FXUS66 KMTR 202031
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
1231 PM PST Tue Jan 20 2026

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

.KEY MESSAGES...
Updated at 1225 PM PST Tue Jan 20 2026

 - Late night and morning fog across the North and East Bay

 - Light rain across the Santa Lucia Range Late Wednesday into
   Thursday

 - Mild and mostly rain-free for the Bay Area and Central Coast

&&

.SHORT TERM...
Issued at 1225 PM PST Tue Jan 20 2026
(This evening through Wednesday)

Fog continues to gradually dissipate across the region as 
mid/upper level moisture continues to lift northward. While the 
cloud shield is expansive, examination of our morning sounding and
aircraft sounding data from SFO reveals that the low levels 
remain quite parched. What this translates to is high confidence 
that widespread rainfall is unlikely over the short term. 

The primary hazard in the short term will be the re-development 
of fog, largely across the North and East Bay. While mid-level 
ridging remains in place across the area, surface flow will 
gradually become more southerly which may support more in the way 
of onshore flow. At a minimum, this should promote fog development
in the favored spots of the North and East Bay, if not give us a 
higher chance. In fact some of the higher resolution model 
guidance advertises a repeat of fog across the US HWY 101 corridor
across Sonoma County, similar to today. 

&&

.LONG TERM...
Issued at 1225 PM PST Tue Jan 20 2026
(Wednesday night through next Monday)

While general mid/upper level ridging will equate to a largely 
"quiet" long term forecast period, there will be some brief shifts 
in the main upper pattern (in addition to morning fog). These 
shifts will offer slight changes in the day to day sensible 
weather elements. Late Wednesday into Thursday, model guidance is
a bit more honed in on the exact track of an upper low forecast 
to intensify and move eastward through Southern California. This 
will induce upslope flow across the higher terrain of the Central 
Coast. While moistening will transpire, it doesn't appear that the
column will completely saturate for a prolonged period of time. 
In addition, the speed of this upper trough will mean a low 
residence time across our area. At this time, there doesn't appear
to be a significant amount of spread in the track of this feature
so confidence is high that overall rain amounts will remain on 
the light side through Thursday night. In fact, some of the 
higher end scenarios (90th percentile) paint up to a tenth of an 
inch of rainfall for this event. 

By Friday evening/Saturday morning the main upper low will kick 
eastward. In the wake of this system, a more defined push of 
offshore winds is anticipated. At this juncture, the potential for
winds above 35 mph is under 10%, though it may be a little higher
across some of the favored gaps and passes. The story will be a 
little different across the marine zones as the surface pressure 
gradient does tighten. In fact 925mb flow does ramp up here to 
around 35 knots (40 mph) across Pacific waters west of 
Marin/Sonoma counties. 

Beyond day 7, model guidance remains largely consistent with broad 
ridging remaining in place. At times, however, storm systems may 
attempt to dampen out the amplitude of the ridge and place our 
region within a more zonal pattern. Some of the ensemble guidance 
systems remain aggressive with rainfall above 1/2." While the upper 
air pattern depiction is largely similar from ensemble suite to 
ensemble suite, the spread among individual systems (e.g., EPS) 
remains rather high as the spread ranges from no rain to three 
inches of rain in some locales. As always, we'll continue to 
monitor, message, and adjust as necessary.  

&&

.AVIATION...
(18Z TAFS)
Issued at 1112 AM PST Tue Jan 20 2026

LIFR vis due to valley fog is showing signs of lifting at KSTS
with VFR expected to prevail through the remainder of the after
noon and evening, with valley fog/Tule Fog expected to fill back
in over the North Bay terminal later this evening into the
overnight hours. VFR is expected to prevail through the remainder
of the forecast period for Bay Area and Monterey Bay terminals.

Vicinity of SFO...VFR is expected to prevail through the remainder
of the forecast period. A transition from weak offshore flow to
weak onshore flow (potential for stratus/fog) returns late in the
forecast period.

SFO Bridge Approach...Similar SFO.

Monterey Bay Terminals...VFR is expected to prevail through the
forecast period with weak offshore flow limiting low stratus
development.

&&

.MARINE...
(Today through Sunday)
Issued at 1112 AM PST Tue Jan 20 2026

Gentle to moderate northerly breezes continue across the waters
through Wednesday. Winds increase later this week to a strong
breeze by Friday. A weak storm system over the eastern Pacific
will result in moderate northwest swell, promoting rough seas
across the northern outer waters on Friday into the weekend.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...None.
&&

$$

SHORT TERM...Bain
LONG TERM....Bain
AVIATION...JM
MARINE...DialH

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Prepared by Boulder-Creek.com Weather at: Tue Jan 20 14:30:02 PST 2026