780
FXUS66 KMTR 080926
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
126 AM PST Mon Dec 8 2025

...New SHORT TERM, LONG TERM...

.KEY MESSAGES...
Updated at 117 AM PST Mon Dec 8 2025

 - Another round of fog this morning in the valleys across the Bay
   Area.

 - Fairly consistent weather pattern continues this week.

 - Next chance of rain around the 15th.

&&

.SHORT TERM...
Issued at 117 AM PST Mon Dec 8 2025
(Today and tonight)

Tule Fog remains in the Central Valley and there is slightly more
low stratus coverage across the Bay Area than this time last
night. There is a good chance for these clouds to expand down to
the surface as the the wind continues to calm through the morning.
The most likely locations to develop fog are the North Bay
Valleys, Interior East Bay Valleys, and the Santa Clara Valley.
There`s more uncertainty along the Bay side of San Mateo County.
Typically this area is less prone to fog than the cooler valleys,
but is the best set-up for it. Light NNE winds over the Bay don`t
suffer from the downslope drying of winds with a westerly
component. For example, historically the visibility at SFO drops
below 3 miles 8% of the time in December between 5AM and 10AM. If
we add a NNE wind direction constraint to these variables, the
probability increases to 17%. On the other hand, NW winds bring
this low visibility only 4% of the time. The otherwise clear
skies and long December nights also help with the radiative
cooling to generate this fog, and cloud top radiation helps to
sustain it. The complicating factor is the relatively warm water
in the Bay (55F) compared to the air temp (45F). This induces some
instability that will work to lift fog into a low cloud deck if
the low level winds don`t calm this morning. In the end, I`ll give
a 30% chance for fog in Eastern San Mateo, 50% for the Santa
Clara Valley, and 80% across the North and East Bay Valleys.
Otherwise the weather will be quite similar to the last couple
days as the fog and low clouds gradually clear through the day and
temperatures under the morning stratus struggle to get into the
60s in the afternoon.

&&

.LONG TERM...
Issued at 117 AM PST Mon Dec 8 2025
(Tuesday through Sunday)

The long term forecast is mostly boring. There is a hint of rain at
the end of the 7-day window, but let`s spend a minute discussing
this dry spell first. If you`ve read any of our AFDs in the last
week, you know it`s caused by a "ridiculously resilient ridge" over
California that is keeping the storm track well to our North. It`s
now been 17 days since San Francisco recorded any rain, and that
streak will very likely extend into the mid 20s. How unusual is
that? Well it turns out a long dry spell in the middle of the
rainy season is actually pretty common. There have been 54
instances of a dry spell lasting 24 days or more between November
and March since 1849. In other words, there is a 31% (54/176)
chance of getting a dry spell at least this long any given Winter.
The longest such streak was 60 days from November 17, 1876 to
January 15, 1877. We`re not expected to get anywhere near that
record as it looks like a pattern change is finally on the
horizon. More and more ensemble members are agreeing that the
ridge will finally break down with more a more active zonal
pattern bringing the jet stream and chances for rain back to the
Bay Area. 40/50 ECMWF ensemble members bring some rain to the Bay
Area next week, with an outside chance for heavier rain the
following weekend. We`ll wait and see for now, but our official
forecast finally has a decent probability of precipitation
starting on the 15th.

&&

.AVIATION...
(06Z TAFS)
Issued at 948 PM PST Sun Dec 7 2025

Low clouds and fog are building in the North Bay and interior East
Bay, but have cleared over STS and LVK while APC remains IFR. Expect
cloud cover to lower further into the night while the fog to becomes
thicker. The OAK and SJC see CIGs arrive overnight with some
reductions in visibilities. Winds stay mostly light to moderate
through the morning, with wind directions determined by more
localized affects. Cloud cover and fog will be slow to erode into
Monday, but conditions look to improve into the afternoon. However,
the North and East Bay will continue to struggle with low clouds and
fog, causing some doubt for clearing at STS and LVK, while APC
doesn`t clear. As cloud cover erodes, pockets of haze will affect
much of the region. CIGs and fog reform for STS and APC into the
evening.

Vicinity of SFO...VFR through the TAF period. Winds become lighter
into the night with scattered low clouds building into the area,
along with some slight reduction in visibilities. Cloud cover thins
into the afternoon and winds become slightly more moderate and
northwesterly.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR through the TAF period. Winds stay
light for MRY through the TAF period, while SNS sees moderate
northwest winds become light into the night. North to northwest
winds return Monday afternoon with the potential for haze.

&&

.MARINE...
(Tonight through next Saturday)
Issued at 948 PM PST Sun Dec 7 2025

Moderate to fresh north to northwest winds continue for the
southern waters. Moderate seas with wave heights 6 to 8 feet
across the outer waters will subside to around 4 to 6 feet for
Monday. Winds and swell increase Monday night, with strong  gusts
possible across the waters. A new, long period northwesterly
swell is anticipated by Wednesday and looks to exit late Friday.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory until 9 AM PST Tuesday for Pt Pinos to Pt
     Piedras Blancas 0-10 nm.

     Small Craft Advisory from 3 PM this afternoon to 3 AM PST
     Tuesday for Pt Arena to Pt Reyes 10-60 NM.

     Small Craft Advisory from 3 PM this afternoon to 9 AM PST
     Tuesday for Pigeon Pt to Pt Pinos 10-60 NM.

&&

$$

SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...Murdock
MARINE...Murdock

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