021
FXUS66 KMTR 030143
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
643 PM PDT Tue Sep 2 2025

...New UPDATE...

.KEY MESSAGES...
Updated at 238 PM PDT Tue Sep 2 2025

 - A non-zero (10-15%) chance of isolated showers and
   thunderstorms continues through this afternoon along the
   interior eastern portions of Contra Costa, Alameda, Santa
   Clara, San Benito, and Monterey Counties.

 - Gradual cooling trend through the remainder of the week.

&&

.UPDATE...
Issued at 626 PM PDT Tue Sep 2 2025

The 00Z sounding for September 3rd is in. Precipitable water was
measured at 1.00 inches which is near the 90th percentile (1.03
inches) for this date and time. While that is an impressive
amount, dry air at and above 700 millibars has effectively slammed
the door shut on anymore elevated convection chances. While there
is still low-level moisture, the lack of a lifting mechanism with
conditionally unstable lapse rates at best will not be conducive
for thunderstorm development. The low-level moisture will likely
be realized via a healthy marine layer and stratus feed tonight,
kicking off the cooling and moistening trend that will continue
through the week.

Sarment

&&

.SHORT TERM...
Issued at 238 PM PDT Tue Sep 2 2025
(This evening through Wednesday)

Thunderstorm chances are diminishing across the CWA due to an influx
of drier air into the middle and upper levels of the atmosphere.
Satellite imagery shows the cut-off low located just off the coast
from San Mateo County down to Monterey County with guidance showing
the cut-off low progressing inland and weakening late tonight into
tomorrow. As the low approaches our coastline, it is transporting
much drier air into the middle and upper levels of the atmosphere
along the coastline (shown quite well in the Mid-level water vapor
satellite imagery). Aircraft soundings from SFO, OAK, and SJC all
show RH values dropping in the mid and upper levels of the
atmosphere between early this morning and this afternoon. The most
recent SJC aircraft sounding (taken at 2030Z or 1:30 PM PDT) shows
mid level-RH values around 31% and high level RH values between 17%
to 29%. Comparatively, mid to high level RH values were closer to
45% around 4AM to 5AM PDT this morning at SJC when thunderstorms
were observed across eastern portions of Santa Clara County. A look
at SPC mesoscale analysis shows a corridor of higher CAPE values
across the Central Valley while CAPE values are much lower (~500-
1000 J/kg) across the Bay Area and Central Coast. Given the
instability present, there continues to be a non-zero chance that an
isolated thunderstorm may develop along the far easternmost portions
of Napa, Contra Costa, Alameda, Santa Clara, San Benito, and
Monterey Counties. The incoming dry air mass has helped to prevent
more widespread convection from developing across our CWA with High
Resolution convective allowing models (CAM) showing thunderstorms
staying well to our east over the Sierra Nevada Range. A non-zero
(10-15%) chance of thunderstorms will thus continue through the
remainder of the afternoon across the easternmost portions of the
Bay Area and Central Coast.

Tuesday night into Wednesday, the offshore cut-off low will weaken
and weak troughing will persist along across the coastline. This
will help kick off a cooling trend for the Bay Area and the Central
Coast. Temperatures will cool by 2-4 degrees on Wednesday with highs
in the 80s to low 90s across the interior and in the 60s to low 70s
along the coastline. Weak upper-level troughing will help to keep
the marine layer deeper (~1500-1800 ft) which will allow widespread
stratus along the coast and across the interior tonight. Patchy
coastal drizzle is expected tomorrow morning with max accumulations
of a few hundredths of an inch possible.

&&

.LONG TERM...
Issued at 238 PM PDT Tue Sep 2 2025
(Wednesday night through next Monday)

Weak upper level troughing continues through the end of this week
with another upper level trough building in over the weekend and
into next week. This will allow our gradual cooling trend to
continue Thursday into Friday with temperatures stabilizing near
normal over the weekend. High temperatures will be in the 80s to low
90s on Thursday before dropping into the 70s to low 80s Friday
through Monday. Coastal drizzle is expected Thursday morning and
again Friday morning as persistent upper level troughing maintains a
a deeper marine layer around 1000 to 1500 ft. CPC shows near normal
temperatures and slightly above normal precipitation continuing
through mid-September. As noted by the night shift, however,
September is still part of our region`s dry season with most
locations seeing no rain from late spring into the fall. Any
accumulating precipitation, even that of a few hundredths of an
inch, would therefore be considered above normal for our region.

&&

.AVIATION...
(00Z TAFS)
Issued at 438 PM PDT Tue Sep 2 2025

Some MVFR-IFR stratus lingers at the immediate coastal areas with
VFR conditions inland. Breezy onshore winds will continue through
the the afternoon and evening hours, before diminishing later in the
evening. A very low chance (around 10-20% probability) of convection
remains in the far interior through the next couple of hours, but
will end after sunset. Overnight, stratus will expand into the
inland valleys and drop to a generally IFR-LIFR ceiling, with low to
moderate confidence of stratus impacting the interior. Stratus will
retreat to the immediate coastal region through Wednesday morning,
with breezy onshore winds resuming in the afternoon.

Vicinity of SFO... VFR through the evening with breezy southwest
winds through the evening. MVFR-IFR stratus returns to the terminal
overnight with moderate confidence in timing, with stratus flow
through the Golden Gate potentially setting up to the north of the
terminal. Stratus will clear late Wednesday morning before breezy
west winds return in the afternoon. Low to moderate confidence in
stratus returning Wednesday night, with higher confidence in stratus
returning to OAK before the end of the TAF period.

SFO Bridge Approach... Similar to SFO.

Monterey Bay Terminals... MVFR-IFR stratus has lingered to the north
of the terminals with a potential for imminent impacts to SNS. In
any case, VFR conditions will only continue for the next couple of
hours with IFR-LIFR conditions overnight. Stratus will gradually
retreat to the immediate coast on Wednesday morning, with potential
for stratus to return to MRY at or shortly after the end of the TAF
period. Breezy onshore winds will continue through the evening
hours, becoming light overnight with the onshore winds resuming
Wednesday afternoon.

&&

.MARINE...
(Tonight through next Sunday)
Issued at 438 PM PDT Tue Sep 2 2025

Northwesterly breezes will continue to diminish today, with
moderate to fresh breezes prevailing through Friday. Strong
northwesterly gusts will result in hazardous conditions for small
craft on Wednesday across the outer waters and coastal jet
regions. Moderate seas will prevail through Friday with occasional
rough seas in the outer waters.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory until 3 AM PDT Wednesday for Pt Arena to Pt
     Reyes 10-60 NM.

&&

$$

SHORT TERM...Kennedy
LONG TERM....Kennedy
AVIATION...DialH
MARINE...DialH

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