FXUS66 KMTR 132017
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
1217 PM PST Sat Dec 13 2025

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

.KEY MESSAGES...
Updated at 1216 PM PST Sat Dec 13 2025

 - Stratus persists over the North and East Bay valleys and the
   coastal regions

 - Wetter conditions return for the upcoming work week

&&

.SHORT TERM...
Issued at 1216 PM PST Sat Dec 13 2025
(This evening through Sunday)

The upper level ridge that brought us the days of stable weather is 
beginning to move off to the east, and an shortwave trough has set 
up across the State. Satellite imagery reveals that stratus
persists across the valleys over the North Bay and interior East 
Bay valleys in addition to the southeastern part of San Mateo 
County, outgrowths of the Tule Fog across the Central Valley, and 
the immediate coastal regions near Point Reyes, San Mateo County, 
and south of Point Sur, as a southerly surge takes hold just off 
the coast. There is low to very low confidence in any clearing 
across the coastal regions, the Sonoma County valleys, the 101 
corridor of Marin County, and the northern and eastern parts of 
Contra Costa County. Everywhere else, some clearing is possible 
through the rest of the afternoon. 

High temperatures will be broadly similar to those yesterday. The 
coldest regions will be the eastern part of Marin County and the 
northern and eastern regions of Contra Costa County, where highs 
will remain in the middle to upper 40s. At the other extreme, the 
interior Central Coast valleys, shielded from both the coastal 
stratus and the Tule Fog, reach the middle to upper 70s. In general, 
the Bay Area valleys see highs in the middle 50s to lower 60s, 
although lingering stratus in the Sonoma County valleys makes the 
high temperatures a little uncertain, with the potential for the 
region to remain in the 40s if the stratus persists all day.

On Sunday, the shortwave trough crawls across the state and ridging 
redevelops on its backside, with conditions expected to be broadly 
similar to today's. 

&&

.LONG TERM...
Issued at 1216 PM PST Sat Dec 13 2025
(Sunday night through next Friday)

As we head into the early part of upcoming work week, the ridge will 
continue to flatten, a pattern change that brings more zonal flow to 
the region by Tuesday. This new pattern should help scour out the 
Tule Fog and, when combined with a plume of moisture with PWAT 
values up to 1.25 inches, brings a chance of light rain to the 
region Tuesday and Wednesday, with rainfall totals focused on the 
North Bay. A weak ridge will move through the state on Thursday and 
Friday, but the overall flow should remain generally zonal and 
lingering showers and drizzle are possible from remnant moisture in 
the region.

Towards and beyond the end of the 7-day outlook, the forecast points 
towards another round of wet weather across the region for the 
weekend of the 20th and 21st as a deep trough in the eastern Pacific 
coincides with another plume of moisture moving through the West 
Coast. With a system this far out, details are still very much in 
flux. The CPC maintains a strong lean (60-80% probability) towards 
rainfall totals above the seasonal average in the 6-10 day window 
(December 19-23). Beyond that, the CPC maintains a moderate risk (40-
60% probability) of heavy rainfall and high winds into the Christmas 
holiday period with a 60-70% probability of rainfall totals above 
the seasonal average through the 8-14 day period (December 21-27), 
which would impact any holiday travel plans.

&&

.AVIATION...
(18Z TAFS)
Issued at 1035 AM PST Sat Dec 13 2025

It's going to be slow to very slow going today regarding fog and
low stratus /VLIFR-IFR/ mixing out in the North Bay and the East 
Bay. With offshore pressure gradients 8.0 mb WMC-SFO, 1.5 mb SAC-
SFO and weak surface low pressure located over the coastal waters,
the surface to near surface pattern will continue to strongly 
favor tapping the Central Valley source of cold air, fog and low 
stratus today. With a weak 500 mb trough overhead and a weakening 
of the lower level temperature inversion, it's up to the Sun to do
what it can to try to break through the solid pool of incoming 
fog and low stratus; pilot reports indicate ~ 1,000 feet thickness
from bases to tops. Peak surface heating this time of year is ~ 3
pm, only 5 hours from now. Away from the current fog and stratus,
the sky is mainly VFR with a few lingering cold pockets of air 
around the Bay Shoreline and the south San Francisco Bay. Maritime
fog and stratus /LIFR-IFR/ continues to brush along the coastline;
local, limited inland intrusions remain possible with this area 
of fog and stratus through late morning and afternoon. 

Below normal water vapor distribution mainly in the mid to upper 
levels of the troposphere supports nocturnal radiative cooling 
tonight to early Sunday, this will reinforce the ongoing Central 
Valley tule fog/low stratus. Neither the GFS nor the ECMWF are 
forecasting cirrus tonight and Sunday morning. Light offshore 
winds will quite likely transport fog and low stratus to areas 
that have nightly/daily fog and low stratus.

Vicinity of SFO...A patch of stratus lingers just north of KSFO
nudged southward and westward with an influx of colder air arriving
on light NE-E surface winds. This patch should continue to mix out, 
favoring moderate to high confidence VFR for the remainder of the 
morning and afternoon. Surface wind may shift back around to light 
northwest during the afternoon to early evening, then back to light 
northeast tonight and Sunday morning. Low confidence VFR forecast 
tonight and Sunday morning primarily because of the meso-scale 
models having difficulty forecasting this ongoing pattern.

SFO Bridge Approach...Low stratus clouds are getting nudged southward
from influx of NE-E winds arriving at KSFO terminal. Cold surface
air continues to wobble around the SF Bay, and this may result in
redevelopment of low stratus patches back in the approach zone
from late morning to early afternoon. Otherwise higher probability
of VFR for the afternoon to early evening. Slantwise visibility
may be reduced to moderate to poor at sunset and sunrise due to
hazy conditions. 

Monterey Bay Terminals...LIFR due to stratus and mist rotated
inland from the northwest reaching KMRY, this will need another ~
1/2 hour to mix out to VFR at KMRY. Otherwise moderate to high
confidence VFR for the afternoon. Low to moderate confidence IFR
due to stratus developing tonight and Sunday morning.

&&

.MARINE...
(Today through Thursday)
Issued at 859 AM PST Sat Dec 13 2025

A weak surface low pressure circulation over the outer coastal
waters will result in 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...None.
PZ...None.
&&

$$

SHORT TERM...DialH
LONG TERM....DialH
AVIATION...Canepa
MARINE...Canepa

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Prepared by Boulder-Creek.com Weather at: Sat Dec 13 12:30:02 PST 2025