904
FXUS66 KMTR 110346
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
846 PM PDT Fri Oct 10 2025

...New AVIATION, MARINE...

.KEY MESSAGES...
Updated at 149 PM PDT Fri Oct 10 2025

 - Isolated showers in the North Bay through the late evening

 - Cool, dry, and mostly clear weekend

 - Widespread rainfall Monday through Wednesday with chances for
   thunderstorms Monday and Tuesday

&&

.UPDATE...
Issued at 655 PM PDT Fri Oct 10 2025

While the official forecast trend has been increasing for the
rain Mon through Tue, there is still a lot of timing and intensity
uncertainty that`s important to mention. For example the 12Z ECMWF
ensemble spread (10th-90th percentile) showed a range of 0.5" to
3.0" at SFO. The 18Z ensemble decreased that spread to 0.1" to
2.0". On the other hand, the GEFS 12Z ensemble spread was 0.1" to
2.0" and the 18Z run actually increased to 0.25" to 2.25". Two
things jump out. Most importantly the spread is quite large. We
have the same chance of getting 1/10th of an inch or 2 inches.
Secondly, the trend is increasing in the GEFS and decreasing in
the ECMWF ensemble. Even though we are only 3 days away, it`s
still tough to say what will happen with any certainty.

&&

.SHORT TERM...
Issued at 149 PM PDT Fri Oct 10 2025
(This evening through Saturday)

A band of showers associated with a cold front tied to a low
pressure system off the coast of Oregon is making its way through
the northern outer waters off of Sonoma County, and expected to make
its way across the North Bay through the afternoon and evening
hours. Rain totals from this band are expected to be rather light
with a tenth of an inch being the highest expected total across
favored locations in the Sonoma County mountains. Otherwise, the
rest of the day should remain mostly clear and chilly, with high
temperatures in the lower to middle 70s in the Bayside and interior
valleys of the Bay Area and Central Coast, the upper 60s to the
lower 70s in the North Bay valleys, and the lower to middle 60s
along the Pacific Coast.

On Saturday, winds are expected to pick up as a landfalling upper
level low tightens the pressure gradient aloft. The strongest gusts
of around 30 to 40 miles per hour are expected along the coast,
especially south of Davenport and Point Sur and within the Salinas
valley, while gusts of 15 to 25 miles per hour are generally
expected across the region. This is below Wind Advisory criteria,
but it is still advised to secure or bring in loose items in your
yard, lest you find that they have become your neighbor`s loose
items in their yard, or worse, through their windows. Otherwise,
conditions remain generally clear outside patches of low clouds over
the higher elevations, with highs generally similar to today`s.

&&

.LONG TERM...
Issued at 149 PM PDT Fri Oct 10 2025
(Saturday night through next Thursday)

Sunday`s conditions will be rather similar to Saturday`s, including
the gusty winds, the strongest of which will spread to the north to
impact the North Bay Pacific coastline. The only change in sensible
weather is that highs in the North Bay valleys tick up to the lower
to middle 70s.

Confidence continues to increase that a notable early season
rainstorm will impact the Bay Area and Central Coast next Monday
through Wednesday. An upper level trough over western Canada will
split off a cutoff low over southwestern British Columbia, which
will maintain itself as it travels southward paralleling the
Pacific coast until it arrives basically due west of California. As
it does so, it will pull moisture in from the polar jet stream and
direct it at the state. The current forecast depicts the main rain
band coming to the Bay Area Monday afternoon, moving through the
night before exiting the Central Coast on Tuesday evening, with
lingering showers through Wednesday. The latest forecast update has
continued a recent trend of higher rainfall totals across the Bay
Area and Central Coast, with most of the lower elevations seeing
rain totals above 1 inch, possibly approaching 2 inches in favored
locations. The higher elevations could see 2 to 3 inches of rain,
particularly across the Santa Cruz Mountains and the Santa Lucia
range. These amounts have caused our colleagues at the Weather
Prediction Center to issue a Marginal risk for excessive rainfall
for the Bay Area and Central Coast on Monday and Tuesday, meaning
that there is at least a 5% probability that rain totals will lead
to flash flooding in the area. One final note, there is also the
chance for slick roadways, especially in areas where we haven`t seen
rain last month, as the summer has allowed for oil buildup on the
pavement. Slowing down and leaving extra room for braking is a good
idea for any rainstorm, doubly so for this one.

There is also a risk for thunderstorms with the system, with the
latest model output placing chances for convection as high as 20-30%
for the Monday and Tuesday timeframe. Contributing to the convective
potential is the fact that the storm will be very cold. By Monday
morning, the 850 mb temperature over OAK will be 7 degrees Celsius,
or around the 10th percentile for this time of year, and the
temperature will only drop as the low pressure system approaches.
This will support steep lapse rates, which contribute to enhanced
instability, one of the three key ingredients for the convection. As
for the other two: moisture should be self-evident, recall the
forecast rain totals above. Meanwhile, this system brings two
sources for lift across the region. The first is the potential for
air to be lifted into the atmosphere by the local terrain, what we
would call orographic forcing. This is more prominent in the Sierra
Nevada, but can definitely happen in our region. The second source
for shift is a couple of surface-level fronts that will come through
the region, which will provide their own sources of lift as
airmasses lift over and undercut each other.

From the previous forecaster Sarment (updated to reflect the current
forecast): It is important to note that this system is not
unprecedented, it is not an atmospheric river, and it is not from
typhoon remnants. These distinctions are important so that decision
fatigue and recency bias are kept in check through our wet season so
that when there is a system that calls for stronger wording, it has
not lost all meaning. It`s all about context. Are we forecasting a
month`s worth of rain over the course of three days? Yes. It is
important to keep in mind though that October is the start of the
wet season and water year; thus, it is not too difficult to achieve
the feat that we are forecasting. For example: SFO`s average
precipitation during the month of October between 1945 and 2024 is
0.97 inches - their rainfall total forecast between Monday and
Wednesday is 1.64 inches. Their maximum precipitation for the month
of October is 7.30 inches dating back to 1962 where 7.29 inches fell
over the course of four days during the Columbus Day Storm. All in
all, this system should generally be beneficial from a water storage
and fire weather standpoint.

Beyond Wednesday, the upper level low moves inland and tracks
through the Great Basin. At this point, the models do diverge on the
strength and positioning of the low. The forecast for Thursday could
credibly include some northerly offshore flow or some lingering
rainfall depending on how quickly the storm moves and where its
track lies. But for now, let`s get the rainstorm out of the way
before we deal with what comes next.

&&

.AVIATION...
(06Z TAFS)
Issued at 845 PM PDT Fri Oct 10 2025

The last few showers are dissipating along the week front that has
passed into the SF Bay. Cloud cover along the boundary will continue
to carry south, leading to overcast mid-level clouds and spotty MVFR-
level clouds. Moderate to breezy winds linger into the night before
reducing. More consistent MVFR CIGs affect the Monterey Bay
terminals into the night, but scatter into early Saturday morning.
Cloud cover across the region begins to erode into late Saturday
morning as moderate to gusty winds build. These winds reduce into
Saturday night, but CIGs fail to return.

Vicinity of SFO...VFR through the TAF period. Expect moderate winds
to reduce into the evening as scattered low clouds arrive. These
clouds clear into the late morning on Saturday with gusty winds to
follow during the mid afternoon. Gusts cut of and winds become light
again into early Saturday night.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Winds are reducing and will become light
into the night. Expect MVFR CIGs to fill over the terminals into the
late night and last into early Saturday before scattering. Moderate
northwest winds are expected by the late morning and look to
last through Saturday evening before becoming light again.



&&

.MARINE...
(Tonight through next Thursday)
Issued at 845 PM PDT Fri Oct 10 2025

The last of the showers are eroding. Winds stay light into the
night but steadily increase across the southern waters early
Saturday morning, and spread north during through the day.
Hazardous conditions are expected across the waters as winds
approach gale force, with the Monterey Bay and the adjacent
coastal zone going above gale force. The increased winds will
result in rough seas by Sunday. An unsettled pattern for the first
half of next week will bring more wind and rain to the forecast.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Gale Warning from 3 PM Saturday to 3 AM PDT Sunday for Mry Bay-
     Pigeon Pt to Pt Pinos 0-10 nm-Pt Pinos to Pt Piedras
     Blancas 0-10 nm.

     Small Craft Advisory from 3 PM Saturday to 3 AM PDT Sunday for
     Pt Arena to Pt Reyes 0-10 nm-Pt Arena to Pt Reyes 10-60 NM-
     Pt Reyes to Pigeon Pt 0-10 nm.

     Small Craft Advisory until 3 PM PDT Saturday for Pt Pinos to Pt
     Piedras Blancas 0-10 nm.

     Small Craft Advisory from 9 AM Saturday to 3 AM PDT Sunday for
     Pigeon Pt to Pt Pinos 10-60 NM.

&&

$$

SHORT TERM...DialH
LONG TERM....DialH
AVIATION...Murdock
MARINE...Murdock

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