Marin County California Weather Forecast Discussion

263
FXUS66 KMTR 101107
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
407 AM PDT Fri Oct 10 2025
...New AVIATION...
.KEY MESSAGES...
Updated at 328 AM PDT Fri Oct 10 2025
- Isolated rain showers linger today in the North Bay
- Thunderstorm chances Monday into Tuesday
- Widespread rainfall Monday through Wednesday
&&
.SHORT TERM...
Issued at 328 AM PDT Fri Oct 10 2025
(Today through Sunday)
A surface low pressure system in the Eastern Pacific Ocean will
continue to allow for isolated rain showers with the chance of an
embedded thunderstorm today - particularly in the North Bay and over
the waters. The upper-level low will weaken through the day today,
becoming an upper-level longwave trough tomorrow. The positively
tilted axis will swing through the region tomorrow, bringing
stronger onshore winds than the usual sea breeze circulation. The
region will remain under the influence of the upper-level longwave
trough through the weekend. Well below seasonal normal temperatures
and an increase in northwesterly winds will have the sensible
weather feeling cold, at least relatively speaking. This is
supported by the 00Z October 10th KOAK sounding which observed an
850 millibar temperature of 8.55 degrees Celsius which is near the
tenth percentile of 7.5 degrees Celsius for this date and time. If
you are looking for weekend plans, Sunday would be a great day to
clean out your gutters and properly secure outdoor items that could
be blown away so that your Halloween decorations don`t become your
neighbors!
&&
.LONG TERM...
Issued at 328 AM PDT Fri Oct 10 2025
(Monday through Thursday)
Global ensemble clusters are in agreement that a reinforcing upper-
level low will dig from the Washington/Canada border down off our
coast beginning Monday. The uncertainty with this system comes in
strength and location, and thus, timing. A stronger solution closer
to the coast will be wetter and result in more impacts than a weaker
solution farther inland which will be drier and result in less
impacts. As of now, it does look like everywhere will get over an
inch of rainfall between Monday and Wednesday with locally higher
totals of two to three inches in the higher terrain. As this is the
first widespread wetting rainfall (0.10 inch or greater) of the wet
season, there will likely be an increase in car accidents due to oil
separation on the roads. Please allow extra patience, space, and
time when driving. There is also a marginal risk (at least 5%
chance) of rainfall exceeding flash flood guidance meaning that
isolated flash floods will be possible, but localized and primarily
affecting places that can experience rapid runoff with heavy
rainfall. This system is also going to be cold. The 850 millibar
temperature that was discussed in the short term is forecast to drop
through Tuesday with the deterministic ECMWF and GFS forecasting it
to be somewhere in the 3-4 degrees Celsius range which would be near
the daily minimum. The cold air aloft will also allow for further
destabilization of the atmosphere, so with the critical thunderstorm
ingredients of lift, instability, and moisture all in place, there
is a slight chance for thunderstorms Monday into Tuesday before cold
air funnels in at the surface behind the cold front.
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 1.36 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.
&&
.AVIATION...
(12Z TAFS)
Issued at 343 AM PDT Fri Oct 10 2025
Cooler air under clear skies has led to valley fog developing in
vicinity of KSTS overnight, with better visibility expected later
this morning as a cold front approaches the area, however clouds
will linger through today and tonight with mostly VFR conditions
expected.
Vicinity of SFO...Stratus has shifted east of the terminals with a
slow moving surface boundary early this morning with VFR expected
to prevail through the remainder of the forecast period.
SFO Bridge Approach...Similar SFO
Monterey Bay Terminals...MVFR ceilings this morning should give
way to VFR soon after sunrise, with VFR conditions expected for
the remainder of the day and evening. A return to MVFR stratus
ceilings late tonight into early Saturday morning is possible.
&&
.MARINE...
(Today through Wednesday)
Issued at 328 AM PDT Fri Oct 10 2025
Light showers with scattered thunderstorms are moving into our
northern waters this morning and will persist through much of the
day before shifting inland tonight. Light winds will continue
today and tonight, then begin to steadily increase across our
southern waters first on Saturday morning, and spread north during
the day. The increasing northwesterly winds across our area
Saturday 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 Watch from Saturday afternoon through late Saturday night
     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 AM to 3 PM PDT Saturday for Pt Pinos
     to Pt Piedras Blancas 0-10 nm.
&&
$$
SHORT TERM...Sarment
LONG TERM....Sarment
AVIATION...JM
MARINE...JM
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NWS MTR Office Area Forecast Discussion