UPDATE
Issued at 935 PM PDT Thu Sep 12 2024

The northerly pressure gradient ACV-SFO has slowly increased today to currently 5 mb, while the SFO-SAC pressure gradient (onshore wind)
has leveled off to a little less than 2 mb. Evening satellite imagery shows a clear sky with exception of a couple small patches of stratus and/or fog forming on the coastline. Drier conditions have arrived at the surface per lower dew point temperatures and the evening Oakland upper air sounding showed a precipitable water at 0.60", a little lower than this morning. Thus, there's good outgoing radiative cooling potential tonight and early Friday morning for coastal stratus and/or fog to redevelop. The lower level temperature inversion is otherwise present, a little more compressed and stronger which will slow air temperatures from lowering much in the hills and mountains overnight.

SHORT TERM
(This evening through Friday)
Issued at 1238 PM PDT Thu Sep 12 2024

Today will bring near seasonable temperatures and gusty winds in the afternoon and evening, particularly through gaps and passes.
Tomorrow will be slightly warmer and the warmest day of the week as the low-amplitude ridge reaches its peak.

While fire concerns are overall on a downward trend due to cooler conditions and increased relative humidities, fuels are still conducive for large fires. Northerly/offshore winds will prevail through the overnight and morning hours in the higher elevations resulting in further warming and drying of fuels. In fact, a few stations briefly observed Red Flag criteria this morning; however the temporal requirement of eight hours was not met. In the past two days alone there have been two new fire starts in the East Bay. An important reminder to remain vigilant when it comes to exercising fire safety as the region is very much in the thick of fire season.

LONG TERM
(Friday night through next Wednesday)
Issued at 1238 PM PDT Thu Sep 12 2024

Saturday will be quite nice. Temperatures will be near seasonal normals inland, and a few degrees cooler than normal at the coast where clouds may linger all day. That puts us in the 60s at the coast and mid 80s inland. Winds will be light in the morning and moderate in the afternoon.

Everything changes on Sunday. Models are in good agreement that an early jet stream dip will reach central California. By Monday morning the 500 mb height may drop to a level never recorded in the month of September. The GFS, ECMWF, and their respective ensemble means all have the 500 mb height dropping between 5,550m and 5,600m. That puts us between the 99.5% and 100% (lowest on record) for all Oakland balloon launches in the month of September (roughly 4,600 samples going back to 1948). While that sounds extreme, it's important to add that this wouldn't be nearly as unusual in Winter. If we compare to all January soundings, the forecasted 500 mb height would only fall between the lowest 74 and 85 percentile. So it's the fact that this is happening in September that's out of the ordinary.

Lower than normal temperatures starting Sunday are virtually guaranteed. There will also be gusty winds both Sunday and Monday, especially as the cold front moves through. There is a 25-50% chance of rain, and that has been increasing over the last couple days. It's worth noting that there is still only a 5% chance of exceeding 1/10th of an inch, so we're not expecting a washout.
Then we get to thunderstorm potential. The ingredients are there through the day Monday with cold air aloft, good daytime heating (longer and more direct sunlight compared to winter), and some elevated bands of moisture. The models are finally starting to show some CAPE, but it's less than 25 J/kg right now. Not too impressive. The NBM gave us around a 10% chance of thunderstorms Monday, and that seems reasonable before the higher resolution guidance starts to reach the event. While the probability is currently low, there is a scenario where the cold front moves through overnight, the clouds clear during the morning and the moisture plume lines up with a post-frontal trough. That gives us instability, moisture and a trigger that could generate a few thunderstorms through the day Monday.

That's not the end of the fun. Continued meridional flow of the jet stream will bring a reinforcing trough and renewed rain chanced mid-week. This system will have more moisture to tap into compared to Monday, but the dynamics don't look as strong.
Everything looks to clear up by next weekend, but the uncertainty really starts to balloon so don't hold me to that.

AVIATION
(00Z TAFS)
Issued at 538 PM PDT Thu Sep 12 2024

24 hour trends show for the most part warmer and drier conditions across the forecast area late this afternoon. VFR prevails per visible satellite imagery and surface observations. The marine layer depth varies from a few hundred feet to near 1000 feet per profilers and surface observations. Plentiful diurnal mixing has mixed out the coastal stratus and fog from earlier today.

Presently we are cycling through lower surface dew point temps (less water vapor) on northwest winds, though coastal dew point temps are still within a few degrees Fahrenheit of sea surface temps. With radiative cooling commencing tonight and either neutral to gradual strengthening of the lower to mid level thermal ridge, stability and a slight compression of the marine layer will result in areas of LIFR-IFR in coastal stratus and fog. A few inland patches of low lying fog may develop under radiative cooling as well. Stratus and fog mixes out by late Friday morning, otherwise VFR for Friday afternoon and early evening.

Vicinity of SFO...Near high confidence VFR prevails through the evening, west wind near 20 knots with a few higher gusts to 25 knots. Low to moderate confidence VFR for the rest of tonight and Friday. West wind decreasing to 5 to 10 knots tonight and Friday morning, shifting to light northeast wind Friday morning then shifting to northwest 20z Friday and increasing to 15 knots.

SFO Bridge Approach... Similar to SFO.

Monterey Bay Terminals...Moderate to high confidence VFR prevails through early to mid evening, satellite imagery shows upstream conditions also VFR. After sunset radiative cooling will result in patches of IFR stratus and fog which may near the terminals by late evening and/or later overnight into Friday morning. For now 00z TAFs advertise VFR, will amend as needed if stratus and/or fog patches develop quicker than models forecast.

MARINE
(Tonight through next Wednesday)
Issued at 912 PM PDT Thu Sep 12 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. These systems will bring a slight chance to chance of showers.

MTR WATCHES/WARNINGS/ADVISORIES
CA...None.
PZ...Small Craft Advisory until 9 AM PDT Friday for Pigeon Pt to Pt Pinos 10-60 NM-Pt Arena to Pt Reyes 0-10 nm-Pt Pinos to Pt Piedras Blancas 0-10 nm.

Small Craft Advisory until 3 AM PDT Friday for Pt Reyes to Pigeon Pt 0-10 nm.

Gale Warning until 3 AM PDT Friday for Pt Arena to Pt Reyes 10- 60 NM.