FXUS66 KMTR 120820
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
120 AM PDT Wed Aug 12 2026
...New SHORT TERM, LONG TERM, FIRE WEATHER...
.KEY MESSAGES...
Updated at 118 AM PDT Wed Aug 12 2026
- Near-critical fire weather conditions Wednesday afternoon
through Thursday early morning across Monterey and San Benito
Counties due to high based thunderstorms
- Up to a 15% chance for elevated rain showers/thunderstorms
Wednesday across the Central Coast
- Cooling trend through Friday will bring well below normal
temperatures, gradual warming trend through the weekend
- Minor coastal flooding for bayshore locations along the San Francisco
Bay, San Pablo Bay, and Monterey Bay through Thursday morning
&&
.SHORT TERM...
Issued at 118 AM PDT Wed Aug 12 2026
(Today and tonight)
Today's forecast attention turns 300 miles southwest of Point
Conception where a low pressure system is bringing up to a 15%
chance for elevated rain showers/thunderstorms Wednesday across the
Central Coast. This has the potential to be a low probability/high
impact scenario and will bring near-critical fire weather conditions
Wednesday afternoon through Thursday early morning across Monterey
and San Benito Counties. So far, lightning has remained confined to
the circulation with mid-to-high level clouds beginning to stream
in. An upstream upper-level trough, mid-level lapse rates around 7.5
degrees C/km creating sufficient MUCAPE values around 500-1000 J/kg,
and PWATs in excess of one inch are the thunderstorm ingredients.
Confidence is lowest in the lift/trigger as guidance/models are
struggling with the circulation and cutoff lows are notorious to
forecast due to their independence from the jet stream. Uncertainty
lies in what these two features will do when they collide, including
track and potential dry air entrainment which would lower rain
shower/thunderstorm chances. With moisture confined to the mid-to-
upper-levels and a dry slot in the low levels, DCAPE values of 500-
1000 J/kg will be capable of producing erratic/gusty outflow winds
of 35-45 mph with any rain showers/thunderstorms that develop. The
main concern is lightning which will pose the risk for new fire
starts and has the potential to hinder operations on the Timber
Fire. Smoke from the Timber Fire will continue to be transported
into the Central Coast via light southwesterly winds. The smell and
appearance of smoke will be the worst under the inversion at night
and in the morning. Sensible weather wise, the cooling and
moistening trend will continue through Friday with temperatures well
below normal and a deep marine layer bringing a chance of drizzle.
Minor coastal flooding is expected during the highest high tide
(evening/overnight) for bayshore locations along the San Francisco
Bay, San Pablo Bay, and Monterey Bay through Thursday morning.
&&
.LONG TERM...
Issued at 118 AM PDT Wed Aug 12 2026
(Thursday through Tuesday)
Cooling and moistening trend continues through Friday as global
ensemble clusters are in agreement that upper-level troughing will
remain over the West Coast. By the weekend, heights will begin to
rise as upper-level ridging builds in from the Southeast while
simultaneously a cutoff low develops off the Pacific Northwest.
Sensible weather wise, this will result in a gradual warming trend
and an increase in onshore flow; however, temperatures will remain
below normal.
&&
.AVIATION...
(06Z TAFS)
Issued at 914 PM PDT Tue Aug 11 2026
Satellite looks very similar to 24hrs ago, including local
offshore eddys likely to complicate inland progression of a
gradually increasing marine layer depth. The eddys are preventing
spillage over the terrain toward KSFO which has struggled to
manifest MVFR/IFR ceilings despite being surrounded by stratus.
Will continue with that thinking for this update.
Vicinity of SFO...Coverage and inland progression should increase
somewhat tonight with a gradually increasing marine layer depth.
Expect status to lift and VFR conditions by late morning with
breezy onshore flow by the afternoon into the early evening.
Stratus may return earlier with a more robust marine layer in
place.
SFO Bridge Approach...MVFR/IFR ceilings likely on approach even
if not reported at KSFO.
Monterey Bay Terminals...More typical diurnal trend for this time
of year than the sites to the north. Expect the MVFR/IFR ceilings
to lift by late morning with breezy onshore flow in the afternoon
with primarily VFR until early evening before sunset, when stratus
ceilings return.
&&
.MARINE...
(Tonight through next Monday)
Issued at 914 PM PDT Tue Aug 11 2026
Fresh to strong northwest winds continue into early Thursday with
near gale gusts possible at times tonight over the northern outer
waters with wind driven rough seas up to 12 feet. Long period
southerly swell will gradually increase late in the week, peaking
between 16 to 18 seconds late Friday through early Sunday. Expect
haze and smoke to limit visibility at times along the Big Sur
coast due to the Timber fire.
&&
.FIRE WEATHER...
Issued at 118 AM PDT Wed Aug 12 2026
Near-critical fire weather conditions Wednesday afternoon through
Thursday early morning across Monterey and San Benito Counties due
to high based thunderstorms. Confidence remains low for a low
probability/high impact scenario of elevated rain
showers/thunderstorms across the Central Coast from a low pressure
system that is currently 300 miles southwest of Point Conception.
Erratic/gusty outflow winds of 35-45 mph can be expected with any
elevated rain showers/thunderstorms. Lightning will pose the risk
for new fire starts and has the potential to hinder operations on
the Timber Fire in Big Sur. The areas of most concern will be in the
higher terrain above the marine layer (1,500 feet), namely the Santa
Lucia Mountains and Gabilan Range. Poor minimum daytime relative
humidity around 30% is expected in the afternoons with little-to-no
recovery overnight.
Sarment
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Coastal Flood Advisory until 2 AM PDT Thursday for CAZ006-506-
508-529-530.
PZ...Small Craft Advisory until 9 AM PDT Thursday for Pt Arena to Pt
Reyes 10-60 NM.
&&
$$
SHORT TERM...Sarment
LONG TERM....Sarment
AVIATION...JM
MARINE...JM
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Prepared by Boulder-Creek.com Weather at: Wed Aug 12 04:30:02 PDT 2026
From the National Weather Service
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