FXUS66 KMTR 160431
AFDMTR

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

...New AVIATION, MARINE...

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

 - Cooler temperatures through Thursday

&&

.UPDATE...
Issued at 847 PM PDT Tue Sep 15 2026

Satellite imagery this evening looks a little different than 24
hours ago. We've talked about the marine layer making a come back
and the evening satellite fog product shows it's definitely
happening. Stratus remains just north of Pt Reyes and stretches 
southward Monterey Bay. Interestingly enough coastal stratus then
disappears along the Big Sur coast. Why? Most likely lingering 
Big Sur jet combined with weak eddy effect per wind directions 
noted on weather stations. Do expect this area to eventually fill
in with clouds overnight. We have clouds, but what's the initial 
marine layer depth? Profilers and an ACARs flight from OAK show 
the depth in the 1100-1400 foot range. For the rest of the 
evening expect stratus to expand and move farther inland thanks 
to a solid 3mb plus onshore gradient.

No update needed this evening.  

Other item of note this evening is a solid heat signature. While
not in our forecast area, the Dome Fire near Yosemite has notable
heat on it cause some pixel saturation on a few ABI satellite
bands.  In other words, it's hot.  

&&

.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...
(06Z TAFS)
Issued at 925 PM PDT Tue Sep 15 2026

Currently a mixed bag of IFR-VFR. Stratus is at or within five miles 
of all terminals and rapidly expanding. As such, all terminals are 
expected to get MVFR ceilings tonight with a late-morning clearing 
time. Reasonable worst case scenario is an increase in onshore winds 
will delay or even prohibit clearing. Diurnal winds will prevail.

Vicinity of SFO...Currently VFR with westerly flow. Stratus is 
quickly advancing and is expected to form a ceiling shortly. MVFR 
conditions will likely prevail through the morning with an early 
return slated for tomorrow night. Diurnal winds will prevail.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Currently MVFR with westerly flow at both 
terminals. MVFR conditions will likely prevail through the morning 
with an early return tomorrow night. Stratus will stick close to the 
coast during the afternoon with a possibility that the terminals, 
especially MRY, remain MVFR through the TAF period. Diurnal winds 
will prevail. 

&&

.MARINE...
(Tonight through next Monday)
Issued at 925 PM PDT Tue Sep 15 2026

Moderate northwest to northerly winds prevail across the outer
waters as occasional fresh to strong gusts gradually diminish by
Wednesday afternoon. West to southwest moderate breezes dominate
over the Bays with strong gusts resulting in hazardous conditions
for small crafts on Wednesday. By Friday, widespread northwest to
northerly breezes increase to fresh. Wind driven rough seas
subside to moderate by Wednesday evening and will remain slight to
moderate into the weekend.

&&

.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...Sarment
MARINE...Navarrete

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Prepared by Boulder-Creek.com Weather at: Wed Sep 16 00:30:03 PDT 2026