FXUS66 KMTR 162011
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
111 PM PDT Wed Sep 16 2026
...New SHORT TERM, LONG TERM...
.KEY MESSAGES...
Updated at 110 PM PDT Wed Sep 16 2026
- Cooler temperatures through Thursday
- Warming trend Friday into the weekend
&&
.SHORT TERM...
Issued at 110 PM PDT Wed Sep 16 2026
(This evening through Thursday night)
The marine layer was measured to be 2,400 feet deep by the 12Z
sounding from Oakland. Surface observations show it's closer to
3,000 feet deep along the Central Coast, where all of Santa Cruz
County remains under overcast skies. The NW wind direction created
some downslope dry air that managed to keep Silicon Valley clear,
but that was the exception. The morning stratus reached inland to
Healdsburg, Dublin, and Paso Robles. As a cut-off low over Northern
California continues to drift south, the marine layer will expand
over the next 24 hours. The deeper cloud layer will be enhanced by
some 700 mb vorticity and typical orographic lift to support some
drizzle overnight through Thursday morning. The signal is better
over the Bay Area than the Central Coast, but all of the typical
drizzle locations have a chance. In fact, all 50 of the ECMWF
ensemble members have some precipitation reaching SFO Thursday
morning. This is still drizzle though, so the accumulation will
range from a trace to a few hundredths before the atmosphere mixes
in some drier air by midday.
In addition to drizzle, the cut-off low will bring some strong
onshore winds this evening. The SJSU 2km WRF ensemble shows high
confidence in a onshore 8-10 mb SFO-WMC gradient developing quickly
this afternoon. This is a result of northerly winds west of the low
cooling the coast, while warm southerly winds east of the cut-off
support thermal exhaust and surface low pressure over northern
Nevada. The RRFS shows gusts up to 40 mph this evening in favored
gaps such as Pacheco Pass, Altamont Pass, and the Carquinez
Straight. Gusts up to 25 mph are likely in both the San Francisco
and Monterey Bays later today as well. These winds will peak in the
early evening, but continue well into the overnight hours. The
strongest winds are not widespread enough to warrant a wind
advisory, but expect to feel them while driving through the areas
mentioned.
Outside of drizzle and wind, the cut-off low will bring cool, humid
conditions to the entire Bay Area. Coastal temperatures will be in
the 60s along the coast and 70s inland. Some of the warmer places
will reach the upper 70s, but that's still roughly 10 degrees cooler
than normal for this time of year. We'll get a repeat of these cool
temperatures on Thursday before a slight warming trend kicks off
Friday.
&&
.LONG TERM...
Issued at 110 PM PDT Wed Sep 16 2026
(Friday through next Tuesday)
The cut-off low will have weakened and start to be reabsorbed into
the jet stream by Friday. This will open the door for a short wave
ridge to briefly build through Saturday. While short lived, this
feature will cause the marine layer to compress and temperatures to
return back to normal. We may even see some weak offshore winds at
this time. The SFO-WMC gradient is expected to flip negative by
Saturday morning, but there is no indication it will be very strong.
By Sunday a much weaker cut-off low will move in from the west. This
feature will likely keep conditions pretty consistent and relatively
benign through mid week. The 850 mb temperature is expected to hover
around 20C early next week, which is around the 75th percentile for
this time of year. By mid week the uncertainty balloons. Since the
pattern depends on multiple, hard to track cut-off lows, chaos
theory goes into full swing quickly. Ensembles shows an equal chance
of 850 temps reaching 5C or 25C next Thursday, which literally spans
the entire range of historical possibilities for this time of year.
While I like to look for signs of early rain or strong offshore
winds this time of year, with the high uncertainty it's not really
possible to say what's likely to happen beyond the 7 day forecast.
&&
.AVIATION...
(18Z TAFS)
Issued at 1024 AM PDT Wed Sep 16 2026
An interesting stratus pattern across the region today. Stratus
should clear within the hour at STS and APC with questions as to
if OAK and SFO will clear at all today. Model guidance suggests
patchy reductions in cloud cover are possible for OAK, SFO, MRY,
and SNS with the general prevailing conditions being MVFR. Kept
the TAFs slightly more pessimistic with MVFR conditions prevailing
through the day but there may be brief periods where cloud cover
is reduced and conditions improve during the day. Breezy onshore
winds are expected this afternoon and evening with winds between
10 to 15 knots and gusts to around 25 knots at OAK and SFO. With
the marine layer deepening to 3000 ft tonight it is likely that
OVC/BKN clouds will be at a higher depth tonight. This may result
in cloud cover on the border of MVFR / VFR criteria.
Vicinity of SFO...Stratus continues to push into SFO after
clearing briefly earlier this morning. Maintained a slightly more
optimistic TAF with VFR returning this afternoon. MVFR conditions
return late tonight and prevail through tomorrow morning. Gusts to
around 25 knots are expected this afternoon.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...MVFR conditions at both sites. Given the
continued push of stratus towards the interior, kept MVFR
conditions prevailing through the day at both sites. This may need
to be adjusted if cloud cover is able to break up more than
originally anticipated. Breezy westerly winds between 10 to 15
knots expected this afternoon with lighter winds overnight.
&&
.MARINE...
(Today through Monday)
Issued at 1024 AM PDT Wed Sep 16 2026
Moderate northwest to north winds prevail across the outer waters
with occasional fresh to strong gusts to diminish today. West to
southwest moderate breezes dominate over the Bays with gusts up to
25 knots resulting in hazardous conditions for small crafts late
morning through this evening. By Friday, widespread northwest to
north breezes increase to fresh with occasional strong gusts.
Wind driven rough seas to 9 to 10 feet subside to moderate by the
evening and will remain slight to moderate into the weekend.
&&
.FIRE WEATHER...
Issued at 412 AM PDT Wed Sep 16 2026
A passing trough is allowing the marine layer to expand with the
depth estimated around 2000-2500 feet in the area around the Timber
and Plaskett Fires. Above the marine layer inversion, humidities
remain low with relative humidity values as low as 15% observed. The
trend for deeper marine layers is expected to continue into Friday,
with humidities in higher elevations expected to rise as a result.
Some nocturnal drying is possible in the highest peaks, but minimal
coverage is expected.
DialH
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory until 9 PM PDT this evening for SF Bay N of
Bay Bridge.
Small Craft Advisory until 9 PM PDT this evening for Mry Bay.
Small Craft Advisory until 3 PM PDT this afternoon for Pt Arena
to Pt Reyes 10-60 NM.
&&
$$
SHORT TERM...MM
LONG TERM....MM
AVIATION...Flynn
MARINE...Flynn
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Prepared by Boulder-Creek.com Weather at: Wed Sep 16 14:30:03 PDT 2026
From the National Weather Service
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