FXUS66 KMTR 212357
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
457 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

&&

.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...
(00Z TAFS)
Issued at 457 PM PDT Mon Sep 21 2026

Marine layer remains entrenched along the coast and locally inland
already as of 00z and will spread inland efficiently and early 
this evening with a trough over the region. Low cigs to linger at 
least through 18-19z Tuesday before mixing out.

Vicinity of SFO...Low cigs to push over the terminal early this
evening, as early as 03z then linger at least through 18-19z
Tuesday.

SFO Bridge Approach...Stays clear through around 04z before
losing visuals through at least 17z Tuesday.

Monterey Bay Terminals...KMRY and KSNS already have ceilings and
high confidence they continue at least through 18-19z Tuesday.

&&

.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 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...RW
MARINE...Navarrete

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Prepared by Boulder-Creek.com Weather at: Mon Sep 21 20:30:04 PDT 2026