FXUS66 KMTR 130915
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
115 AM PST Sat Dec 13 2025
...New SHORT TERM, LONG TERM...
.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...
(06Z TAFS)
Issued at 949 PM PST Fri Dec 12 2025
Fog returns across the Bay Area and stratus returns to the Central
Coast. For the Bay Area, followed a persistence forecast with fog
arrival and dissipation timing for STS, APC, LVK, and SJC with fog
not reaching SFO or OAK. Current obs show dense fog developing
across the the Bay Area so have made the TAFs more pessimistic with
more widespread 1/4SM visibilities. A southerly surge of stratus is
bringing stratus south to north across the southern coastal waters
with stratus to reach HAF, MRY, and SNS overnight. There is some
potential for fog to develop, at least temporarily, at MRY and SNS
overnight. Another round of stratus and fog is likely again tomorrow
night.
Vicinity of SFO...VFR. Currently not anticipating stratus or fog to
reach SFO tonight. Confidence is increasing that stratus will
redevelop along the coastline by tomorrow night and will likely
reach SFO early Sunday morning. Winds remain light and generally
offshore through the TAF period but may become more variable at
times.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...IFR-LIFR overnight. Kept the TAFs slightly
more pessimistic with dense fog expected at least temporarily early
tomorrow morning. The NBM is most supportive of dense fog developing
and seems to be doing a good job with the arrival of the southerly
surge. Winds generally stay light and offshore through the TAF
period but may shift more onshore during the afternoon/evening hours.
&&
.MARINE...
(Tonight through next Thursday)
Issued at 844 PM PST Fri Dec 12 2025
Dense fog is possible across the southern coastal waters tonight
as a southerly surge of marine stratus moves northward. Gentle
northeasterly winds will gradually shift southerly on Saturday
morning before moderate northerly winds return again on Monday.
Seas continue to subside with slight waves continuing through the
weekend. Unsettled weather returns by the middle of next week
with seas to build and light rain possible across the coastal
waters.
&&
.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...Kennedy
MARINE...Kennedy
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Prepared by Boulder-Creek.com Weather at: Sat Dec 13 02:30:02 PST 2025
From the National Weather Service
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