UPDATE
Issued at 823 PM PDT Sat Sep 7 2024

Temperatures across our area did cool slightly from yesterdays highs across our area, mostly around 2 to 5 degrees, with that trend continuing through the middle of next week. Many spots that are in the 90s to low 100s today will be 20 to 25 degrees cooler by Wednesday afternoon.

Low marine stratus is a little slower working south from the Bay Area to Monterey Bay this evening but will eventually impact most of our coastal locations with low ceilings, patchy to dense fog at times and light drizzle through the remainder of the overnight hours into Sunday morning.

SHORT TERM
Issued at 112 PM PDT Sat Sep 7 2024 (This evening through Sunday)

A ripple through the longwave pattern over the West Coast this afternoon thanks to an upper low sweep eastward through N CA/S OR.
While this feature is bringing showers and thunderstorms up north it's resulted in sunny, but cooler conditions across the Bay Area and Central Coast. Afternoon visible satellite shows marine layer stratus hugging the coast, but inland sunshine. Latest 24 hour trends shows many locations running similar to or colder than 24 hours ago. Despite the "cooldown" there are still many locations well above 90 degrees. One notable place is the Big Sur RAWS sitting at 98 degrees. Pretty impressive given the proximity to the coast. Definitely has a shot at eclipsing 100 degrees. Other far interior locations away from the coast range from 95-105 degrees. These locations have a moderate HeatRisk, but coverage isn't widespread enough to warrant another Heat Advisory.

Fire Weather concerns continues this afternoon as well. Area of greatest concern are the interior N Bay Mts. Last night featured no humidity recovery with places lingering in the single digits for RH. The same is true this afternoon with places holding steady below 10%. Winds are breezy with gusts up to 15 mph. Bone dry conditions with some wind will result in elevated fire weather conditions.

Tonight and Sunday: Marine layer along the coast will gradually move in. Similar to this morning, fog will once again be possible.
Only subtle changes in the overall sensible weather on Sunday.
Coastal clouds with inland sunshine - repeat from Saturday.
Pockets of 100+ deg possible over interior Monterey/San Benito.

LONG TERM
Issued at 130 PM PDT Sat Sep 7 2024 (Sunday night through next Friday)

Additional, but weaker, ripples roll through the upper level pattern through early next week. The lower 500mb heights and cooling airmass will allow for additional cooling both day and night. Onshore flow will persist, which will lead to a deepening of the marine layer and increasing night/morning low clouds. Have added coastal drizzle to the forecast (late night/early morning)
through this timeframe as well.

A more pronounced upper level trough develops by Tuesday night/Wednesday over the PacNW. Given all of the wildfire over OR a lot of hope was riding on this system being wetter, but sadly recent trends are drier. As for our area, some drizzle is possible, but looking more inside slider-ish for Wednesday/Thursday. Temperatures remain cooler on Wednesday/Thursday, but more importantly winds begin to shift with an offshore component. At this point it doens't appear to be a big offshore setup, but something we'll be watching nonetheless.

AVIATION
(00Z TAFS)
Issued at 541 PM PDT Sat Sep 7 2024

The marine layer depth varies from several hundred feet to as much as 1300 feet vicinity Bodega Bay (profiler data) along the coastal North Bay to at least northern San Mateo County sloping down to near sea level from southern San Mateo County to Monterey County. Visible imagery shows the stratus/no stratus line is nearly stationary from Pacifica extending out southwest across the coastal waters, the advected stratus clouds on northwest winds are eroding upon arrival into the warmer thermal ridging aloft to the south.

Model forecasts including HRRR are currently over-forecasting areal stratus coverage and ceilings to the south of the aforementioned stratus/no stratus line. At least two things would allow the stratus and fog to develop to the south tonight (1. radiative cooling which will happen after sunset and 2. a weakening of the thermal ridging aloft), a weakening of the ridge aloft does not look favorable based on same model forecasts thus stratus and fog development along the coast to the south will be slower to develop than models currently forecast. This is a good example of how complex meteorology is and this is only an example of a smaller scale here in our forecast area. When we get to the mix of warm and cool season transition, the complexity and interactions can increase much more.

For the 00z TAF cycle, decided to slow stratus /IFR/ forecast arrival/development times for SFO, OAK, SJC, MRY and SNS in inherited TAFs. If the thermal ridge aloft and lower level temperature inversion described above holds together fairly well tonight, it may take even longer for stratus and fog to develop otherwise under radiative cooling and compressed marine layer expect increasing probability of coastal and nearby coastal valley fog /VLIFR-IFR/ tonight and Sunday morning. Highest probability is for VFR to extend well into the evening for all TAF sites, with IFR currently limited to the immediate coastline from approx Half Moon Bay-Pacifica and the coast northward. This is also a great example as to how helpful satellite imagery is, including surface observations, spotter reports, etc.

Vicinity of SFO...Moderate to high confidence VFR prevails this evening, 300 degree surface wind may help continue VFR at SFO.
Upstream stratus is nearby per satellite imagery, but is hung up along the coast upon meeting the lower level thermal ridge hovering over the SF Peninsula and areas to the south and east. Best guess is for BKN stratus ceiling developing by 08z tonight and continuing to 17z; the RAP model may be overdoing the stratus /IFR/ duration tonight too beginning 04z, but don't want to completely rule out stratus at SFO tonight either, thus factoring in "later is better" into the forecast. Otherwise west to northwest wind near 20 knots eases to near 10 knots tonight and Sunday morning, increasing to 15 to 25 knots Sunday afternoon and early evening.

SFO Bridge Approach...Similar to SFO except the approach may have longer duration VFR into early Sunday morning before any chance of IFR ceiling development. Not seeing anything unusual about potential stratus feed Sunday morning thus mix out time should be quick Sunday morning.

Monterey Bay Terminals...Low to moderate confidence VFR prevails through the evening, chances of VFR holding look good so far based on satellite imagery. Post sunset radiative cooling however is when things may change quicker, thus the low to moderate confidence in the evening forecast. Data at buoy 92 Monterey Bay is very helpful, currently showing the air temp is a little warmer then the sea surface temp (vs if the air temp was cooler at the air/sea interface, upward heat flux) which matches the clear sky currently observed over the Monterey Bay. In short it will take a little time for the stratus to advect here into the thermal ridging aloft. By late evening and overnight expect at least patchy stratus and fog /VLIFR-IFR/ nearby and potentially reaching the Monterey Bay terminals. Any stratus and fog Sunday morning will mix out to VFR by late morning. Mainly onshore winds 5 to 12 knots except light and variable tonight and Sunday morning.

MARINE
(Tonight through next Thursday)
Issued at 446 PM PDT Sat Sep 7 2024

Moderate to breezy north and northwest winds continue through the weekend with occasional gusty winds over the southern waters.
Northwest winds strengthen and wave heights increase over the California coastal waters ahead of an incoming low pressure system by the middle of next week with unsettled conditions persisting through the end of the week.

MTR WATCHES/WARNINGS/ADVISORIES
CA...None.
PZ...None.