839
FXUS66 KMTR 131800
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
1100 AM PDT Sun Sep 13 2026

...New SHORT TERM, LONG TERM...

.KEY MESSAGES...
Updated at 1056 AM PDT Sun Sep 13 2026

 - Breezy onshore winds this afternoon becoming lightly offshore
   overnight

 - Gustier, dry offshore winds across the Vaca Range and Altamont
   Pass tonight into Monday

&&

.SHORT TERM...
Issued at 1056 AM PDT Sun Sep 13 2026
(This afternoon through Monday)

Troughing continues to dominate the forecast as a deep upper level
low continues to move through the PNW and into the Intermountain
West. The marine layer remains at a depth of approximately 1500 ft
but dry air in the wake of last night`s dry cold frontal passage
prevented stratus from redeveloping across the interior last night.
The drier airmass is expected to remain in place through at least
midweek with another night of largely clear skies expected tonight.
Patchy stratus is possible directly along the coastline where
conditions are more supportive for stratus development. Wondering
why? Buoys are reporting warmer ocean temperatures than air
temperatures above it. This indicates an upwards transfer of
heat/moisture from the ocean to the air, resulting in a warming and
saturating layer of air where stratus is able to develop. Any
coastal stratus that does develop will be patchier with the San
Mateo coastline and coastal Monterey Peninsula the most likely areas
to see at least temporary cloud cover overnight.

Another factor in tonight`s stratus forecast is the wind. Winds
through this evening will be breezy and onshore with the SFO-WMC
gradient forecast to peak around +9 to +10 mb today. This largely
correlates to wind gusts between 20 to 35 mph with gusts up to 45
mph across the favored windy spots. By tonight the wind setup
changes from onshore to offshore. Offshore winds should remain on
the lighter but two locations, the highest ridgelines of the Vaca
Mountains and the Altamont Pass, will experience gustier offshore
winds between 20 to 35 mph overnight tonight through Monday.
Offshore winds will keep the region slightly drier overnight which
increases confidence that stratus will struggle to
redevelop/spread across the interior tonight. That being said, not
anticipating any major fire weather concerns given the short
duration and localized nature of the stronger offshore.

A slight warming trend will kick off Monday as the center of the
upper low moves into the Intermountain West and more zonal flow
dominates Central California. This will see a return of near normal
temperatures for much of the region with interior highs in the 80s
and coastal areas in the 60s to 70s. The hottest locations (far
interior East Bay and interior Central Coast) will reach the upper
80s to potentially the 90 degree mark. Winds to decrease throughout
the day with light to moderate onshore winds returning during the
day.

&&

.LONG TERM...
Issued at 1056 AM PDT Sun Sep 13 2026
(Monday night through next Saturday)

Zonal flow to slight troughing continues through midweek before
another deeper trough pushes into the Oregon and Northern California
border. Temperatures remain relatively seasonal on Tuesday with
highs in the upper 70s to 80s across the interior and 60s along the
coastline. The arrival of the upper level trough will bring a cooler
airmass into the Bay Area and Central Coast with 850 mb temperatures
peaking around 12 C during the day. That is roughly between the 10th
and 25th percentiles for this time of year with the median 850 mb
temperature ~17 C. The NBM was running a bit warmer than other
models for Wednesday/Thursday but the latest run has started to
lower temperatures, coming into better agreement. A reasonable lower
end temperature forecast for Wed/Thurs would be interior highs in
the low to mid 70s while the higher end forecast (the current
forecast) keeps high temperatures in the mid 70s to low 80s. Given
the arrival of the much cooler airmass, would not be surprised if we
saw the forecast high temperatures for Wed/Thurs to decrease by a
few more degrees (slightly closer to the lower end forecast). The
caveat to this being temperatures may be warmer if the trough/cold
air mass slows down and arrives later than currently forecast.
Regardless, the cool down will be most evident Wednesday and
Thursday with near normal temperatures in the upper 70s to mid 80s
Friday into the weekend. The arrival of the trough should allow the
marine layer to deepen enough that stratus will become more
widespread mid to late week. Diurnally breezy winds continue across
mountain gaps and passes, the coast, and along the highest
ridgelines. Northern California and the Sierra Range have some
potential for thunderstorms mid to late week as the trough moves in
but chances are negligible across our region (<5%) with
thunderstorms not expected.

&&

.AVIATION...
(18Z TAFS)
Issued at 1023 AM PDT Sun Sep 13 2026

VFR conditions persist at most terminals this morning except for
patchy coastal stratus along the San Mateo County coastline.
Onshore winds increase late this morning and will mix out the
stratus. These stronger winds will prevail through the evening
before easing into the night and into Monday morning. There is
moderate confidence for stratus to return to the coast late
tonight into early Monday morning. Any stratus that does develop
with dissipate by late Monday morning with increasing onshore
winds by the afternoon.

Vicinity of SFO...VFR. Northwesterly winds increase throughout
the morning and into the afternoon with gusts up to 33 kt
expected. Winds ease late in the night with some offshore wind
potential early Monday morning before returning to onshore by mid-
to-late morning.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR. Northwesterly winds increase
throughout the morning and into the afternoon with gusts up to
around 25 kt. Winds ease after sunset and potentially become
variable early Monday morning. There is also moderate confidence
for IFR-MVFR conditions to develop early Monday morning.

&&

.MARINE...
(Today through Friday)
Issued at 858 AM PDT Sun Sep 13 2026

Hazardous conditions for small craft prevail through the middle of
the week. Northwest winds strengthen through tonight with gale
force gusts across the southern inner coastal waters and the
Monterey Bay. A few gale force gusts are possible across the
northern inner coastal waters. Winds will remain elevated through
midweek before diminishing late week. Moderate seas will build to
between 8 and 10 feet and become rough by Monday in response to
the stronger winds.

&&

.FIRE WEATHER...
Issued at 422 AM PDT Sun Sep 13 2026

Patchy coastal stratus has developed overnight. 925 mb relative
humidities are higher over the Timber and Plaskett fires early
this morning. Tonight into Monday night, a light to gentle
offshore breeze (with occasionally moderate gusts) is forecast to
develop in the higher terrain as the mid/upper level trough axis
shifts into the northern Rockies. 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 SF Bay N of Bay Bridge.

     Small Craft Advisory until 3 PM PDT this afternoon for Mry Bay-
     Pigeon Pt to Pt Pinos 0-10 nm.

     Gale Warning 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 Monday for Pt Arena to Pt
     Reyes 0-10 nm-Pt Reyes to Pigeon Pt 0-10 nm.

     Small Craft Advisory from 3 PM Monday to 3 AM PDT Tuesday for Pt
     Arena to Pt Reyes 0-10 nm.

     Gale Warning until 3 AM PDT Monday for Pt Pinos to Pt Piedras
     Blancas 0-10 nm.

     Small Craft Advisory from 3 PM this afternoon to 3 AM PDT
     Tuesday for Pt Arena to Pt Reyes 10-60 NM.

     Small Craft Advisory until 3 AM PDT Tuesday for Pigeon Pt to Pt
     Pinos 10-60 NM.

&&

$$

SHORT TERM...Kennedy
LONG TERM....Kennedy
AVIATION...RGass
MARINE...RGass

Visit us at www.weather.gov/sanfrancisco

Follow us on Facebook, X, and YouTube at:
www.facebook.com/nwsbayarea
x.com/nwsbayarea
www.youtube.com/nwsbayarea