FXUS66 KMTR 151806
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
1106 AM PDT Tue Sep 15 2026
...New SHORT TERM, LONG TERM...
.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...
(18Z TAFS)
Issued at 1029 AM PDT Tue Sep 15 2026
MVFR-IFR conditions continue directly along the coastline with VFR
across the interior. MVFR stratus returns again tonight with IFR
conditions possible directly along the coastline. Confidence is
low to moderate that stratus will not reach SJC or LVK overnight
but if it does occur, it likely will not be until after 09Z.
Highest potential for IFR conditions directly along the coastline. Onshore
winds pick up this afternoon to around 10-15 knots with light to
variable southerly winds overnight.
Vicinity of SFO...VFR through the rest of today with MVFR CIGs
likely to return overnight. Patchy stratus could arrive as early
as 04Z but it more likely will not arrive until 10Z. Onshore winds
to around 15 knots expected this afternoon and evening with
occasional stronger gusts possible. Stratus clears by late
tomorrow morning with a return of onshore winds around 15 knots
expected during the afternoon and evening.
SFO Bridge Approach...Generally similar to SFO but gusts may be
slightly stronger (closer to 20 knots) over the bay on the Bridge
Approach.
Monterey Bay Terminals...MVFR-IFR conditions persist at MRY
through the rest of today. Confidence is low that a ceiling will
fully mix out at MRY due to persistent onshore push of stratus
into MRY from the Monterey Bay region. If stratus does clear, it
likely will not be until after 19Z. MVFR-IFR CIGs return to SNS
overnight. Winds remain light through mid morning with westerly
winds around 12 knots expected today. Winds weaken overnight
becoming light to variable.
&&
.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 until 3 PM PDT Wednesday for Pt Arena to Pt
Reyes 10-60 NM.
&&
$$
SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...Kennedy
MARINE...Kennedy
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Prepared by Boulder-Creek.com Weather at: Tue Sep 15 14:30:03 PDT 2026
From the National Weather Service
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