FXUS66 KMTR 160032
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
532 PM PDT Tue Sep 15 2026

...New AVIATION...

.KEY MESSAGES...
Updated at 1051 AM PDT Tue Sep 15 2026

 - Cooler temperatures through Thursday

&&

.SHORT TERM...
Issued at 1051 AM PDT Tue Sep 15 2026
(This afternoon through Wednesday)

The marine layer has returned, but it's not uniform. A mesoscale, 
low-level, cyclonic eddy has formed off the coast of San Mateo 
County. This is producing easterly, offshore winds in the North Bay 
and westerly, onshore winds across the Central Coast. The morning 
sounding measured the marine layer at 1,200 feet deep over Oakland. 
Surface-based marine layer detection shows it's more shallow to 
the north and deeper to the south. The marine layer in Sonoma 
County is around 800 feet deep with near normal temperatures and 
mostly clear skies. Meanwhile it's around 2,000 feet deep across 
Monterey County, where max temperatures are around 10 degree 
below normal, and most of the coast was under overcast skies this 
morning.

The synoptic scale pattern features a trough over the west coast and 
a ridge over the SE US. At 500 mb, the trough will become cut-off 
over the next 24 hours and settle somewhere over Northern 
California. As a result, temperatures will be cooler than normal 
today and even more so tomorrow. This low will also bring stronger
winds Wednesday afternoon, with gusts of 20-30 mph expected across
the East Bay Hills, Salinas Valley, and both the San Francisco 
and Monterey Bays.

&&

.LONG TERM...
Issued at 1051 AM PDT Tue Sep 15 2026
(Wednesday night through next Monday)

The cut-off low will certainly deepen the marine layer Wednesday 
night, and it may provide enough lift to support some drizzle 
Thursday morning. After losing upper level support, the low will 
gradually weaken through the day. As a result, winds will be much
calmer Thursday afternoon. Temperatures will remain roughly 5-10 
degrees below normal.

The remnants of the cut-off will gradually move off to the NE and 
eventually get caught up in the jet stream flow again. We should be
on the backside of the trough by Friday, with a weak short-wave 
ridge building Saturday. This feature looks short-lived as another
cut-off low approaches the coast Sunday or Monday. Temperatures 
should return back to normal Fri-Sun with typical marine layer 
impacts.

By next week the forecast confidence really falls apart. A few of 
the deterministic global models show deep, reinforcing trough 
bringing much cooler weather. The ensembles, however, reveal that 
this is an outlier in the total guidance envelope. The ensemble mean 
is much more benign, and there is at least an equal likelihood of
warm ridging next week. The ECMWF ensemble actually shows 
anywhere from 5C to 25C 850 mb temperatures by the middle of next 
week. That pretty much covers all of the historical possibilities 
for this time of year. In other words, we don't yet know what's 
going to happen next week.

&&

.AVIATION...
(00Z TAFS)
Issued at 530 PM PDT Tue Sep 15 2026

MVFR stratus is hugging the coastline as seen on current satellite 
imagery, mainly impacting HAF and Monterey Bay sites. Interior East 
and South Bay valley terminals may be the only sites spared from 
stratus impacts this TAF cycle, with a 30-40% chance of development 
as advertised by the SCT decks. Coastal terminals are the first to 
develop MVFR ceilings which may drop to IFR overnight, followed by 
the remaining sites around midnight. Medium confidence on the exact 
arrival of stratus for Bay Area terminals. Moderate onshore breezes 
ease to gentle overnight before increasing to moderate again by 
Wednesday early afternoon.

Vicinity of SFO...Stratus has begun to flow through the Golden Gate 
which will allow for borderline MVFR-IFR ceilings to develop at OAK 
shortly after sunset. SFO will follow with MVFR ceilings around 
midnight. Both will remain under these conditions until late 
Wednesday morning. Moderate to gentle diurnally driven onshore 
breezes prevail through the TAF period. Medium confidence on the 
exact onset of MVFR ceilings and scattering times as they may occur 
a couple hours later than specified.

SFO Bridge Approach...Stratus coverage may be patchy overnight, but 
generally will be similar to SFO.

Monterey Bay Terminals...MVFR stratus is currently settling into the 
terminals. As the evening goes on, ceilings will gradually drop to 
IFR overnight and lift to MVFR Wednesday morning before scattering 
out by the early afternoon. Gentle onshore winds prevails through 
the TAF cycle. Medium confidence on the exact timing of when 
ceilings transition to IFR, back to MVFR, and eventually VFR.

&&

.MARINE...
(Today through Sunday)
Issued at 1029 AM PDT Tue Sep 15 2026

Northwesterly winds diminish across the inner and southern
coastal waters today with a moderate breeze and occasional fresh
gusts prevailing through late week. Strong northwesterly winds
diminish more slowly across the northern outer waters with
hazardous conditions continuing into Wednesday. Rough seas linger
through Wednesday before seas abate and become moderate.

&&

.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 from 9 AM to 9 PM PDT Wednesday for SF Bay 
     N of Bay Bridge.

     Small Craft Advisory from 3 PM to 9 PM PDT Wednesday for Mry Bay.

     Small Craft Advisory until 3 PM PDT Wednesday for Pt Arena to Pt 
     Reyes 10-60 NM.

&&

$$

SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...Navarrete
MARINE...Kennedy

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Prepared by Boulder-Creek.com Weather at: Tue Sep 15 18:30:03 PDT 2026