FXUS66 KMTR 031152
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
452 AM PDT Thu Sep 3 2026
...New AVIATION, MARINE...
.KEY MESSAGES...
Updated at 1010 PM PDT Wed Sep 2 2026
- Beneficial rain Thursday, mainly in the North Bay
- Hazardous beach conditions this weekend
&&
.SHORT TERM...
Issued at 1010 PM PDT Wed Sep 2 2026
(Tonight through Friday)
The marine layer is gone. A cold front is currently located about
200 miles west of San Francisco and is supported in the upper levels
by a 500 mb trough and a meridional jet stream. We saw some pre-
frontal rain in the North Bay on Wednesday. Santa Rosa received
0.08" and Mt. Tam recorded 0.64". That was just the warm-up. The
main event will occur Thursday morning as the front moves through.
Models have been trending wetter, and we're now expected around 1/4"
in Santa Rosa through the morning and up to 1" in the mountains.
That's not enough to cause any major issues, but it is real rain,
especially for this time of year. The normal rainfall for the
entire moth of September is 0.15" at Santa Rosa. Expect slick
roadways in the North Bay Thursday morning.
As the front continues to march south, the associated rain should
cross the Golden Gate by midday, but it will be losing upper level
support and start to rapidly weaken. While a good portion of the
Bay Area should see some rain, it likely won't be more than a few
hundredths outside of the coastal mountains. The Central Coast
will be even drier, and will have to wait until Thursday evening
to see if any rain will fall.
In addition to rain, this front will bring NW winds with gusts up to
25 mph for most. Stronger gusts of 30-40mph are expected through
favored gaps/passes, and in the Salinas Valley. We'll obviously have
more cloud cover Thursday as well, keeping the temperatures cool
and comfortable.
After the front passes, skies will clear from North to South through
Thursday evening. As the skies clear, radiational cooling overnight
will help the marine layer will start to reform. While there is
still some uncertainty with the timing, marine layer stratus will
likely return to the North Bay by Friday morning, but may take until
Saturday or even Sunday to reach the Central Coast.
&&
.LONG TERM...
Issued at 1010 PM PDT Wed Sep 2 2026
(Friday night through next Wednesday)
Speaking of this weekend, a Beach Hazards Statement is in effect
from Friday through Monday as southerly swell from multiple distant
hurricanes reaches the coast. Days when it's nice outside but the
ocean is energetic are typically the most deadly. On top of that
it's a holiday weekend. Please refer to the Beaches section below
for more details.
Behind the front the temperature outside of the newly forming marine
layer will start to warm up through the weekend. At 500 mb the
trough over the coast will become a pseudo cut-off low. While it
detaches from the 500 mb flow, a healthy jet finger will continue to
provide some upper level divergence through the weekend.
Nevertheless the low pressure will gradually fill. Meanwhile high
pressure over the Southwest will start to influence the weather,
allowing temperatures to rebound back to normal by Monday.
As Hurricane Marie now looks like it will take a more westerly track
out to sea, the residual moisture impacts are much less likely to
reach any further north than Southern California this weekend.
Models are also showing a slug of dry air Sunday over the Central
Coast that blocks the less dense tropical air mass from moving
north. The deterministic ECMWF shows 1.8" of PW at Los Angeles
Sunday afternoon, while Monterey has 0.5". That would give
Southern California a decent amount of rain, but nothing for the
Bay Area. It's a similar story with instability. Not a single
member of the ECMWF ensemble has MUCAPE over 50 J/kg north of Pt.
Conception. There is always the chance that the tropical moisture
rides over this dry air mass, opening the door for dry
thunderstorms. There's no direct support for that in the model
guidance at the moment, but pattern recognition suggests it's
worth keeping an eye on.
While this weekend now looks like it will probably be dry, there is
a hint in the guidance that the remnants of Marie will eventually
get caught up in the persistent SW flow if the cut-off low can
survive long enough. If that happens, we may see some associated
moisture by the middle of next week.
&&
.AVIATION...
(12Z TAFS)
Issued at 438 AM PDT Thu Sep 3 2026
A weak cold front is on track to move through the Bay Area this
morning. This cold front will bring scattered showers in the
North Bay (higher confidence) and potentially into the rest of the
Bay Area (lower confidence). Stratus coverage is patchy this
morning with the incoming cold front keeping the marine layer
mixed out. After cold frontal passage, the marine layer looks to
return tonight (around 2000 ft) and will see MVFR CIGs along the
coast. Confidence is slightly lower as to the timing of stratus
return for interior sites with guidance indicating a later return
between 09-12Z likely. Winds initially start more southwesterly
this morning before shifting northwesterly again after cold
frontal passage. Breezy to gusty winds are possible as the front
moves through with winds to diminish after frontal passage.
Vicinity of SFO...Shower chances increase by late this morning and
continue through the early evening. The cold front is expected to
pass through SFO this afternoon/early this evening with the
highest potential for showers during this time. Breezy
southwesterly winds become more northwesterly by this evening
after cold frontal passage. Guidance is mixed as to if stratus
will return to SFO tonight as the marine layer redevelops. For the
12Z TAFs opted to keep VFR conditions at SFO tonight (similar to
the HRRR) but this may need to be updated.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...Patchy MVFR-IFR CIGs have been developing
over the Monterey Peninsula with ceilings coming in and out at
both MRY and SNS. Moderate confidence that ceilings will continue
to fluctuate at SNS through mid morning while confidence is low to
moderate that a ceiling will, at least temporarily, return to MRY
this morning. VFR conditions expected this afternoon with breezy
SW to W winds expected. The cold front will largely have fallen
apart by the time it reaches MRY/SNS but there is still a low
potential for drizzle to impact both sites late this evening.
&&
.MARINE...
(Today through Tuesday)
Issued at 438 AM PDT Thu Sep 3 2026
Rain chances increase over the northern waters this morning as a
weak cold front moves through the region. Southwesterly winds
gradually shift northwesterly by this afternoon after cold frontal
passage. Winds generally stay gentle to moderate but diurnal fresh
to strong gusts return within the Monterey Bay and along the Big
Sur Coast Friday into the weekend. More widespread fresh to
strong gusts return by early next week across the coastal waters.
Long period southerly swell associated with tropical activity well
to our south will reach the coastal waters Friday and continue
into early next week.
&&
.BEACHES...
Issued at 1010 PM PDT Wed Sep 2 2026
A Beach Hazards Statement is in effect Friday-Monday as the swell
from both Hurricane Marie and Hurricane Karina reaches the coast.
As these hurricane whip up the ocean, the waves travel over 1,000
miles in all directions, gradually flattening out to low
amplitude, but long period swell. These waves aren't very
intimidating if you're on a boat in the open ocean, but the long
period allows them to push much farther up the beach. Long period
swell always increases the chance for sneaker waves, but that's
particularly true this weekend since there are 2 different sets of
southerly swell, and their energy will periodically combine. In
addition to sneaker waves, expect strong rip currents and
breaking waves up to 10 ft. As this is a holiday weekend and the
weather is otherwise pretty nice, the exposure to the risk is
high. Avoid the water if you're not an expert swimmer. If caught
in a rip current, swim parallel to shore until you stop feeling
the pull. Even on the beach, be prepared for sneaker waves this
weekend and never turn your back on the ocean.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Beach Hazards Statement from Friday morning through Monday
evening for CAZ006-505-509-529-530.
PZ...None.
&&
$$
SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...Kennedy
MARINE...Kennedy
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Prepared by Boulder-Creek.com Weather at: Thu Sep 3 06:30:03 PDT 2026
From the National Weather Service
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