330
FXUS66 KMTR 132254
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
354 PM PDT Sun Sep 13 2026

...New AVIATION, MARINE...

.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 (

NWS MTR Office Area Forecast Discussion