FXUS66 KMTR 202109
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
209 PM PDT Tue Oct 20 2026

...New SHORT TERM, LONG TERM...

.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...
(18Z TAFS)
Issued at 1053 AM PDT Tue Oct 20 2026

Near term forecast will be VFR and relatively light winds as a
storm system off the coast spreads mid and upper level clouds.
After 00-06Z tonight mid and lower clouds will begin to increase,
but still VFR. Precip slowly arrives 12-15Z Wed impacting Monterey
Bay, HAF, and poss SFO. After 15-18Z -shra has a better chance
impacting other terminals with MVFR to VFR conditions. Overall
conf for Wed is low to moderate as a lot depends of shower
activity.

Vicinity of SFO...VFR with light easterly flow this afternoon and
then switching onshore by this evening. VCSH possible by 12Z, but
better conf for -shra afternoon 15-18Z Wed.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR with lighter winds this afternoon.
Stronger onshore NW returns this afternoon/evening. Will bring
MVFR with -SHRA 12Z and beyond for Wednesday. 

&&

.MARINE...
(Today through Sunday)
Issued at 856 AM PDT Tue Oct 20 2026

A storm system off the California Coast will keep light to gentle
southerly breeze through early Thursday with winds gradually
becoming more northerly Wednesday into Thursday. Fresh to strong
northerly winds return Thursday and continue into the weekend with
near gale force gusts possible across the northern outer waters.
Moderate seas prevail with localized rough seas across the outer
waters Tuesday and Friday. Rain showers with a chance of
thunderstorms are expected late Tuesday through Thursday,
especially south of Pigeon Point.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...None.
&&

$$

SHORT TERM...MM
LONG TERM....MM
AVIATION...MM
MARINE...MM

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Prepared by Boulder-Creek.com Weather at: Tue Oct 20 14:30:03 PDT 2026