings the return of strong NW winds and rough seas to the exposed coastal waters. && .MTR WATCHES/WARNINGS/ADVISORIES... CA...None. PZ...Small Craft Advisory until 9 AM PDT Tuesday for Pt Arena to Pt Reyes 10-60 NM. Small Craft Advisory until 9 PM PDT this evening for Pigeon Pt to Pt Pinos 10-60 NM. && $$ SHORT TERM...RGass LONG TERM....Behringer AVIATION...Canepa MARINE...Flynn 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