457
FXUS66 KMTR 021735
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
1035 AM PDT Tue Sep 2 2025

...New AVIATION, MARINE...

.KEY MESSAGES...
Updated at 156 AM PDT Tue Sep 2 2025

 - A 15-20% chance of isolated showers and thunderstorms through
   the early 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 918 AM PDT Tue Sep 2 2025

We had a bit of excitement for any early risers this morning thanks
to some thunderstorms that developed along our CWA border. The bulk
of the thunderstorms were to our east over the Central Valley but we
did have a handful of lightning strikes within Contra Costa,
Alameda, Santa Clara, San Benito, and Monterey Counties. The
majority of these storms are producing dry lightning (i.e. any
precipitation that is associated with the thunderstorm evaporates
before reaching the surface) so we have seen a few small fires pop
up in Monterey/San Benito Counties. A 20-30% chance of thunderstorms
continues through 6PM today for the far most eastern portions of
Napa, Contra Costa, Alameda, Santa Clara, San Benito, and Monterey
Counties. The caveat to this is that we are seeing a drier air mass
move in into the middle and upper levels of the atmosphere. This
should help limit our overall thunderstorm potential today, but, we
will continue to monitor as a low, 20-30% chance, of thunderstorms
does continue for the far eastern portions of our CWA.

&&

.SHORT TERM...
Issued at 156 AM PDT Tue Sep 2 2025
(Today and tonight)

Today`s forecast is interesting as we have an upper level low just
off to our west, which will slowly make its way towards our coast
today. While that is occuring, monsoonal moisture surges northward
across the Golden State. What we`ve seen so far, is convection
flaring across the southern San Joaquin Valley and just to our
south, along San Louis Obispo`s interior mountains. Expect this
trend to continue with storms drifting northward this morning and
chances gradually increasing to 15-20% around daybreak. We`ll have
lift from either the upper low or the terrain and decent moisture as
PWATs should be around 0.75 to 1.25 inches today. The limiting
factor should CAPE as models tend to keep amounts light, around a
couple hundred J/kg or less and there should be some CIN. Mid level
lapse rates today favor around 7-8C, total totals around 44-58, and
LI`s around 0 to -3. If we`re able to overcome any drier air and
bust the cap, a few showers and isolated thunderstorms will be
possible through the early afternoon hours. Chances then begin to
decrease mid to late afternoon as the upper low traverses north and
we lose daytime heating.

Outside of the thunderstorms, expect fairly quiet conditions today,
with temperatures starting their gradual cooling trend today. &&

.LONG TERM...
Issued at 156 AM PDT Tue Sep 2 2025
(Wednesday through Monday)

Not much change in the extended as the gradual cooling eventually
steadies by the weekend. Cluster analysis shows upper level troughing
holding over meandering over the E PAC and gradually approaching the
PacNW late in the weekend or early next week. We can expect the
marine layer to deep and as a result, we`ll see stratus developing
each evening and then spreading inland overnight. The CPC continues
to show near normal temperatures and above average precipitation.
Note: Above average precipitation this time of year does not amount
to much and may be in the form of coastal drizzle and/or light rain.
Thus, widespread rainfall is not expected.

&&

.LONG TERM...
Issued at 156 AM PDT Tue Sep 2 2025
(Wednesday through Monday)

Not much change in the extended as the gradual cooling eventually
steadies by the weekend. Cluster analysis shows upper level troughing
holding over meandering over the E PAC and gradually approaching the
PacNW late in the weekend or early next week. We can expect the
marine layer to deep and as a result, we`ll see stratus developing
each evening and then spreading inland overnight. The CPC continues
to show near normal temperatures and above average precipitation.
Note: Above average precipitation this time of year does not amount
to much and may be in the form of coastal drizzle and/or light rain.
Thus, widespread rainfall is not expected.

&&

.AVIATION...
(18Z TAFS)
Issued at 1034 AM PDT Tue Sep 2 2025

Mostly clear skies expected for much of the region through this
afternoon before an early return of stratus this evening and moreso
into the early Wednesday morning. Any high-based convection should
remain east of the region today, yet cannot rule out isolated
convection across the East Bay this afternoon.

Vicinity of SFO...VFR. Northwesterly winds increase this afternoon
with gusts up to 25kt. There is moderate to high confidence for
IFR/MVFR ceilings to return to the region late tonight and continue
through about 17Z-19Z Wednesday. Onshore winds will increase by
around 21Z Wednesday with VFR conditions expected to return to the
region.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR. Onshore winds increase a bit this
afternoon and will generally remain out of the west-northwest. High
confidence for LIFR/IFR ceilings and/or visibilities returning to
the region this evening and continuing through midmorning on
Wednesday.

&&

.MARINE...
(Today through Sunday)
Issued at 1034 AM 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.

&&

.FIRE WEATHER...
Issued at 156 AM PDT Tue Sep 2 2025

An upper level low continues to move northeastward passing over
the region today. The low is surrounded by some drier air off
shore, but has started to tap into residual mid level moisture
this morning. The primary concern is there is a low, 15-20%,
chance of high impact weather if any elevated showers, isolated
dry lightning and storm associated gusty outflow winds develop.
The best chances look to be across our interior and most eastern
regions, favoring Contra Costa, Alameda, Santa Clara, San Benito, and
Monterey counties. For interior portions of the North Bay, chances
are less than 10%, but will still be monitored.

Otherwise a cooling trend sets in Tuesday and will continue through
the remainder of the extended forecast. Daily highs and lows will
be closer to normal for early September.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory until 9 PM PDT this evening for Pt Arena to
     Pt Reyes 10-60 NM.

&&

$$

SHORT TERM...KR
LONG TERM....KR
AVIATION...RGass
MARINE...RGass

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