FXUS66 KMTR 220417
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
917 PM PDT Tue Oct 21 2025
...New AVIATION, MARINE...
.KEY MESSAGES...
Updated at 252 PM PDT Tue Oct 21 2025
- Slight chance for isolated showers and thunderstorms from Bay
Area southward tonight into Wednesday morning.
- Seasonal temperatures through much of the week.
- Rain chances return Friday evening through the weekend across
the Bay Area. Widespread cooler temps beginning this weekend.
&&
.UPDATE...
Issued at 902 PM PDT Tue Oct 21 2025
No changes to the forecast this evening as we watch fast moving
elevated showers move northwest across the Central Coast. The
elevated showers with a slight chance of convection will persist
through much of the overnight into Wednesday morning. There has
been lightning detected farther south over the past couple of
hours in Ventura and Los Angeles County. Similar moisture is in
place there as the upper level low moves slowly toward land.
There is better instability to the south than we'll see in our
area, but lapse rates around 8C/km with decent divergence aloft
and vorticity advection in the midlevels may come together to
create enough instability to get isolated thunderstorms over
Monterey County and San Benito County over the next couple of
hours. The primary concerns with any dry thunderstorms would be
lightning with gusty and erratic wind gusts up to 40mph.
&&
.SHORT TERM...
Issued at 252 PM PDT Tue Oct 21 2025
(This evening through Wednesday)
Light, offshore flow continues across the higher elevations through
late tonight. The KSFO-KWMC gradient peaked at -9.60 mb (moderately
offshore) this morning and is down to -6.80mb as of 1PM. The KSFO-
KWMC gradient is expected to continue weakening into this evening
before becoming positive (onshore) late tonight/early Wednesday
morning. This shift coincides with a pattern shift and the arrival
of a cut-off low pushing into the Central Coast. Guidance continues
to look promising enough to include a slight (15%) chance of
thunderstorms to the forecast for the Central Coast, Santa Cruz
Mountains, and portions of the South and East Bay. High resolution
guidance shows around 300 J/kg of most unstable CAPE, decent mid-
level lapse rates between 7C/km and 8C/km, and an influx of mid-
level moisture tonight into tomorrow morning. The most likely area
for storms to develop would be Monterey and San Benito Counties with
recent guidance showing some potential for showers to move in along
the Santa Cruz/Monterey coastline overnight. We may see a few
hundredths of an inch of precipitation along the coast and interior
Central Coast as these showers move in. If a thunderstorm is able to
develop, locally higher rain totals and gusty winds are expected.
Remember - when thunder roars, go indoors. Thunderstorm chances drop
off by the late morning with drier air moving in and MUCAPE values
decreasing during the afternoon hours. Skies will remain partly
cloudy through the rest of the day before overcast low clouds and
potential fog return Wednesday night.
High temperatures trend downwards tomorrow into the mid to upper 60s
across the interior, 50s to mid 60s along the coast, and upper 50s
along the elevated terrain. Nighttime temperatures will cool as well
with morning lows forecast to be in the low 50s to upper 40s on
Wednesday. The marine layer is currently at a depth of around 500 ft
but is expected to deepen to 1000-2000 ft as the cut-off low moves
into California. Fog and low clouds are possible along the coastline
tomorrow morning but a deepening marine layer should improve (raise)
visibilities and ceiling heights compared to this mornings dense
fog. Another complicating factor is that any early morning showers
that do develop may locally disrupt the marine layer and prevent fog
formation or disperse fog faster than it otherwise would have. Early
morning commuters in the Central Coast, Santa Cruz, and South Bay
regions should be prepared for isolated showers and sudden drops in
visibility/fog.
&&
.LONG TERM...
Issued at 252 PM PDT Tue Oct 21 2025
(Wednesday night through next Monday)
Shortwave ridging will allow temperatures to rise into the upper 60s
to low 70s across the interior Thursday and Friday with coastal
highs in the mid to upper 60s. The warmest locations in the Bay Area
and Central Coast will see high temperatures in the mid to upper 70s
with a few interior Central Coast locations reaching the low 80s.
Our next system will arrive late Friday into the weekend as a deep
upper level trough pushes into the West Coast bringing cooler,
wetter conditions with it. Temperatures drop into the mid to upper
60s again across the interior with coastal areas in the upper 50s to
low 60s Saturday through Monday. Overnight lows will drop as well,
with low temperatures in the 40s to low 50s across the board. All
together this weekend will see a return of slightly below normal
temperatures and bring a return of chilly fall weather.
The track of this system is well to the north of the Bay Area with
far northern California and Oregon receiving the heaviest amounts of
rain. The QPF forecast has not changed too much for the Bay Area
with the afternoon forecast update. The North Bay is slated to
receive between 0.5"-0.75" across the coastal ranges and northern
portions of Sonoma and Napa Counties with up to 0.5" from Santa Rosa
southwards in the North Bay. San Francisco, the San Mateo Peninsula,
Santa Cruz Mountains, and the East Bay will see between 0.1"-0.25"
while the South Bay only sees up to 0.1". Rain totals across the
Central Coast are minimal with only coastal areas seeing a trace of
rain. This system will arrive Friday afternoon with the heaviest
rain overnight Friday into Saturday in the North Bay. Saturday will
be rainy across the Bay Area with persistent light rain for much of
the day. Rain will taper off throughout the day on Sunday before
drier weather returns Monday. Minor nuisance flooding (ponding on
roads and underpasses) is possible with any heavier rain but no
major flooding or river flooding is expected with this system.
Locally breezier conditions are expected along the coast, mountain
gaps and passes, and across the Altamont Pass this weekend but winds
will stay below Wind Advisory criteria.
&&
.AVIATION...
(06Z TAFS)
Issued at 915 PM PDT Tue Oct 21 2025
Currently a mixed bag of IFR-VFR at the terminals with isolated rain
showers streaming in from the south with lightning activity
remaining confined to Southern California for now. If a thunderstorm
were to develop, the best chances would be in the vicinity of the
Monterey Bay terminals, but the best chances seem to be to the south
and west of the terminals. Nonetheless, isolated rain showers will
remain possible through the morning with activity waning by the
afternoon. For VCSH, highest confidence is in MRY and SNS, but rain
showers will be within the general area.
Vicinity of SFO...Currently VFR with northwesterly flow. Moderate
confidence in an MVFR ceiling developing tonight. Winds will back to
become southwesterly tonight. VFR and westerly winds will prevail by
tomorrow afternoon.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...Currently MVFR with southwesterly flow at
MRY and MVFR with northwesterly flow at SNS. High confidence in
ceilings deteriorating to IFR tonight.
&&
.MARINE...
(Tonight through next Monday)
Issued at 915 PM PDT Tue Oct 21 2025
There's a slight chance for rain showers and thunderstorms
through tomorrow morning. A gentle to moderate breeze will prevail
through tomorrow, building to become moderate to fresh Thursday
through Saturday. Rough seas in the inner waters and very rough
seas in the outer waters will prevail through Saturday.
&&
.BEACHES...
Issued at 252 PM PDT Tue Oct 21 2025
A long period northwesterly swell will continue to bring an
increased risk for sneaker waves and rip currents through Wednesday.
Additionally, breaking waves of 12-17 feet and locally as high as 25
feet will be possible. Sneaker waves can unexpectedly run
significantly farther up the beach than normal, including over rocks
and jetties. Rip currents are typically more frequent and stronger
in the vicinity of jetties, inlets, and piers. Swimmers should
always swim near a lifeguard. Remain out of the water to avoid
hazardous swimming conditions and never turn your back on the ocean!
Sarment
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Beach Hazards Statement through late Wednesday night for CAZ006-
505-509-529-530.
PZ...Small Craft Advisory until 9 PM PDT Wednesday for Pigeon Pt to
Pt Pinos 10-60 NM-Pt Arena to Pt Reyes 10-60 NM.
&&
$$
SHORT TERM...Kennedy
LONG TERM....Kennedy
AVIATION...Sarment
MARINE...Sarment
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Prepared by Boulder-Creek.com Weather at: Tue Oct 21 22:30:02 PDT 2025
From the National Weather Service
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