FXUS66 KMTR 131151
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
351 AM PST Sat Dec 13 2025
...New AVIATION, MARINE...
.KEY MESSAGES...
Updated at 111 AM PST Sat Dec 13 2025
- Dense fog impacts are expected through the morning in the
Interior North Bay and East Bay Valleys as well as coastal
locations
- Wet conditions return next week
&&
.SHORT TERM...
Issued at 111 AM PST Sat Dec 13 2025
(Today and tonight)
Today's main impact will be a tale of two fogs: advection and
radiation. Let's start with the persistence side of the forecast, or
what has been happening. Portions of the region will continue to be
affected by tule fog (a type of radiation fog) as it seeps out of
the Central Valley through neighboring gaps and passes which will
continue to facilitate below normal temperatures. There is also a
southerly surge underway which will bring more traditional advection
fog to coastal locations. This will not only be a change to the
warm, sunny afternoons that have been observed the last week, but
also to the cold overnight conditions that have been made possible
by clear skies which allow for radiational cooling. We will continue
to monitor the evolution of both of these through the morning to see
if the Dense Fog Advisory that is in effect through 11AM for North
Bay and East Bay Interior Valleys needs to be extended in area,
time, or both. Consider delaying unnecessary travel until
visibilities improve. If travel is necessary, make sure to use your
low-beam headlights, increase following distance between vehicles,
and allow extra time and patience. Elsewhere, sensible weather will
yield temperatures up to 15 degrees above normal as the region
remains under the influence of upper-level longwave ridging.
Offshore flow will also continue with high pressure across the Great
Basin and a coastal trough off the West Coast.
&&
.LONG TERM...
Issued at 111 AM PST Sat Dec 13 2025
(Sunday through Friday)
The aforementioned upper-level longwave ridge will exit to the east
Sunday, giving way to zonal flow for most if not all of the long
term forecast. Passing disturbances to the north and an increase in
precipitable water and integrated vapor transport values will renew
rain chances as early as Monday with no clear cut dry day beyond
that. Rainfall in the beginning of the week is expected to be light
and beneficial for those who are lucky enough to get any. This will
also act to disrupt the pattern that we have been in that has
allowed for relentless tule fog. Towards the tail end of the long
term forecast period, a relatively more significant system is
forecast as a surface low pressure system and its attendant cold
front encroach the California Coast. It is too soon to delve into
the specifics, but expect a change in the pattern and a return to
wet conditions next week. This weekend would be a great time to
clean out gutters and trim trees away from your home and power lines.
But wait, there's more! Beyond the long term forecast of seven days,
it continues to be advertised that there's a moderate risk of heavy
precipitation and high winds Saturday, December 20th through Friday,
December 26th. This will not only be important for daily life, but
also holiday travel.
&&
.AVIATION...
(12Z TAFS)
Issued at 349 AM PST Sat Dec 13 2025
Currently a mixed bag of LIFR-VFR at the terminals with tule fog in
the Central Valley spilling into Interior North Bay and Interior
East Bay terminals and a southerly surge bringing fog/stratus to
coastal terminals. Satellite trends continue to show fog/stratus
expanding, so there is still a potential for terminals that are VFR.
Coastal terminals will have stratus return early tonight with later
times if at all for interior terminals. Offshore winds overnight and
onshore winds during the afternoon are expected to be light.
Vicinity of SFO...Currently MVFR with northeasterly flow. An
isolated patch of stratus is developing on the western side of the
terminal. The TAF may need to be amended if this continues to grow,
with the ceiling likely on the cusp of MVFR/IFR. Aside from that,
moderate confidence on VFR prevailing through the TAF period with
sub-VFR conditions becoming possible towards the end of the TAF
period. Northeasterly flow will back to become onshore with the
afternoon sea breeze. Winds will largely become variable after the
sea breeze diminishes.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...Currently VFR with drainage flow at both
terminals. Low to moderate confidence on the terminals being able to
hold out and remain VFR through the afternoon. The things to watch
will be the small patch of stratus/fog that is about 10 miles
north of the terminals as well as the southerly surge over the
Pacific Ocean and if it is able to curl around Monterey Bay.
Drainage winds will back to become onshore with the afternoon sea
breeze. An early return of stratus (and potentially fog) is
expected tonight as the bank is already there and northwesterly
flow will feed it right in.
&&
.MARINE...
(Today through Thursday)
Issued at 349 AM PST Sat Dec 13 2025
A southerly surge will bring gentle to moderate southerly breezes
and the potential for dense fog today. Winds will veer to become
northwesterly tomorrow night. Seas will remain moderate through
Monday, building to become rough in the outer waters and northern
inner waters by Tuesday. Several disturbances passing to the north
next week will bring renewed rain chances and wind shifts.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Dense Fog Advisory until 11 AM PST this morning for CAZ506-510.
PZ...None.
&&
$$
SHORT TERM...Sarment
LONG TERM....Sarment
AVIATION...Sarment
MARINE...Sarment
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Prepared by Boulder-Creek.com Weather at: Sat Dec 13 06:30:03 PST 2025
From the National Weather Service
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