FXUS66 KMTR 202327
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
427 PM PDT Tue Oct 20 2026
...New AVIATION, MARINE...
.KEY MESSAGES...
Updated at 100 PM PDT Tue Oct 20 2026
- Cooling trend continues through the end of the week
- Beneficial light rain showers expected mid-week
- Thunderstorms possible mid-week across Central Coast
- Warm and dry next weekend
&&
.SHORT TERM...
Issued at 104 PM PDT Tue Oct 20 2026
(This evening through Wednesday night)
Near term - mid and upper level clouds continue to stream over
the forecast area this afternoon. There have been a few pockets of
sunshine, but temperatures are still seasonably cool. Highs this
afternoon will top out in the 60s and 70s. We have a few spots
interior Central Coast with a forecast to reach close to 80, but
that may be tricky given temps are holding in the 60s currently.
The much talked about storm system is knocking on our door step.
Afternoon visible satellite imagery shows a solid circulation well
offshore. That's the surface low getting its energy from a robust
upper level jet rounding the base of the trough. We've actually
begun to pick a few lightning strikes near the center of the
surface low. Between the surface low and the coastline is another
swath of clouds. This particular location is where the digging
northern jet is phasing with the southern jet. In other words,
solid lift. KMUX radar continues to pick up bands of shower
activity associated with this region of lift and moisture. So
what's it look like this evening through Wednesday? The upper
level jet dynamics will continue to fuel the cut-off low at 500mb
and the occluded surface low. Both of these features will inch
their way eastward between now and early Wednesday. As such, a
few scout/leading edge showers will be possible across the region.
Still some dry air in the mid levels so it will take some time to
get the precip to hit the ground. Where it becomes more
interesting dynamically is early Wednesday. As the low moves
toward the coast stronger flow aloft takes aim at the Central
Coast with a jet max showing up from 700-500mb leading to deeper
layer shear. The increasing shear also lines up with some higher
lapse rates and decent CAPE (Cali standards). The other wildcard
at play are the SSTs in that region. Current SPORT data shows
values in the upper 60s or 8-10 degrees above normal. One would
think that will add some extra fuel. Given the setup, dynamics,
and pattern recognition some low-topped supercells are within the
realm of possibilities. Rotating cells could spawn a waterspout
that may make landfall. The threat for thunderstorms have been
extended northward to Santa Cruz, but the most likely area for
waterspouts remains off Big Sur. SPC has now included the Big Sur
Coast in a 2-4% chc of tornadoes. Peak convective threat remains
from 12Z Wed through 03-06Z Thursday. Outside of this threat and
this region, elsewhere around the CWA will see rain shower
throughout the day.
Timber and Plaskett burn area concerns - given the convective
threat there are definitely concerns for debris flows. Given the
low conf and more cellular nature will not issue any flash flood
watch at this time. Persons along the Big Sur Coast should still
be prepared. Needless to say, we'll watching this situation
closely.
Convective threat end Wednesday night into Thursday, but
lingering showers remain.
&&
.LONG TERM...
Issued at 204 PM PDT Tue Oct 20 2026
(Thursday through next Monday)
The cut-off low transitions to an open wave on Thursday as it
moves eastward. While the best dynamics may have eased, lingering
showers remains through the day Thursday. Rainfall totals: remain
the same for most of the Bay Area 0.05-0.25", while Central Coast
into Santa Cruz has increased 0.25-0.75" locally up to 1.25" Santa
Lucia.
By Friday the open wave/trough will be east of the area with
upstream ridging moving in. The ridging will usher in a warming
and drying trend for the upcoming weekend. The pattern over the
weekend into next week has some omega blocking vibes, but upstream
lows keep trying to knock into the ridge. Dry through the rest of
the extended.
Totally fantasy land at this point, but longer range guidance
keeps the pattern active with wet weather returning toward the end
of the month just in time for Halloween. Stay tuned...
&&
.AVIATION...
(00Z TAFS)
Issued at 427 PM PDT Tue Oct 20 2026
VFR ceilings with high clouds prevail into the evening and early
overnight hours. A gentle to moderate onshore wind push will
continue into the evening hours. The first band of showers
associated with an approaching low pressure system will approach the
Central Coast early Wednesday morning and spread northward into the
Bay Area through the day. Behind the band, scattered showers will
persist particularly in favored mountain areas. Confidence in wind
speed and direction on Wednesday is low as the exact conditions may
rely heavily on the low pressure system's exact track and its
resulting interaction with local terrain.
Vicinity of SFO... VFR with high clouds continues into the overnight
hours. A moderate northwest winds will develop this afternoon and
last into the evening. Isolated to scattered showers may develop
overnight before a band of showers comes through Wednesday morning
and afternoon. Low confidence in the winds on Wednesday afternoon as
high resolution models are diverging on the direction of the flow,
which could depend critically on the evolution of the low pressure
system.
SFO Bridge Approach... Similar to SFO.
Monterey Bay Terminals... VFR with high clouds and a gentle to
moderate northwest wind through the evening hours. Isolated showers
will start to build in the overnight hours before an organized band
arrives Wednesday morning, bringing MVFR ceilings along with it.
Expecting some moderate east to southeast winds to develop on
Wednesday but confidence is low in the exact direction.
&&
.MARINE...
(Tonight through next Sunday)
Issued at 427 PM PDT Tue Oct 20 2026
A storm system off the California Coast will maintain light to
gentle breezes through early Thursday with southeast winds
gradually backing to a more northerly flow Wednesday into
Thursday. Fresh to strong north winds return Thursday and
continue into the weekend with gale force gusts possible across
the northern outer waters. Moderate seas prevail with localized
rough seas across the outer waters today and Friday. Rain showers
with a chance of thunderstorms are expected late tonight through
Thursday, especially south of Pigeon Point.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...None.
&&
$$
SHORT TERM...MM
LONG TERM....MM
AVIATION...DialH
MARINE...DialH
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Prepared by Boulder-Creek.com Weather at: Tue Oct 20 18:30:02 PDT 2026
From the National Weather Service
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