FXUS66 KMTR 311129
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
429 AM PDT Mon Aug 31 2026
...New AVIATION, MARINE...
.KEY MESSAGES...
Updated at 1004 PM PDT Sun Aug 30 2026
- Morning drizzle likely along the coast
- Cooler weather continues this week
- Chance for light rain Wed-Thu in North Bay
&&
.SHORT TERM...
Issued at 1004 PM PDT Sun Aug 30 2026
(Tonight through Tuesday)
Marine layer stratus is becoming much better organized than 24
hours ago. Low clouds now cover the entire coast south of Pt.
Reyes. With a deeper, more organized marine layer, high
resolution models are suggesting coastal drizzle is likely Monday
and Tuesday morning.
The troughing pattern will deepen over the next 48 hours, keeping
high temperatures 5-15 degrees below normal for this time of year.
According to the ECMWF ensemble, there is an 90% chance the 850
mb temperature will drop below 12 C on Tuesday, which is the
bottom 10th percentile for this time of year. As the trough axis
approaches the coast, there is a good chance (roughly 70%) that
the North Bay will get some light rain starting Tuesday.
&&
.LONG TERM...
Issued at 1004 PM PDT Sun Aug 30 2026
(Tuesday night through next Sunday)
The jet stream becomes highly meridional (north to south) by
Wednesday. This makes the forecast more challenging. The 500 mb
low will linger around the Pacific Northwest, but the exact track
becomes much less certain, and the models are split if it will
become completely cut-off from the jet stream later this week or
not. Either way the cooler weather and occasional light rain
chances will continue through the rest of the week. The exact evolution
of this low could have a major impact on the weather next weekend
and beyond.
First some background. There is an tropical disturbance in the
Eastern Pacific well offshore SW Mexico. The National Hurricane
Center is currently calling this system Invest Area 95E as they
monitor for the further development needed to earn a name. This
system has a 90% chance of formation into a tropical depression in
the next 48 hours, and has a high chance of becoming a hurricane
by mid-week. This would be the 13th tropical cyclone in the
Eastern Pacific this season, so why am I writing about it?
Tropical cyclones are steered by large scale, mid-tropospheric
pressure systems. Typically the semi-permanent high pressure over
the subtropical eastern Pacific blocks tropical cyclones from
leaving the deep tropics. They very often ride the trade winds and
follow a WNW track after forming, moving into the open ocean.
With the deep trough over the west coast breaking up the typical
pattern, the door is open for this particular tropical cyclone to
take a more northerly track, roughly parallel to the coast of
Mexico.
To be clear, there is virtually no chance that a hurricane will
make landfall in California. While it's true the SST anomalies
are quite high (3-5 F) along this track, the actual water
temperature (70-74 F) is still far too cold to support a
tropical cyclone north of the Vizcaino Peninsula (mid Baja).
However, if the cut-off low stays strong enough, and moves far
enough south to pull the tropical cyclone quickly to the north,
there could be impacts from the remnants of this system in about a
week. The primary impacts would be rain and wind, and they would
likely be focused across Southern California. There are so many
moving puzzle pieces that it's impossible to be more specific at
this time, but the chance is there, and we'll keep an eye on it.
&&
.AVIATION...
(12Z TAFS)
Issued at 429 AM PDT Mon Aug 31 2026
Extensive stratus development with varying ceilings (MVFR to LIFR)
is occurring across the region and further expansion is expected
through sunrise. Stratus will retreat to the immediate coastal
region after sunrise with a moderate onshore pattern wind developing
in the afternoon and evening hours. Tonight through Tuesday morning
brings some uncertainty into the stratus forecast, as a decaying
cold front approaches the region and could mix out the lower layers,
thus inhibiting stratus formation. Opted to lean towards a more
persistent forecast for the time being.
Vicinity of SFO... VFR at present. Continuing to monitor MVFR
stratus around the terminal area with the possibility of SFO
remaining clear through the night. Low confidence on clearing time
if stratus does form over the terminal; high resolution model output
is attempting to keep the stratus over the terminal into the
afternoon hours. Moderate west-northwest winds develop in the
afternoon and evening before gradually diminishing into the
overnight period. Moderate confidence in stratus development tonight
through Tuesday morning.
SFO Bridge Approach... Slightly greater confidence in the
development of stratus that would impact the approach path.
Otherwise similar to SFO.
Monterey Bay Terminals... IFR stratus continues through the morning
hours. Clearing time post-sunrise is uncertain with some high
resolution model output keeping MRY under IFR-MVFR ceilings well
into the afternoon hours. Gentle to moderate northwest winds are
expected this afternoon and evening, with moderate confidence of
stratus developing in the late evening-early overnight hours.
&&
.MARINE...
(Today through Saturday)
Issued at 429 AM PDT Mon Aug 31 2026
Fresh northwest winds continue to diminish to a moderate breeze,
allowing rough seas to subside to moderate by the late morning.
The long period southerly swell continues to decrease through
Tuesday. Northwest to westerly winds remain gentle to moderate
through the remainder of the work week as slight to moderate NW
seas prevail.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory from 9 AM this morning to 3 AM PDT Tuesday
for SF Bay N of Bay Bridge.
Small Craft Advisory from 3 PM this afternoon to 9 PM PDT this
evening for Mry Bay.
Small Craft Advisory until 9 AM PDT this morning for Pt Arena to
Pt Reyes 10-60 NM.
&&
$$
SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...DialH
MARINE...DialH
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
Prepared by Boulder-Creek.com Weather at: Mon Aug 31 10:30:04 PDT 2026
From the National Weather Service
|