FXUS66 KMTR 210455
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
955 PM PDT Tue Oct 20 2026
...New AVIATION, MARINE...
.KEY MESSAGES...
Updated at 801 PM PDT Tue Oct 20 2026
- Cooling trend continues through the end of the week
- Beneficial light rain showers expected Wednesday through Thursday
- Thunderstorms possible Wednesday afternoon into Thursday morning
across Central Coast
- Warm and dry next weekend
&&
.UPDATE...
Issued at 801 PM PDT Tue Oct 20 2026
It is the calm before the storm. Weather admirers are anxiously
waiting for the first rain to reach the surface. The most recent 12Z
and 00Z KOAK soundings observed precipitable water values nearly
double from 0.44 inches yesterday to 0.84 inches today. Mid-level
moisture is increasing, though dry air at the surface remains. As a
result, today's cloud buffet consisted of cirrostratus and
altocumulus castellanus with virga (rain that evaporates before
reaching the ground) at the cloud base. Soon enough this moisture
will saturate the lower levels setting the stage for the first rain
showers to develop overnight.
Navarrete
&&
.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...
(06Z TAFS)
Issued at 955 PM PDT Tue Oct 20 2026
VFR with increasing high clouds and light winds through the next few
hours. A low pressure system offshore will bring showers to the area
on Wednesday with a slightly delayed onset time compared to the
previous forecast. As before expecting a main rain band to come up
from the Central Coast through Wednesday with convective activity
behind the band. Low confidence (20-30% probability) for
thunderstorm activity along the coast south of MRY. Confidence in
the wind speeds beyond the next few hours remains low, as hinted at
by the large differences in the wind field output from subsequent
runs of the high resolution models. With the low pressure system so
close to shore, the wind pattern may very well depend on how the low
travels as it meanders off the coast and how it interacts with local
terrain. Any thunderstorm activity on Wednesday is also liable to
bring localized strong gusts and visibility reductions to the
immediate area.
Vicinity of SFO... VFR with high clouds and light winds continues
into the overnight hours. A band of showers arrives Wednesday
morning and afternoon followed by a period of more scattered
showers. . Low confidence in the winds on Wednesday 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 light drainage
winds through the overnight hours. Isolated showers will start to
build early Wednesday morning before an organized band arrives
later that morning, bringing the chance for MVFR ceilings along with
it. Low confidence in the winds throughout Wednesday, and for a
isolated thunderstorm (10-20% probability) to impact the terminals
Wednesday.
&&
.MARINE...
(Tonight through next Monday)
Issued at 955 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 Friday into the weekend. Rain
showers with a chance of thunderstorms are expected through
Thursday, especially south of Pigeon Point.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory from 3 PM Thursday to 9 AM PDT Friday for
Pt Arena to Pt Reyes 10-60 NM.
Small Craft Advisory from 9 PM Thursday to 9 AM PDT Friday for
Pigeon Pt to Pt Pinos 10-60 NM.
&&
$$
SHORT TERM...MM
LONG TERM....MM
AVIATION...DialH
MARINE...DialH
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Prepared by Boulder-Creek.com Weather at: Wed Oct 21 00:30:02 PDT 2026
From the National Weather Service
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