Guerneville California Weather Forecast Discussion

655
FXUS66 KMTR 121124
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
424 AM PDT Sun Oct 12 2025
...New AVIATION, MARINE...
.KEY MESSAGES...
Updated at 236 AM PDT Sun Oct 12 2025
- Cold and windy conditions today
- Widespread rainfall and a chance for thunderstorms Monday and
   Tuesday
- Warming and drying trend kicks off Wednesday
&&
.SHORT TERM...
Issued at 236 AM PDT Sun Oct 12 2025
(Today and tonight)
It`s going to be a cold start to the morning. Conditions are cooler
and drier than they were 24 hours ago. This is thanks to light
offshore flow and subsequently clear skies that are allowing for
enhanced radiational cooling. Sensible weather today will remain
cool and windy with below normal temperatures and moderate
northwesterly breezes. Wind gusts will be locally stronger along the
coast, down northwest-southeast oriented valleys (namely the North
Bay Interior Valleys, Santa Clara Valley, and Salinas Valley), and
through gaps and passes with gusts up to 30 mph to be expected. If
you have not done so already, there is still time today to clean out
your gutters and secure/store any outdoor items that are sensitive
to rain and/or wind ahead of tomorrow`s system.
&&
.LONG TERM...
Issued at 236 AM PDT Sun Oct 12 2025
(Monday through Saturday)
A gale force low pressure system with support aloft in the form of a
reinforcing upper-level low will begin sliding down the West Coast
today - this will be the driver of our weather the next two days.
The Polar Jet Stream also dives south with this feature, but the
placement doesn`t seem to be too notable for our region in terms of
jet theory. The forecasted arrival time and heaviest rainfall period
has trended earlier as this is now looking like a Monday-Tuesday
event with the heaviest rainfall on Monday rather than a Monday-
Wednesday event with the heaviest rainfall on Tuesday. The brief
drying trend and offshore flow from this weekend has effectively
erased the marine layer with further drying expected through
tonight. As such, virga in the pre-frontal environment will work to
saturate the column early Monday morning ahead of the bulk of the
rainfall with the cold frontal passage Monday afternoon and evening.
The Weather Prediction Center has highlighted the region for a
marginal risk (at least 5%) of rainfall exceeding flash flood
guidance Monday morning through Tuesday morning. This means that
isolated flash floods are possible, but that they would be localized
and primarily affect places that can experience rapid runoff with
heavy rainfall. Post-frontal rain showers and thunderstorms will be
the name of the game on Tuesday. The thunderstorm potential on
Monday and Tuesday can be characterized by low CAPE (500 J/kg) and
conditionally unstable lapse rates (7 degrees Celsius/kilometer).
The Storm Prediction Center has highlighted the region for general
thunder during this time; remember, all thunderstorms are capable of
producing lightning and flooding, when thunder roars, go indoors!
and see a flash, dash inside! When all is said and done, a majority
of the area will accumulate 1.00" with locally higher totals of
1.50"-2.50" in the higher terrain of the East Bay, South Bay, and
Central Coast. This system is also going to be a cold one with 850
millibar temperatures putting daily minimum records in jeopardy.
Near-freezing temperatures are forecast for far interior portions of
Monterey and San Benito Counties Tuesday night. With the recent
rainfall, conditions will feel colder than what the thermometer
reads - please take care of people, pets, and plants. Patchy frost
is also possible during this time for the Interior East Bay and
Interior South Bay.
Global ensemble clusters are in agreement that the upper-level low
will weaken and shift east Wednesday into Thursday with the help of
upper-level shortwave ridging nosing in from the Eastern Pacific
Ocean. This feature will quickly get flattened by an upper-level
shortwave trough, putting the region under the influence of the
eastern periphery of the high pressure over the Eastern Pacific
Ocean. This will translate to a warming and drying trend Wednesday
through Saturday.
&&
.AVIATION...
(12Z TAFS)
Issued at 424 AM PDT Sun Oct 12 2025
Patchy low clouds and/or fog /MVFR-IFR/ otherwise VFR continues
today. Increasing mid and high level clouds tonight and early
Monday morning. Moderate to heavy showers (and potential t`storms)
accompany the arrival of a cold core low from Alaska/Western
Canada Monday-Tuesday.
Vicinity of SFO...VFR today and tonight. MVFR in moderate to heavy
showers accompany the arrival of a cold core low beginning Monday
morning. West wind 5 to 10 knots increasing to 15 to 25 knots this
afternoon to tonight, wind shifting to southwest 10 to 12 knots
Monday morning.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...VFR. Light east to southeast winds shifting
to west to northwest near 10 knots by early afternoon. West to
northwest winds becoming south to southwest 5 to 10 knots tonight
and Monday morning.
&&
.MARINE...
(Today through Friday)
Issued at 408 AM PDT Sun Oct 12 2025
Gusty northerly winds will continue across the waters today ahead
of a strengthening storm system arriving from the north Monday
into Tuesday. Expect hazardous conditions with gales developing
over the northern waters and southern inner waters later today and
this evening. Winds remain breezy Monday then ease Tuesday, then
become breezy from the northwest later in the week.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory from 3 PM this afternoon to 9 PM PDT this
     evening for Mry Bay.
Small Craft Advisory until 3 PM PDT this afternoon for Pt Arena
     to Pt Reyes 0-10 nm-Pt Arena to Pt Reyes 10-60 NM-Pt Pinos
     to Pt Piedras Blancas 0-10 nm.
Gale Warning from 3 PM this afternoon to 9 PM PDT this evening
     for Pt Arena to Pt Reyes 0-10 nm-Pt Arena to Pt Reyes 10-60
     NM-Pt Pinos to Pt Piedras Blancas 0-10 nm.
Small Craft Advisory until 9 AM PDT Monday for Pigeon Pt to Pt
     Pinos 0-10 nm-Pigeon Pt to Pt Pinos 10-60 NM-Pt Reyes to
     Pigeon Pt 0-10 nm.
&&
$$
SHORT TERM...Sarment
LONG TERM....Sarment
AVIATION...Canepa
MARINE...Canepa
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NWS MTR Office Area Forecast Discussion