Tamalpais Valley, California Weather Forecast Discussion

026
FXUS66 KMTR 060431
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
931 PM PDT Fri Sep 5 2025
...New AVIATION, MARINE...
.KEY MESSAGES...
Updated at 232 PM PDT Fri Sep 5 2025
- Cooler, slightly below normal temperatures continue into next
   week.
- Chance for light rain early next week across the North Bay.
&&
.SHORT TERM...
Issued at 232 PM PDT Fri Sep 5 2025
(This evening through Saturday)
Gradual cooling trend continues as upper level troughing builds into
the West Coast. Temperatures are anticipated to drop another 1 to 3
degrees across the interior. This will keep highs largely in the 70s
to low 80s across the interior and 60s to low 70s along the
coastline. Widespread stratus coverage is expected tonight with
cloud cover to dissipate by mid to late morning. Coastal drizzle is
likely again Saturday morning with accumulations around a hundredth
of an inch anticipated. Patchy fog is possible directly along the
coastline but any fog or areas of reduced visibility are expected to
be localized and not widespread. Breezy onshore winds continue today
and again tomorrow with gusts between 25 to 30 mph across favored
breezy areas (Altamont Pass, Salinas Valley).
&&
.LONG TERM...
Issued at 232 PM PDT Fri Sep 5 2025
(Saturday night through next Thursday)
Some uncertainty continues for if/how much rain we receive early to
mid next week as upper level troughing deepens over the West Coast.
To start off, upper level troughing will continue to build in Sunday
with troughing becoming more established over the West Coast by
Monday. The trough will then deepen Monday through Wednesday with a
surface low pressure system and associated cold front pushing inland
Sunday night into Monday morning. Rain chances increase across the
North Bay early Monday morning through Monday evening with another
chance of rain early Tuesday and Wednesday mornings. Any
precipitation that we do see will walk the fine line between drizzle
and light rain with total accumulations of less than 0.1" expected.
The bulk of the rain from this system will fall well to our north as
precipitation clips the northernmost edge of our CWA. Temperatures
will remain fairly stable Sunday through Thursday with high
temperatures in the 70s to low/mid 80s across the interior and 60s
along the coast. Winds will strengthen Monday through Thursday as
the upper level trough moves into the region with gusts between 25
to 30 mph (locally higher across favored terrain) becoming more
widespread along the coast and across the interior.
The main question is how much will this trough amplify, how long
will it persist, and where the center of the trough will set up. The
ECMWF and GFS maintain slightly different solutions as to where the
center of this trough will set up, how much it will amplify, and
long it will remain over the West Coast. The GFS supports a deeper
upper level low than the ECMWF with troughing lingering over the
West Coast into Thursday. The ECMWF supports a slightly weaker
trough that progresses eastward at a faster rate where it exits the
region on Wednesday. Earlier runs of the GFS showed the center of
the upper level low drifting further south, over the Bay Area, in
comparison to the ECMWF which kept it further north, closer to the
PNW. This would result in two different scenarios for potential
convection across the region. If the GFS were to be correct, a
further southward centered low would result in more widespread
convection across the CWA whereas, under the ECMWF, a further
northward oriented low would keep any convection largely confined to
the North Bay. Confidence is low that the scenario suggested by the
GFS will play out as described above. In fact, the recent 12Z run of
the GFS has started to agree with the ECMWF that the center of the
upper level low will be over the PNW but disagreements as to the
strength of the low and how long it will persist for continue. The
NBM places around a 5% chance of thunderstorms across northern
Sonoma and Napa Counties Tuesday morning and again Wednesday
morning. Thunderstorm chances (and rain potential) are likely to
change as models continue to come into agreement as to the amplitude
of the trough and the speed at which it progresses eastward.
&&
.AVIATION...
(06Z TAFS)
Issued at 928 PM PDT Fri Sep 5 2025
Currently a mixed bag of IFR-VFR at the terminals. The marine layer
is currently being observed at 2,500 feet with patchy and rather
unhealthy looking stratus across the region. The reason for this is
a surface low pressure system that is encroaching the Northern
California Coast is advecting some dry air. As such, confidence
has lowered since last TAF issuance to moderate that all terminals
will develop sub-VFR ceilings - likely on the cusp of IFR/MVFR and
later than originally forecast.
Vicinity of SFO...Currently VFR with westerly flow. Moderate
confidence on an MVFR ceiling redeveloping tonight. Westerly winds
will prevail through the TAF period.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...Currently IFR with westerly flow at MRY and
MVFR with westerly flow at SNS. Moderate confidence on SNS further
deteriorating to IFR. VFR will prevail by tomorrow afternoon with
low stratus sticking close to the coast.
&&
.MARINE...
(Tonight through next Thursday)
Issued at 928 PM PDT Fri Sep 5 2025
Gentle to moderate northwesterly breezes and moderate seas will
prevail through Tuesday. Locally strong gusts can be expected with
the afternoon sea breeze.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...None.
&&
$$
SHORT TERM...Kennedy
LONG TERM....Kennedy
AVIATION...Sarment
MARINE...Sarment
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NWS MTR Office Area Forecast Discussion