161
FXUS66 KMTR 100119
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
619 PM PDT Thu Oct 9 2025

...New UPDATE...

.KEY MESSAGES...
Updated at 106 PM PDT Thu Oct 9 2025

 - Cool conditions continue

 - Chances for light rain in the North Bay on Friday

 - Increasing confidence for widespread significant rainfall early
   next week

&&

.UPDATE...
Issued at 555 PM PDT Thu Oct 9 2025

Very light rain is being reported across the North Bay and these
passing showers will continue through the next 24-36 hours. When
all is said and done the North Bay should have received somewhere
from a couple hundredths to around 1/10th of an inch from this
weak boundary. The rest of the Bay Area will be lucky to get more
than a trace as the moisture plume mixes with drier air as it
slides south through the day Friday. The associated mid-level
trough axis will move through Saturday, bringing some stronger
afternoon gusts than normal. Sunday will be a good day to clean
the gutters before much more widespread rain arrives early next
week. The details are still unclear, but unless the forecast
changes dramatically, it looks likely that from Mon-Tue much of
the cwa will receive somewhere around the normal amount of rain
for the entire month of October. That`s mostly a testament to the
time of year this rain storm is arriving. If this was mid-January
it would feel more routine. Overall it looks like a mostly
beneficial event that will put a significant damper on fire season
while bringing mostly minor impacts, and no real chance for main
stem river flooding.

&&

.SHORT TERM...
Issued at 106 PM PDT Thu Oct 9 2025
(This evening through Friday)

Satellite imagery depicts extensive cloudiness over the North Bay,
some cumulus clouds over the higher elevations of the Bay Area and
Central Coast, and lingering clouds over the Big Sur coastal region.
Radar returns show a narrow stationary boundary extending south
from Marin County to the inner waters off the San Mateo Peninsula
and the outer waters off the Central Coast. Our colleagues at the
WPC are analyzing this as a stationary front on land associated with
a surface low pressure system in the North Coast and a weakening
cold front offshore. Any remaining rainfall totals from this system
will be very light, a few hundredths of an inch at most. Another
shot of light rain comes through Friday, generally limited to Sonoma
County where a few hundredths of an inch will fall in the valleys,
and a tenth to two tenths of an inch in the coastal ranges.
Otherwise, the weather remains remains dominated by an upper level
low currently meandering off the Pacific Northwest, leading to cool
conditions with breezy and gusty winds each afternoon and evening
along with periods of clouds as weak cold fronts pass through the
region. High temperatures today and Friday range from the lower to
middle 70s in the interior valleys of the central bay Area, the
South Bay, and the Central Coast, along with the Bayside regions, to
the upper 60s in the North Bay valleys and the lower to middle 60s
along the Pacific coast.

&&

.LONG TERM...
Issued at 106 PM PDT Thu Oct 9 2025
(Friday night through next Wednesday)

The chance for rain clears out Friday night as the low pressure
system moves eastward into the Pacific Northwest, and leads into a
dry, but cool, weekend with highs broadly similar to the previous
couple of days, except in the North Bay where temperatures warm to
the lower to middle 70s on Sunday. Incidentally, around this time,
the remnants of Typhoon Halong in the northwestern Pacific will
begin to dissipate into the jet stream circulation to the south of
the western Aleutians and lose any sense of identity through the
weekend as its remnants make their way towards Alaska. Sometimes,
the remnants of western Pacific typhoons will recurve and impact
the West Coast, but in this specific instance, it looks like any
impacts will be well to our north.

Confidence is increasing in some form of rainfall, particularly
significant rainfall, for the early part of the next work week. As
the current low pressure system begins to dissipate heading into the
Northern Rockies, a second trough will start to develop over British
Columbia, gravitate offshore, and deepen in to a new low pressure
system that descends parallel to the West Coast, eventually reaching
the coast off central California. The result will be a rather fast
moving storm, but one which will likely be our first significant
rainmaker since the previous winter season. Probabilities of
precipitation in the Monday to Tuesday timeframe top out around 60-
80% across the forecast area. A preliminary estimate of potential
rainfall totals suggests that 1 inch of rain is possible in the
lower elevations with rainfall totals in the coastal ranges reaching
the 2-3 inch range. Please note that there is still a fair bit of
uncertainty with the rainfall totals at present. Taking the National
Blend of Models as a baseline, from Monday at 5 AM to Wednesday
at 5 AM, there is an 80% chance that the rainfall total in
downtown San Francisco falls between 0.0" and 2.4", and a 50%
chance that the total falls between 0.09" and 1.65". For Monterey
Airport, those ranges are 0.01"-2.2" and 0.58"-1.68" respectively,
while at downtown Santa Rosa, the respective ranges are 0.0"-2.1"
and 0.01"- 1.50". Right now, a significant rain event is more
likely than not, but the forecast does have time to evolve.

Wind threats will also arrive as the storm system comes through the
region, with strong gusts potentially developing on Monday and
Tuesday as the descending low causes a strong pressure gradient and
a jet streak comes through the region with 40-50 kt winds at 500 mb.
It is a little far out to mention any specifics, but gusts reaching
or exceeding 30 mph can not be ruled out. In addition, model data is
also suggesting a slight (10-20% probability) chance for
thunderstorms Monday night and Tuesday. This is when the region
would sit on the left exit region of the jet streak, a favored
region for divergent flow that would support a chance for
convection. Again, too early to say what the specifics will be. We
will continue to follow the system as it evolves and refine the
forecast as more model data arrives into the weekend.

&&

.AVIATION...
(00Z TAFS)
Issued at 425 PM PDT Thu Oct 9 2025

Broad area of cloud deck moving through the North Bay along a
stationary front will gradually move away and mix out through this
evening. Largely anticipating VFR through the period as the marine
layer remains mostly mixed out. Some patchy areas of stratus can
be expected around the Bay during the overnight hours, but not
expecting coverage to be too impactful. Slight chance of brief,
isolated showers invof North Bay terminals by mid-morning Friday.

Vicinity of SFO...Moderate confidence in VFR through the period.
The timeframe that makes it moderate instead of high would be
around sunrise Friday morning as patchy stratus lingers around the
terminal. Other impact to the terminal would be the southerly
component of wind Friday morning. Wind expected to stay in the
range of 4-7 kts during this time, but will likely have an
extended period of winds out of the S/SW.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Anticipating VFR through the period as
cigs that may form overnight should be aoa FL030. Otherwise mostly
quiet conditions with onshore flow during the afternoons. Some
SE drainage flow at KSNS likely during the early morning.

&&

.MARINE...
(Today through Tuesday)
Issued at 852 AM PDT Thu Oct 9 2025

Light precipitation is expected over the coastal waters,
especially south of Point Reyes. Light to locally moderate winds
will be shifting through tonight. Light rains build into the
waters through friday and exit into that night. Northwesterly
winds arrive Saturday and increase to a strong breeze that night,
building rough seas by Sunday. Disturbed conditions with periods
of rain and gusty winds are expected across the waters early next
week.

&&

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

$$

SHORT TERM...DialH
LONG TERM....DialH
AVIATION...Behringer
MARINE...MM

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