502
FXUS66 KMTR 011148
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
448 AM PDT Mon Sep 1 2025

...New AVIATION...

.KEY MESSAGES...
Updated at 1234 AM PDT Mon Sep 1 2025

 - Moderate risk for heat-related illnesses continues today,
   gradual cooling Tuesday through the remainder of the week

 - Elevated fire weather concerns today across the interior
   mountains with low RH and moderate onshore winds

 - Hazardous coastal conditions through tonight due to long period
   southerly swell

&&

.SHORT TERM...
Issued at 1234 AM PDT Mon Sep 1 2025
(Today and tonight)

Another night of mostly clear skies across our area. However, there
is increased coverage of stratus in proximity to the coastline from
24 hours ago. We`ll see one more day of above normal temperatures
under the influence of the ridge to our east. The evening shift
update provided excellent context for tomorrows temperatures:

"In our forecast area we`ll have another hot day again tomorrow for
Labor Day with 90s inland, cooler at the coastline and bays. Record
highs tomorrow are very unlikely at the long term sites for at least
one reason, it`s the 8th anniversary of the blistering hot heatwave
in 2017 that e.g. resulted in San Francisco reaching current all-
time record high 106F (triple digits inland). The dynamic,
radiative, thermodynamic pattern was different then, we do not
expect the same for tomorrow by any means. Instead, San Francisco
forecast high tomorrow is lower 70s with a sea breeze, it should be
a nice day along the coast if planning to escape the inland hot
weather. During the warm season especially, please try to stay out
of direct sunlight and stay properly hydrated."

A low probability-high impact window of non-zero chances for
elevated convection opens later tonight and will extend through
Tuesday evening. Also summed up nicely by the evening shift:

"The 00z NAM model sounding forecasts show mid tropospheric
convective potential, but the forecasts are also showing convective
inhibition, even sampling forecasts along the back side sharply
defined moisture gradient is showing inhibition.
Cross sections southeast-northwest across our forecast area show
strong vorticity but mostly above 500 mb then gradually lowering
below 500 mb level Tuesday. 700-500 mb relative humidity is forecast
to stay disconnected from the vorticity. Even in the nearer term
there is still uncertainty if there`ll be convection over our area,
e.g. will there be energy transformation(s) taking place within the
500 mb wave that the models are unable to resolve properly? This
time of year Aug-Sep sometimes Oct too, seemingly innocuous lows can
surprisingly spring to life dynamically when least expecting it this
time of year. For the time being, the components to convection do
look a bit scattered and displaced from each other but it bears
watching and there remains a non-zero chance for convection."

&&

.LONG TERM...
Issued at 1234 AM PDT Mon Sep 1 2025
(Tuesday through Sunday)

Note the convective chances mentioned above bridging the Short Term
and Long Term forecast (into Tuesday). Beyond the thunderstorm
chances, we transition from the influence of the ridging currently
centered over the Four Corners region to the upper level disturbance
approaching from offshore. Tuesday will begin a pleasant cooling
trend through the remainder of the extended forecast. Overall, the
longwave synoptic pattern appears to become more active and
unsettled over the next couple of weeks, which should help keep
temperatures closer to normal or lower at the very least.

&&

.AVIATION...
(12Z TAFS)
Issued at 446 AM PDT Mon Sep 1 2025

IFR-LIFR stratus is returning to the immediate coast through the
morning, with some limited inland intrusion. Stratus is expected to
retreat to the immediate coastal region later this morning with
breezy onshore winds developing in the afternoon and evening. The
current TAF has HAF socked in through the TAF period, but high
resolution models are showing some signs of a temporary clearing. 00-
03Z looks to be the most likely period for VFR conditions if they
develop at HAF today. Tonight, an expanding marine layer should
allow for some more inland development of stratus, with moderate
confidence (30-50% probability) for stratus development at OAK and
low to moderate confidence (20-40% probability) for development in
the North Bay valleys.

Vicinity of SFO... UPDATE: The edge of the stratus deck has made
it to the terminal. Expect stratus impacts to be relatively
short-lived, with the stratus clearing by 15-16Z.

From the previous discussion: Breezy west- northwest winds
develop this afternoon and evening gusting up to 25 knots. High
resolution modeling shows that stratus flowing through the Golden
Gate tonight remains to the north of the terminal, so confidence
in stratus impacts is low. Nevertheless, the TAF ends with an SCT
group to cover the possibility, and will monitor how stratus
evolves in the evening.

SFO Bridge Approach... Lower confidence for stratus impacts
tonight along the approach path. Otherwise similar to SFO.

Monterey Bay Terminals... Satellite imagery shows the stratus deck
lurking offshore, but the terminals remain VFR at this time. A brief
period of LIFR-IFR ceilings is possible this morning, but high
resolution models suggest that the stratus could be patchy in
nature. Thus, the prevailing ceilings for this morning have been
replaced with TEMPO groups. Breezy onshore winds develop this
afternoon, with the stratus returning in the early evening hours,
persisting beyond the end of the TAF period into Tuesday morning
as the winds turn light and variable.

&&

.MARINE...
(Today through Saturday)
Issued at 422 AM PDT Mon Sep 1 2025

Fresh to strong northwesterly breezes will prevail through the
day, posing hazardous conditions for small craft across much  of
the waters. Northwesterly breezes will slowly diminish Tuesday
through Thursday to become moderate to fresh. Moderate seas
continue through Thursday.

&&

.FIRE WEATHER...
Issued at 1234 AM PDT Mon Sep 1 2025

Warm to hot and dry conditions will continue today, especially for
locations above 500ft. Widespread MinRH values between 10%-20%
combined with occasional wind gusts up to 30 MPH will result in
elevated fire weather concerns. We are also monitoring the potential
for elevated thunderstorms later this afternoon into Tuesday.
Chances are low, but if the pattern evolves with improved timing of
the mid-level moisture and best lift associated with the upper level
disturbance we could see some isolated convection across portions of
the Bay Area, East Bay Hills and North Bay, with cloud to ground
lightning and gusty winds the primary concern. Given the elevated
nature of the storms, if they develop, wetting rainfall is not
expected.

&&

.BEACHES...
Issued at 1234 AM PDT Mon Sep 1 2025

Hazardous beach conditions will continue through tonight due to
long period (18 seconds) southerly/southwesterly swell causing an
increased risk for sneaker waves and rip currents along southwest
facing beaches, including but not limited to: Stinson Beach, Santa
Cruz Boardwalk Beach, and Twin Lakes Beach. Sneaker waves can
unexpectedly run significantly farther up the beach than normal,
including over rocks and jetties. Rip currents are typically more
frequent and stronger in the vicinity of jetties, inlets, and
piers. It is always important, but especially so on a warm, dry,
holiday weekend, to exercise caution while at the beach. Never
turn your back on the ocean, remain out of the water to avoid
hazardous swimming conditions, keep pets on a leash, and wear a
life jacket.

Sarment

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Beach Hazards Statement through this evening for CAZ505-509-529.

PZ...Small Craft Advisory until 3 AM PDT Tuesday for Pt Arena to Pt
     Reyes 0-10 nm.

     Small Craft Advisory until 9 PM PDT this evening for Pigeon Pt
     to Pt Pinos 0-10 nm-Pigeon Pt to Pt Pinos 10-60 NM.

     Small Craft Advisory until 9 AM PDT Tuesday for Pt Arena to Pt
     Reyes 10-60 NM.

&&

$$

SHORT TERM...JM
LONG TERM....JM
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