FXUS66 KMTR 180051
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
551 PM PDT Thu Sep 17 2026
...New AVIATION...
.KEY MESSAGES...
Updated at 1211 AM PDT Thu Sep 17 2026
- Cooler temperatures continue today
- Warming trend Friday into the weekend
&&
.SHORT TERM...
Issued at 1241 PM PDT Thu Sep 17 2026
(This evening through Friday night)
A cut-off low overhead allowed the marine layer to expand to
nearly 5,000 feet this morning. This brought stratus to the vast
majority of the Bay Area, where it still remains early this
afternoon. The deep marine layer is also keeping temperatures
around 10 degrees cooler than normal for this time of year.
The cut-off low will gradually weaken and move off the
the NE over the next 24 hours. This will allow a short wave ridge
to slide in from the west, increasing temperatures and compressing
the marine layer Friday through Saturday.
&&
.LONG TERM...
Issued at 1241 PM PDT Thu Sep 17 2026
(Saturday through next Wednesday)
Saturday looks like the warmest day of the weekend as the 500 mb
short wave ridge axis moves over the Bay Area. There is also a
hint of weak offshore winds, but the SFO-WMC gradient isn't
expected to drop much below -2 mb. Another, much weaker, cut-off
low will move over Sunday, dropping the temperatures a bit.
Basically Saturday will be a couple degrees warmer than normal
and Sunday will be a couple degrees cooler than normal.
It now looks like a steady warming trend arrives next week. The
850 mb temperature (measured at about 5,000 ft) is expected to
hover around 20C (68F). That's around the 75th percentile for this
time of year. Not enough for a major heat wave, but enough to
bring surface temperatures 5-10 degrees above normal through the
middle of next week.
&&
.AVIATION...
(00Z TAFS)
Issued at 551 PM PDT Thu Sep 17 2026
VFR everywhere with gentle to moderate onshore breezes around 5-10
kt with breezy terminals reaching closer to 15 kt. Diurnal onshore
winds ease overnight and increase again Friday morning. As the upper
level low moves eastward out of our area, the marine layer will
gradually compress tonight, but will remain deep enough to reach all
(if not, most) terminals. The pattern is rather complex, so there is
currently low to medium confidence on the exact onset of stratus and
ceiling heights. Medium confidence for ceilings to remain within
MVFR territory with STS likely to be IFR. As high pressure builds in
overnight, stratus is expected to scatter out by mid Friday morning.
Vicinity of SFO...VFR. Onshore winds ease overnight as the marine
layer rolls in. Medium confidence on when MVFR ceilings develop.
Some model guidance hints at the possibility of borderline MVFR-IFR
ceilings around BKN007-010. The current TAF advertises BKN013-015 as
significant compression is not expected overnight to allow for IFR
ceilings. Stratus is expected to scatter out earlier around mid
Friday morning as westerly winds increase.
SFO Bridge Approach...Similar to SFO, but may develop MVFR ceilings
before SFO does.
Monterey Bay Terminals...VFR. Onshore winds ease overnight and shift
south to southwest. Low to medium confidence on the arrival of
stratus as well as ceiling heights given the complex pattern. The
marine layer is not expected to compress significantly enough to dip
into IFR territory, though it may be close. Stratus will scatter out
by Friday morning earlier than it did today and a moderate onshore
breeze around 10 kt will develop.
&&
.MARINE...
(Today through Tuesday)
Issued at 911 AM PDT Thu Sep 17 2026
Gentle to light breezes persist through Friday morning with mild
seas across the coastal waters. A shift occurs Friday afternoon as
northwesterly winds ramp up to fresh to strong breezes. Small
Craft Advisories take effect Friday afternoon for Monterey Bay and
the southern coastal waters from Pigeon Point south down to Point
Piedras Blancas. Winds remain enhanced through the weekend,
maintaining hazardous conditions for small craft while overall
seas remain light to moderate through Sunday.
&&
.FIRE WEATHER...
Issued at 421 AM PDT Thu Sep 17 2026
The coolest conditions and deepest marine layer are expected through
the day today as the upper level low stalls over northern
California. Current observations suggest a marine layer depth of
over 3000 feet, allowing for moist conditions to reach very high
into the higher elevations. As the low moves away on Friday, the
marine layer will compress, temperatures will rise, and relative
humidities in the higher elevations will start to decrease. Some
offshore flow is possible on Saturday morning, but it is not
expected to be very strong and the bulk of the offshore influence
will be felt on the higher terrain of the North and East Bay.
DialH
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory from 3 PM Friday to 3 AM PDT Saturday for
Mry Bay-Pigeon Pt to Pt Pinos 0-10 nm.
Small Craft Advisory from 3 PM to 9 PM PDT Friday for Pt Pinos
to Pt Piedras Blancas 0-10 nm.
&&
$$
SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...Navarrete
MARINE...Heinlein
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Prepared by Boulder-Creek.com Weather at: Thu Sep 17 22:30:03 PDT 2026
From the National Weather Service
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