FXUS66 KMTR 141207
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
507 AM PDT Mon Sep 14 2026
...New AVIATION, MARINE, FIRE WEATHER...
.KEY MESSAGES...
Updated at 1240 AM PDT Mon Sep 14 2026
- Gustier, dry offshore winds across the Vaca Range and Altamont
Pass tonight into Monday
- Cooling trend through mid-week
&&
.SHORT TERM...
Issued at 1240 AM PDT Mon Sep 14 2026
(Today and tonight)
Longwave pattern over the West is heavily focused on the wound up
upper low sweeping through the N Rockies. Why is that important?
Behind this upper level feature surface high pressure is slowly
building across the Pac NW and into the Great Basin. Pattern
recognition in the fall-ish months leads to offshore flow
signals. As such, latest sfc pressure gradients from SFO-WMC
reveals a flip from onshore Sunday to trending offshore overnight.
While latest guidance has cut way back on offshore flow potential
from a few days ago to today, it is still offshore nonetheless.
The offshore gradient is projected to be -5ish mb, which is on
the lower end. Regardless current overnight observations show a
switch at Mt St Helena with gusts to 35 mph and lower RH. Not
expecting a big impact given the weakness of the offshore flow,
but will drive sensible weather with breezy, mild, and drier
conditions for N Mts and E Bay Hills through early Monday.
Elsewhere, mostly clear skies prevail overnight. The exceptions
are few high clouds passing overnight and some patchy stratus
along the coast/coastal waters from San Francisco Peninsula south
to Monterey Bay.
Speaking of pressure gradients, the N-S gradient has ramped up
while the W-E gradient has eased. As such, winds too will be less
than what was observed on Sunday. The stronger winds will shift
back over the coastal waters. That being said, still expecting
some breezy conditions for Monterey Bay, Pt Sur, and the Salinas
Valley. Temperatures today will rebound slightly with highs in
the 60s/70s coast and 80s to lower 90s inland...still a few
degrees below normal for mid-September.
Monday night - offshore flow eases as onshore flow returns. As
such, the marine layer will have a better shot at returning.
Latest guidance shows a better marine layer/stratus picture by
Tuesday morning with coastal clouds push locally inland into
Monterey Bay and Salinas Valley.
&&
.LONG TERM...
Issued at 140 AM PDT Mon Sep 14 2026
(Tuesday through Sunday)
Interesting longwave pattern develops heading into mid-week as
another upper low cuts off of the main flow aloft and drifts into
OR/NorCal. Overnight ensemble guidance shows the upper low is
pretty anomalously low for this time of year and packs a cold
punch too. Overall cooling trend is expected Tuesday through
Thursday. In fact, 850mb temps will drop down to 11 to 12 C, which
is an anomaly of -5 to -7. The record heat from last week will be
a distant memory as highs on Wednesday will be locked in the 60s
and 70s, with a few interior spots reaching into the mid 80s.
That's about 5 to 15 degrees below normal. The cold advection will
cause an uptick in winds too. Winds on Wednesday will be
strongest through the Delta, East Bay gaps/passes. The lower 500mb
heights and colder air will help to re-establish a more robust
marine layer Wednesday and Thursday with some guidance suggesting
drizzle along the coast. It does come with a price, nocturnal
drying above the marine layer may return over the highest peaks
Thursday morning.
Heading into next weekend the upper low fades and 500mb heights
rise and 850mb temps warm. Therefore the marine layer will begin
to compress and temperatures will trend back toward seasonable
levels.
&&
.AVIATION...
(12Z TAFS)
Issued at 507 AM PDT Mon Sep 14 2026
It's VFR except for patchy coastal stratus and fog /MVFR-IFR/. A
few additional patches of stratus and fog /MVFR-IFR/ will develop
during the morning hours then mix out by late morning. Coastal
stratus and fog /MVFR-IFR/ redevelop tonight and Tuesday morning.
Vicinity of SFO...VFR during the 12z TAF except BKN007 ceiling
forecast 10z Tuesday. West wind 5 to 10 knots except 15 to 25
knots from this afternoon to mid evening.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...VFR except tempo stratus /IFR/ at KMRY
13z-16z in the morning. A few patches of morning fog are also
possible. Coastal stratus /MVFR-IFR/ redeveloping tonight and
Tuesday morning. Light and variable winds in the morning becoming
onshore 7 to 15 knots in the afternoon to early evening, then
light onshore winds tonight and Tuesday morning.
&&
.MARINE...
(Today through Saturday)
Issued at 436 AM PDT Mon Sep 14 2026
Hazardous conditions for small craft will continue for the coastal
waters and Monterey Bay early in the week. Northwest winds will
ease by mid-week. Moderate to rough seas will prevail through
Tuesday before abating Wednesday.
&&
.FIRE WEATHER...
Issued at 436 AM PDT Mon Sep 14 2026
A few patches of coastal stratus have developed overnight. North
to northeast breezes have developed in the higher terrain. Offshore
winds will diminish by late morning or early afternoon and return
to diurnally breezy onshore winds in the afternoon and evening. A
surface to lower level trough of low pressure will develop along
the north Central Coast Tuesday through Tuesday night. Surface to
lower level troughing will expand to include the Bay Area Wednesday
through Thursday. This will bring higher humidity with good recoveries
from sea level to lower elevations to around 2500 feet, however above
this elevation especially along the higher ridgetops thermal ridging
and much lower humidity will persist during mid-week.
As
such, areas above the marine layer often experience nocturnal
drying that will likely continue through early this upcoming week.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory from 3 PM this afternoon to 9 PM PDT this
evening for Mry Bay-Pigeon Pt to Pt Pinos 0-10 nm.
Small Craft Advisory until 3 AM PDT Tuesday for Pt Arena to Pt
Reyes 0-10 nm-Pt Pinos to Pt Piedras Blancas 0-10 nm.
Small Craft Advisory until 3 AM PDT Wednesday for Pt Arena to Pt
Reyes 10-60 NM.
Small Craft Advisory until 9 AM PDT Tuesday for Pigeon Pt to Pt
Pinos 10-60 NM.
&&
$$
SHORT TERM...MM
LONG TERM....MM
AVIATION...Canepa
MARINE...Canepa
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Prepared by Boulder-Creek.com Weather at: Mon Sep 14 08:30:03 PDT 2026
From the National Weather Service
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