FXUS66 KMTR 201817
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
1117 AM PDT Tue Oct 20 2026
...New AVIATION...
.KEY MESSAGES...
Updated at 1200 PM PDT Mon Oct 19 2026
- Cooling trend continues through the end of the week
- Beneficial light rain showers expected mid-week
- Thunderstorms possible mid-week across southern Central Coast
&&
.UPDATE...
Issued at 856 AM PDT Tue Oct 20 2026
Looking out the window, yes sometimes we do just look out the
window, we can see cirrus and even some accas. Both of these cloud
types can be precursors to a pattern change. If we take a broader
look at satellite the bands of clouds currently west of Monterey
are the leading edge of an occluded low pressure parked off the
coast. Additionally, the bands east of Monterey are likely tied
to the position of an upper level jet (GOES West shows strong
winds in the 250-450mb range). The accas can be a good indicator
of moisture and lower-mid level instability. If you haven't
noticed, KMUX is still in precip mode. We are now picking up a few
echoes well offshore closer to the low pressure and surface
boundaries.
No changes in the near term forecast for today. Will be focusing
on the next 24-48 hour as the offshore system moves east bringing
dynamics with it. Early look still suggest some precip across the
entire CWA. The biggest impacts will be tied to the potential
convective threat along the Central Coast. Will coordinate with
SPC to any updates on SVR Outlooks. Still looks like a low chc for
some rotating cells to develop over the coastal waters off Big Sur
Wednesday.
MM
&&
.SHORT TERM...
Issued at 1203 AM PDT Tue Oct 20 2026
(Today through Wednesday)
The low pressure system is already making its presence known as it
nears our coast. Not only were there various cloud layers present on
Monday, but some measurable rainfall has been observed in
Northern California nearby Eureka (as of 10 PM PDT Monday). Given
that the center of the low is still to our northwest, these
rainbands are not expected to impact us just yet...
Overnight, the upper-level trough continues to dig southward along
the coast and becomes fully cut-off from the main airflow by
Tuesday. During the day, calm conditions are expected with below
normal temperatures and infrequent wind gusts up to 20-25 mph.
Later tonight, the cut-off low begins to slowly propagate
eastward with the first few rainbands possibly impacting the
southern Central Coast first. Through the overnight hours, these
rainbands begin to make their way across the rest of the Central
Coast and Bay Area.
Rain chances increase Wednesday with the North and East Bay less
likely to experience wetting rain (>0.10 inch) during the event.
Chances for wetting rain increase towards the Central Coast where
the center of the cut-off low is expected to enhance moisture
transport onshore. The eastern portion of the system is forecast to
have stronger wind speeds than the western portion, likely due to a
tighter pressure gradient downstream as well as a surge in momentum
flux from the subtropical jet stream. This roughly coincides with
where the most moisture, instability, and lift can be found. As a
result, thunderstorm potential (<20%) remains the greatest along the
Central Coast. The exact timing is uncertain, though model guidance
is suggesting the highest probabilities are during Wednesday
afternoon. Most unstable CAPE has trended slightly downward compared
to yesterday, though it is still highlighting the Big Sur Coastline.
The latest SPC convective outlook has upgraded Central and Southern
California to a marginal severe thunderstorm risk, just south of the
Monterey County border... Even though we are not included in the
marginal severe risk, this is too close to comfort considering the
proximity to the Timber and Plaskett fire burn scars. As the event
draws closer, this will be an area of special interest not only for
the potential of debris flows if rainfall thresholds are met, but
also gusty and erratic winds that may result from low-topped
thunderstorms.
&&
.LONG TERM...
Issued at 1203 AM PDT Tue Oct 20 2026
(Wednesday night through Monday)
Overnight, the center of the low gradually moves across the region
resulting in diminishing rainfall chances, though the Central Coast
still remains with a 60-80%. Lingering rain showers persist through
Thursday as the system continues its eastward track. Across the Bay
Area, 20-40% rain chances persist with a 40-60% chance across the
Central Coast.
By Friday, the trough center will be east of our area as high
pressure gradually builds in its wake with a mean northerly wind
flow. To balance out the recent disturbance, a slightly warmer and
drier air mass settles over the region this weekend with seasonal
temperatures in the 60s and 70s.
&&
.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...Navarrete
LONG TERM....Navarrete
AVIATION...MM
MARINE...MM
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Prepared by Boulder-Creek.com Weather at: Tue Oct 20 12:30:02 PDT 2026
From the National Weather Service
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