FXUS66 KMTR 211825
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
1125 AM PDT Mon Sep 21 2026
...New SHORT TERM, LONG TERM, FIRE WEATHER...
.KEY MESSAGES...
Updated at 1120 AM PDT Mon Sep 21 2026
- Cool today
- Warmer temperatures Wednesday - Thursday
- Cooler weather returns Friday
&&
.SHORT TERM...
Issued at 1120 AM PDT Mon Sep 21 2026
(This afternoon through Tuesday)
The dominant weather feature today is a 500 mb cut-off low currently
centered over Pt. Conception. The 500 mb height at Vandenberg was
5750m as measured from the 12Z sounding. This is just below the 10th
percentile in the climatology for this time of year, so it's fairly
notable, but not extreme. The result is cooler temperatures, a
deeper marine layer, some morning drizzle, and even a few real
raindrops. The marine layer was measured at 2,400 feet deep by
the 12Z weather balloon from Oakland. This deeper cloud layer,
combined with low-level vorticity and orographic lift allowed some
coastal drizzle this morning. Drizzle was reported throughout the
typical areas across the Bay Area and Central Coast. The La Honda
RAWS in the Santa Cruz Mountains was the winner with 0.05" of
precipitation recorded over the last 24 hours. This deep marine
layer, high humidity and ample cloud cover will keep temperatures
5-10 degrees cooler than normal this afternoon.
The cut-off low will continue to slide NE over the next 24 hours,
loosening its grip on the weather here. Tuesday will be a transition
day as the marine layer starts to compress and temperatures increase
back to normal.
&&
.LONG TERM...
Issued at 1120 AM PDT Mon Sep 21 2026
(Tuesday night through next Sunday)
The long term forecast is a little bit of a roller coaster, but like
the kind for kids that doesn't do anything too scary. As the low
continues to exit the area, the upper level pattern will gradually
shift to a strong trough over the Gulf of Alaska and a ridge
stretching from Texas to central Canada. By Wednesday the west
coast will be in between these two features, with a small positive
500 mb height anomaly arriving Wednesday. While the 500 mb height
anomaly is minor, the 850 temperature anomaly will climb to
around 5-7C above normal Wednesday - Thursday. This will bring
inland temperatures into the upper 80s to low 90s, with 70s along
the coast both Wednesday and Thursday. There is some moderate
HeatRisk associated with this event, but it's not widespread and
has been trending down. With the longer nights, short duration,
and continued onshore winds, we are not expecting a heat advisory
with this event at this time. The trough over the Gulf of Alaska
will move into the Pacific Northwest by Friday, reversing the
warming trend and bringing temperatures back below normal for next
weekend.
&&
.AVIATION...
(18Z TAFS)
Issued at 1113 AM PDT Mon Sep 21 2026
Starting the day off with widespread MVFR ceilings with IFR ceilings
in the North Bay. Conditions gradually improve to VFR by the early
afternoon at most sites with lower confidence for clearing at HAF
and MRY. This clearing window will be short-lived as an early onset
of MVFR to IFR stratus is expected to roll in around/after sunset.
Overnight, there is a low chance (20%) for drizzle, mainly at HAF,
MRY, SNS, APC, and OAK. Ceilings are expected to hang on through
late Tuesday morning at the earliest. Moderate onshore winds
increase to 10-15 kt through the afternoon and ease below 10 kt
overnight as they shift south to southwest, depending on the local
terrain.
Vicinity of SFO...MVFR ceilings persist with moderate confidence on
a brief scattering period this afternoon. Southwest winds gradually
become westerly and increase to moderate through the day before
switching back to a light SW flow overnight. Borderline MVFR-IFR
ceilings return early this evening between 5-8 PM PDT with medium to
high confidence on the timing and heights. These conditions will
persist through early Tuesday afternoon as those westerly winds
increase. Current model guidance is hinting at a low, non-zero
chance for drizzle at the terminals early Tuesday morning, but
confidence is too low to include in the TAF.
SFO Bridge Approach...High chance for the bridge to clear before
the terminal and is expected to develop ceilings shortly after SFO.
Monterey Bay Terminals...MVFR stratus hangs on through this
afternoon. Low confidence on whether MRY will fully scatter out due
to the persistent onshore flow and expansive stratus feed over the
ocean. Overall, borderline MVFR-IFR ceilings develop at both
terminals early this evening right before sunset and drop to IFR
overnight through Tuesday morning. Medium confidence on exactly when
IFR ceilings develop. After midnight, there is a 20-30% chance for
drizzle to impact the terminals, though confidence is too low to
include in the TAF.
&&
.MARINE...
(Today through Saturday)
Issued at 1113 AM PDT Mon Sep 21 2026
The gradient between high pressure over the Pacific and lower
pressure over California is supporting a moderate to strong NW
breeze across the coastal waters. Winds are strongest north of Pt.
Reyes. The stronger winds will move south to impact the majority
of exposed waters over the next 36 hours. These winds will build
rough seas, with significant wave height peaking at 10-12 feet
Tuesday night. Conditions will gradually improve Wednesday through
Thursday.
&&
.FIRE WEATHER...
Issued at 423 AM PDT Mon Sep 21 2026
An upper-level low will support near to slightly below normal
temperatures, increased diurnal winds, a deep (2,500') marine
layer. Warming and drying trend kicks off Tuesday with
temperatures up to 10 degrees above normal and areas of moderate
HeatRisk Wednesday and Thursday. The marine layer will compress
below 1,000 feet during this warm-up with nocturnal drying
expected above it.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory from 3 PM this afternoon to 9 PM PDT
Tuesday for Pt Arena to Pt Reyes 0-10 nm.
Small Craft Advisory from 3 PM to 9 PM PDT Tuesday for Pigeon Pt
to Pt Pinos 0-10 nm-Pigeon Pt to Pt Pinos 10-60 NM-Pt Pinos
to Pt Piedras Blancas 0-10 nm.
Small Craft Advisory until 9 PM PDT Tuesday for Pt Arena to Pt
Reyes 10-60 NM.
&&
$$
SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...Navarrete
MARINE...Flynn
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Prepared by Boulder-Creek.com Weather at: Mon Sep 21 16:30:04 PDT 2026
From the National Weather Service
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