FXUS66 KMTR 140052 AFDMTR Area Forecast Discussion National Weather Service San Francisco CA 547 PM PDT Fri Sep 13 2024 ...New AVIATION, MARINE... .SYNOPSIS... Issued at 345 AM PDT Fri Sep 13 2024 Cooler than normal temperatures persist through the majority of the forecast period. Rain chances and gusty winds arrive late Sunday to early Monday. Another chance for wet weather exists in the midweek. && .SHORT TERM... (This evening through Saturday) Issued at 1243 PM PDT Fri Sep 13 2024 Seasonal temperatures continue through tomorrow, with highs ranging from the mid 80s to the low 90s in the inland valleys, the 70s and the low 80s in the Bayshore, and the 60s along the Pacific coast. Tonight's lows range in the 50s across the lower elevations, up to the mid 60s in the highest peaks. Low stratus clouds continue to linger across the immediate coast, with a somewhat compressed marine layer around 1000-1500 feet deep limiting inland intrusion tonight. Patchy fog development is possible early Saturday morning in the Sonoma County valleys and the Salinas Valley as radiative cooling impacts the region overnight. && .LONG TERM... (Saturday through Thursday) Issued at 1243 PM PDT Fri Sep 13 2024 No major updates in the long range forecast. The previous discussion is copied below for reference: As troughing continues, a potent low pressure system (for September, that is) is expected to enter the region Sunday with temperatures continuing to drop. Highs for Sunday and Monday for interior regions will likely feel significantly cooler than those on Saturday, as highs will only be in the 70s. Closer to the bays, highs will be in the 60s. Upper 50s are expected at the coastline. These temperatures will generally be about 10 degrees cooler than normal. However, cooler temperatures is not all. The bigger story is arguably the chance for rain! Though, don't get too excited yet. This is not going to be a super-soaker event. Most locations will only see a few hundredths of an inch of rain, with the wettest locations seeing at most about a tenth of an inch. So while yes, rain will likely be falling, it is not going to be much. Perhaps just enough to make obvious that layer of dust on your car, but not wash it off, and force you to have to take it to the car wash. So why not that much rainfall? Well, part of it has to do with the fact that this system just isn't carrying all that much moisture, especially compared to the mean amount for this particular date in September per KOAK sounding climatology. The mean precipitable water (PWAT) amount for this date is 0.72", and multiple model soundings seem to suggest a max PWAT Sunday of around 0.8". If you're curious, the median PWAT value for this date is 0.70", and the 75th percentile is at 0.84". So really, we're just not looking at a super wet system. Is it carrying a bit more moisture than normal? Sure. But it just isn't up to snuff for a widespread, complete washout type of event. But, nonetheless, it'll be a nice change of pace and a break from just blue skies and sunshine! Current timing places the first chances of rain arriving in the North Bay very late Sunday night, perhaps even the earliest hours of Monday morning, with the Monterey Bay region seeing their first chances more towards sunrise on Monday. There is still uncertainty as to the time of arrival for the rain, but this is a rough ballpark estimate. Aside from the rain, the last piece of this system we need to talk about is winds. Northwesterly winds will strengthen Sunday afternoon and evening ahead of the cold front, with gusts in lower elevations reaching to around 20-25mph, and closer to 35-45mph in higher elevations. Early Monday morning, after the frontal passage, winds begin to decrease and settle. No wind products have been issued at this time, though this will be something we keep our eyes on closely over the next few forecast issuances. Beyond the Sunday/Monday system, another chance of rain exists in the midweek. For now, details on this are still a bit fuzzy. That's all for now folks. Stay tuned to the forecast for updates, and enjoy your Friday! && .AVIATION... (00Z TAFS) Issued at 547 PM PDT Fri Sep 13 2024 It's VFR except along the immediate coastline satellite imagery and surface observations show patchy stratus and fog from the coastal North Bay to just south of Point Reyes and almost continuous stratus and fog /LIFR-IFR/ from the Golden Gate to the Big Sur coast. Visible satellite imagery also shows a narrow intrusion of stratus and fog rapidly moving through the Golden Gate to Berkeley, a signature of increasingly gusty west winds across the SF Bay. The 5 pm SFO-SAC pressure gradient (onshore wind) is only 1.6 mb, however the volume of air initiated to move onshore by the gradient is trapped and forced to move more quickly onshore beneath a shallow (compressed) marine layer temperature inversion. Currently the marine layer depth is compressed to a few hundred feet along the coast except along the Monterey Bay the Fort Ord profiler shows the marine layer depth recently increased to 1000 feet. The 00z TAFs ceiling forecasts are a blend of HRRR/WRF/HREF output. Satellite imagery this afternoon shows clearing upstream over our northern coastal waters and farther north and northwest which indicates drier air advection on northwest winds. However, present coastal stratus and fog coverage along our coastline will likely anchor in through and after sunset, nocturnal cooling thereafter allowing for additional stratus and fog /VLIFR-IFR/ development to sunrise Saturday morning. Conditions improving to MVFR-VFR along the immediate coastline Saturday afternoon; based on recent model output BKN-OVC stratus ceilings are likely to move in early Saturday evening. Expect increasingly gusty onshore winds Saturday afternoon and evening. Vicinity of SFO...Moderate to high confidence VFR continues through early to mid evening, slightly lower confidence VFR holds through tonight and Saturday morning. Based on model output stratus ceilings forming over the East Bay may move across the SF Bay and reach nearby SFO temporarily Saturday morning (SCT tempo for now 13z-16z). West wind 15 to 20 knots with a few highers gusts near 25 knots until mid evening, then wind decreasing and becoming light late tonight and Saturday morning. West wind strengthening to 20 to 27 knots Saturday afternoon and early evening. SFO Bridge Approach...Similar to SFO. Monterey Bay Terminals...Stratus and fog /LIFR-IFR/ are on our doorstep per satellite imagery, the development of an eddy circulation over the northern Monterey Bay may help steer the stratus and fog temporarily away from MRY while northwest wind in the area of SNS may help advect the stratus into the Salinas Valley. Generally anticipating IFR conditions to spread inland by mid evening with a farther inland intrusion of stratus and fog /VLIFR-IFR/ tonight and Saturday morning. Conditions improving to MVFR on the immediate coast along the Monterey Bay Saturday late morning and afternoon, with an early evening intrusion (post 00z MRY and SNS TAFs) of stratus and fog likely Saturday evening. Onshore winds 5 to 15 knots becoming light overnight into Saturday morning, then onshore 5 to 15 knots Saturday afternoon. && .MARINE... (Tonight through next Wednesday) Issued at 451 PM PDT Fri Sep 13 2024 Gusty northwest winds will continue into the weekend, including breezy onshore winds across the bays each afternoon and evening. An unseasonably strong upper level low pressure system arrives from the northwest Sunday, with possibly another on its heels arriving by the middle of next week. Increasing winds and waves will build in tandem with the incoming upper level low. && .MTR WATCHES/WARNINGS/ADVISORIES... CA...None. PZ...Small Craft Advisory from 3 PM to 9 PM PDT Saturday for Pt Arena to Pt Reyes 0-10 nm. Small Craft Advisory until 9 PM PDT Saturday for Pt Arena to Pt Reyes 10-60 NM. Small Craft Advisory from 9 AM to 9 PM PDT Saturday for Pigeon Pt to Pt Pinos 10-60 NM. && $$ SHORT TERM...DialH LONG TERM....AC 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