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Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA
Issued by NWS San Francisco Bay Area, CA
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112 FXUS66 KMTR 180039 AFDMTR Area Forecast Discussion National Weather Service San Francisco CA 539 PM PDT Sat Aug 17 2024 ...New AVIATION... .SYNOPSIS... Issued at 1102 AM PDT Fri Aug 16 2024 Seasonal to slightly cooler than normal temperatures will continue through the weekend into next week. Increased coverage of early morning marine layer stratus, fog and drizzle is likely this weekend. Less than 10% chance for thunder on Saturday. && .SHORT TERM... (This evening through Sunday) Issued at 259 PM PDT Sat Aug 17 2024 Upper-level troughing continues to be the name of the game. Said upper-level trough is responsible for a non-zero (<10%) chance of thunderstorms today across the North Bay in addition to gusty conditions through gaps and passes. While our region is not experiencing critical fire weather conditions, it is imperative to remain vigilant when it comes to exercising fire weather safety as grasses remain dry and winds will easily spread fire. Temperatures will be near normal with tomorrow being slightly warmer than today. Stratus is expected to fill valleys and low-lying areas tonight, receding back to the coastline by tomorrow afternoon. && .LONG TERM... (Sunday night through next Friday) Issued at 259 PM PDT Sat Aug 17 2024 As the upper-level low fills, temperatures will continue to warm up through Tuesday. A brief, gradual cooldown will take place Wednesday through Friday as troughing is expected to take shape. Uncertainty with the forecast primarily lies in the amplitude of the trough - will it be a cutoff low or will it be an open wave? If it is a cutoff low, it will linger longer, the North Bay will have better chances for measurable precipitation, and onshore winds will noticeably increase. If it is an open wave, it will move through quicker with the better chances of precipitation confined to the north. Either way, any kind of troughing offshore will promote moist, onshore flow, which is a good thing. && .AVIATION... (00Z TAFS) Issued at 539 PM PDT Sat Aug 17 2024 It`s a complex pattern resulting in largely variable weather conditions this afternoon, it`s VFR inland while it`s a mix of MVFR and localized IFR (including fog, mist) along the immediate coast. Higher dewpoint air i.e. higher water vapor has arrived across much of the forecast area, surface dewpoint temperatures are in the lower to mid 60s. Additionally a layer of cool air advection aloft with the offshore trough of low pressure has moved in mostly between 925 mb (~2500 feet) and 850 mb (~5000 feet) resulting in low clouds with a mix of stratus/stratocumulus clouds and periodic light drizzle/light rain along the coast. SFO Airport reported a trace just after 4 pm for example. For the 00z TAFs, dry weather is forecast specifically at the terminals Cool air advection aloft has weakened and deepened the marine layer temperature inversion per recent profiler data at Bodega Bay, Fort Ord and Point Sur. A loss of northwest winds and upwelling, including a still steep August sun angle and heat input into sea surface temperatures has also reduced the lower level temperature inversion to sea surface temp difference. The amount of stratus returning tonight including the development of IFR will rest largely on clear sky aloft for radiative cooling and if there`s any additional cool air advection. The 00z TAFs carry a combination of MVFR-IFR ceilings for tonight and Sunday morning, diurnal surface warming should help lift ceilings most locations to MVFR-VFR Sunday afternoon. Vicinity of SFO...North-south band of low clouds /MVFR/ with higher dewpoint air is adhering to the SF Peninsula for the evening. Post sunset cooling will be a combination of residual cool air advection and radiative cooling supporting a continuation of ceiling at the terminal for the overnight hours and Sunday morning. The RAP model is forecasting IFR during the evening, kept in inherited 18z TAF IFR ceiling forecast for 06z this evening. KHAF has reported a low ceiling & visibility in fog, thus LIFR-IFR is there on the other side of SF Peninsula. IFR 06z-16z Sunday then improving to MVFR-VFR thereafter for the remainder of the morning and afternoon. West wind near 12 knots decreasing to 5 to 10 knots tonight and Sunday morning, increasing to near 15 knots Sunday afternoon. SFO Bridge Approach...Similar to SFO. Monterey Bay Terminals...Same as the Bay Area, largely variable weather conditions here this afternoon. Low clouds moving in over KWVI with IFR ceiling reported, KSNS and KMRY holding onto VFR for the time being. Low clouds will initially run into late afternoon diurnal surface warming and vertical mixing with surface temperatures in the lower 70s over the southern Monterey Bay. With the weakened marine layer temperature inversion on recent Fort Ord profiler data, 00z TAFs currently advertise tempo MVFR ceilings, however may need to amend for IFR if the low ceilings over the northern Monterey Bay hold together as the ceilings move south and east. Ceilings mixing out by late Sunday morning with generally MVFR-VFR conditions during the afternoon. Onshore winds 5 to 15 knots during the period. && .MARINE... (Tonight through next Thursday) Issued at 259 PM PDT Sat Aug 17 2024 Light to gentle breezes are anticipated across most of the marine zones tonight. The exception will be across the northern San Francisco and San Pablo Bays where intermittent moderate breezes are forecast. Primary swell will continue to roll in from the west and northwest with wave heights ranging between 2 and 6 feet. Fresh to moderate breezes are expected Sunday afternoon into next week, especially near and south of Point Sur. In response, seas will increase to 6 to 8 feet across southern zones and low-end marine headlines may be needed Sunday afternoon and into early next week. && .MTR WATCHES/WARNINGS/ADVISORIES... CA...None. PZ...None. && $$ SHORT TERM...Sarment LONG TERM....Sarment AVIATION...Canepa MARINE...Bain 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