FXUS66 KMTR 081938
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
1138 AM PST Mon Dec 8 2025
...New UPDATE...
.KEY MESSAGES...
Updated at 1136 AM PST Mon Dec 8 2025
- Another round of fog this morning in the valleys across the Bay
Area.
- A fairly consistent weather pattern continues this week.
- Next chance of rain around the 15th.
&&
.UPDATE...
Issued at 1136 AM PST Mon Dec 8 2025
The morning updating included the lowering of high temperatures
in the North and East Bay due to the lingering stratus. Otherwise,
the forecast remains on track this morning.
&&
.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...
(18Z TAFS)
Issued at 907 AM PST Mon Dec 8 2025
similar to previous TAF forecast - persistence is the name of the
game. Tule fog and marine layer pattern will keep low CATs for N,
E, S, and SF Bay terminals today through tomorrow. There is a low
chance (20-40%) for some clearing this afternoon for N and E bay,
but high confidence for SF and S Bay. Central Coast remains VFR.
Vicinity of SFO...SFO had a briefing clearing earlier, but patch
over approach drifted back over terminal. Will keep CIGs through
20Z then thinning thereafter. Cigs return late tonight.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...VFR with diurnal flow.
&&
.MARINE...
(Today through Saturday)
Issued at 907 AM PST Mon Dec 8 2025
Persistent high pressure off the California coast will maintain
moderate north to northwest winds continue for the coastal waters
and locally to fresh winds south of Point Sur. Moderate seas with
wave heights 6 to 8 feet across the outer waters will subside to
around 4 to 6 feet today. Winds increase to a strong breeze
Tuesday into mid week across the outer waters. A new, long period
northwesterly swell is expected by Wednesday, lasting through the
latter half of the week.
&&
.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
Wednesday for Pigeon Pt to Pt Pinos 10-60 NM-Pt Arena to Pt
Reyes 10-60 NM.
&&
$$
SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...MM
MARINE...MM
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Prepared by Boulder-Creek.com Weather at: Mon Dec 8 12:30:02 PST 2025
From the National Weather Service
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