FXUS66 KMTR 170101
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
601 PM PDT Wed Sep 16 2026
...New AVIATION...
.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
&&
.AVIATION...
(00Z TAFS)
Issued at 601 PM PDT Wed Sep 16 2026
A mixed bag of MVFR to VFR conditions persists across our region.
VFR conditions are not expected to last for long as current
satellite imagery shows stratus beginning to form near the North Bay
coastline. As we approach sunset, the stratus will become more
uniform and push into the Bay Area given the onshore flow support,
thus resulting in widespread MVFR conditions tonight. Monterey Bay
follows shortly after Bay Area terminals. Low to medium confidence
on MVFR ceilings reaching SJC and LVK overnight as the upper-level
low pressure passes overhead and lifts ceiling heights right above
criteria in the interior valleys. The 00Z OAK sounding measured the
top of the marine layer around 3100 feet, thereby indicating
widespread MVFR to borderline MVFR-VFR ceilings tonight. The
synoptic set up also supports the potential for drizzle across Bay
Area terminals, though confidence is too low to include in the TAF.
In terms of wind, gusty onshore breezes continue to impact APC, SFO,
SJC, and LVK into the evening. Overnight, widespread southwest to
southerly gentle breezes settle in before shifting back to the
typical diurnal pattern Thursday morning.
Vicinity of SFO...Leaned on a pessimistic forecast this TAF cycle
given that cloud ceilings are currently on the cusp of MVFR. They
are expected to drop slightly overnight, keeping both OAK and SFO in
MVFR territory. As the upper-level low moves east, ceilings are
expected to gradually lift and scatter out by late Thursday morning
with VFR prevailing through most of the day. Medium confidence on
the exact clearing time tomorrow. Strong gusty onshore winds persist
at SFO until the evening. Overnight, winds ease and become south-
southwest which eventually shift back to westerly by Thursday
afternoon.
SFO Bridge Approach...Similar to SFO with ceilings increasing in
height south of the bridge overnight.
Monterey Bay Terminals...VFR with gentle to moderate west to
southwesterly breezes. MVFR ceilings return around sunset and
persist into late Thursday morning. Winds ease to light breezes and
shift southerly overnight, but return to a west-northwest flow after
stratus scatters out on Thursday. Medium to high confidence for
ceilings to remain MVFR overnight.
&&
.MARINE...
(Tonight through next Monday)
Issued at 237 PM PDT Wed Sep 16 2026
Moderate northwest winds prevail across the outer waters with
occasional fresh to strong gusts. Moderate westerly breezes
dominate over the Bays with gusts up to 25 knots resulting in
hazardous conditions for small crafts through this evening. Winds
will decrease Thursday. 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 Mry Bay-SF
Bay N of Bay Bridge.
&&
$$
SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...Navarrete
MARINE...Flynn
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Prepared by Boulder-Creek.com Weather at: Wed Sep 16 18:30:03 PDT 2026
From the National Weather Service
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