FXUS66 KMTR 220522
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
1022 PM PDT Mon Sep 21 2026
...New AVIATION...
.KEY MESSAGES...
Updated at 1120 AM PDT Mon Sep 21 2026
- Cool today
- Warmer temperatures Wednesday - Thursday
- Cooler weather returns Friday
&&
.UPDATE...
Issued at 826 PM PDT Mon Sep 21 2026
Stratus is expanding across the inland valleys in the wake of the
cut-off low, which is moving across the southern part of the
Central Valley into the Mojave Desert. Coastal drizzle was
reported in the late afternoon hours, and will be added to the
forecast through the night with the upcoming forecast package. No
other changes to the forecast.
DialH
&&
.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...
(06Z TAFS)
Issued at 1021 PM PDT Mon Sep 21 2026
Extensive MVFR-IFR stratus has flowed into the North Bay and the
Monterey Bay region, and continues to develop in the SF Bay region.
A deep marine layer should allow many of the inland valleys to see
ceilings overnight before the stratus retreats back to the immediate
coast late Tuesday morning. Low to moderate confidence in temporary
scatterings at HAF and MRY around the 19-22Z timeframe. Gentle to
moderate onshore winds will develop Tuesday afternoon and evening.
Stratus forecasts for Tuesday night into Wednesday morning are lower
confidence, particularly away from the immediate coast, as a
compressing marine layer should keep stratus away from many inland
regions but lower ceilings and visibilities where it does form.
Vicinity of SFO... A tricky forecast for the next several hours as
the stratus feed through the Golden Gate gets deflected to the
north, leaving the terminal area VFR. High resolution model output
has the terminal getting MVFR ceilings imminently, but the models
have not captured the current situation well and have kept the
terminal VFR for the time being. If stratus forms over SFO expect
the clouds to scatter out late Tuesday morning, with a moderate west
breeze developing in the afternoon and evening hours. Low confidence
of stratus returning Tuesday night, which most of the high
resolution guidance has backed away from and may be impacted by a
compressing marine layer.
SFO Bridge Approach... MVFR stratus may arrive before or after it
gets to the terminal tonight, depending on how exactly the stratus
develops with SF Bay. Otherwise similar to SFO.
Monterey Bay Terminals... MVFR ceilings at present with IFR
conditions expected later tonight. Stratus will scatter out Tuesday
late morning to the early afternoon, with moderate confidence in a
brief scattering period at MRY near 19-22Z. Stratus is expected to
return to MRY late in the afternoon and persist into the evening. At
SNS, confidence is lower as the compressing marine layer may keep
the deck away, but on the flip side, the compression could also
lower ceilings and visibilities at MRY on Tuesday night.
&&
.MARINE...
(Tonight through next Sunday)
Issued at 826 PM PDT Mon Sep 21 2026
Moderate to fresh northwest breezes persist overnight with
locally stronger winds north of Point Reyes. By Tuesday afternoon,
widespread strong northwest winds prevail across the coastal
waters, allowing for seas to build from moderate to rough.
Northwest winds and seas remain elevated through the week with a
brief period of improved conditions Wednesday night into Thursday.
By Friday, winds increase with building rough seas.
&&
.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 until 5 AM PDT Wednesday for Pt Arena to Pt
Reyes 0-10 nm-Pt Arena to Pt Reyes 10-60 NM.
Small Craft Advisory from 3 PM to 9 PM PDT Tuesday for Pigeon Pt
to Pt Pinos 0-10 nm.
Small Craft Advisory from 3 PM Tuesday to 3 AM PDT Wednesday for
Pt Pinos to Pt Piedras Blancas 0-10 nm.
Small Craft Advisory from 3 PM Tuesday to 5 AM PDT Wednesday for
Pigeon Pt to Pt Pinos 10-60 NM.
&&
$$
SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...DialH
MARINE...DialH
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Prepared by Boulder-Creek.com Weather at: Mon Sep 21 22:30:03 PDT 2026
From the National Weather Service
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