959
FXUS66 KMTR 151203
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
503 AM PDT Tue Sep 15 2026

...New AVIATION...

.KEY MESSAGES...
Updated at 1206 PM PDT Mon Sep 14 2026

 - Cooling trend and below normal temperatures through mid-week;
   return to seasonal temperatures late week into weekend

&&

.SHORT TERM...
Issued at 1204 AM PDT Tue Sep 15 2026
(Today and tonight)

Interesting to see some high clouds continue to stream over the
heart of the Bay Area early this morning. In fact, the stream of
clouds have been like a conveyor belt of upper level moisture over
the last 36-48 hrs. Why the conveyor belt like feed? While the
longwave pattern shows broad troughing over the West (including
CA) it also shows a weak upper level disturbance parked well NE
of the Hawaiian Island. The sub-tropical jet is the connection
between that weak disturbance and CA. Impactful? No. Interesting
to see a feed for a longer period of time? Yes. Now onto the
forecast.

Marine layer struggled to fully form last night due to dry air and
offshore flow. Tonight we`re dropping the offshore flow, but dry
air remains. As of midnight stratus still remains patchy, but as
the night moves on there is higher conf that stratus will become
more widespread over the coastal waters and move locally inland.
The limiting factor will be the lingering dry air.

Monday was a warm up day, but Tuesday will be a transition day in
the other direction. Broad upper trough will get a boast from a
highly amplified jet nosing into AK and the downstream trough
deepening. Onshore flow will kick a little more today as well.
Winds will be strongest along the coast, inland valleys, and
inland gaps/passes (Altamont). Onshore flow and dropping 500mb
heights will bring a general cooling of 3 to 5 degrees nudging
much of the region to below normal for this time of year.

Moistening in the low level and falling 500mb heights will once
again promote better stratus development Tuesday night into
Wednesday with the depth increasing of 1000-1500 ft for the marine
layer.

&&

.LONG TERM...
Issued at 1204 AM PDT Tue Sep 15 2026
(Wednesday through Monday)

How a about a little taste of late fall to begin the Long Term.
The aforementioned amplified jet will to drive another upper low
down the PacNW coast and into NorCal/OR. Latest guidance actually
cuts off the low from the main flow pattern. Not only are we
getting another upper low, but this one will be chilly for this
time of year with 850mb temps dropping close to 10C, which is an
anomaly of 5 to 7. So what does this mean for overall sensible
weather? The cooldown in temperatures will continue with highs
dropping another 3 to 8 degrees. Highs will be mainly 60s coast
and upper 60s to mid 80s interior. Onshore flow will bring gusty
conditions again. The marine layer will be a tricky one. Generally
speaking one would expect a deeper marine layer and that`s what
the forecast has Wednesday into Thursday. However, one always has
to watch stratus development as the airmass aloft begins to
approach the current SST. If they`re too close, stratus
production generally declines. Confidence is too low at this point
to remove stratus, but will monitor nonetheless. Given that the
stratus mid- week is tricky and not classic, impacts from extreme
nocturnal drying events are less likely.

The upper level low departs eastward late Thursday into Friday
with shortwave ridging to follow. Subtle warming trend develops
for next weekend with interior locations solidly returning to the
80s and far interior mid 90s.

&&

.AVIATION...
(12Z TAFS)
Issued at 503 AM PDT Tue Sep 15 2026

Coastal stratus and fog /MVFR-IFR-LIFR/ are developing with a
channel of cooler air moving in on northwest winds, including
development benefiting from radiative cooling to space through
daybreak. The marine layer depth varies from 1500 to 2000 feet.
Sep solar angle heat input and vertical mixing will slow stratus
and fog development during the day, then model forecasts show
greater areal coverage of stratus and fog /MVFR-IFR/ returning
tonight and Wednesday morning. Patchy light coastal drizzle is
a possibility, however drizzle is not currently included in the
12z TAFs.

Vicinity of SFO...VFR except tempo stratus /IFR/ 13z-17z this
morning. Stratus /MVFR/ redevelops by early Wednesday morning.
West wind 5 to 10 knots, increasing to 15 to 20 knots during the
afternoon and early evening. West wind decreasing to 5 to 10 knots
tonight and Wednesday morning.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Stratus and fog /MVFR-IFR-LIFR/ during
the morning, ceilings lifting to SCT vs BKN /MVFR/ today. Stratus
/MVFR-IFR/ moves inland early this evening, with stratus prevailing
tonight and Wednesday morning. Mainly onshore winds 5 to 15 knots.

&&

.MARINE...
(Today through Sunday)
Issued at 248 AM PDT Tue Sep 15 2026

Northwesterly breezes will diminish across the inner coastal
waters and the southern coastal waters today. Hazardous conditions
for small craft will linger across the northern outer waters
through Wednesday morning. Moderate to rough seas will prevail
through today before subsiding to become slight to moderate
Wednesday through Friday.

&&

.FIRE WEATHER...
Issued at 345 AM PDT Tue Sep 15 2026

The transport of cooler air on northwest winds relative to sea
surface temperatures in the upper 50s to lower 60s continues to
help produce coastal stratus early this morning. The marine layer
depth varies from 1500 to 2000 feet. Above the marine layer
temperature inversion, humidities are currently in the mid 20% to
30% range. The depth of cool air transport will increase which
will gradually increase humidities at higher elevations during mid
to late week. Before humidities reach a maximum, there may be
temporary, small areas of nocturnal drying at highest elevations,
coverage though should be minimal.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory until 9 AM PDT this morning for Pigeon Pt
     to Pt Pinos 10-60 NM-Pt Arena to Pt Reyes 0-10 nm.

     Small Craft Advisory until 9 AM PDT Wednesday for Pt Arena to Pt
     Reyes 10-60 NM.

&&

$$

SHORT TERM...MM
LONG TERM....MM
AVIATION...Canepa
MARINE...Canepa

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