777
FXUS66 KMTR 010438
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
938 PM PDT Sun Aug 31 2025

...New AVIATION...

.KEY MESSAGES...
Updated at 144 PM PDT Sun Aug 31 2025

 - Moderate risk for heat-related illnesses through Monday

 - Elevated fire weather concerns across the interior mountains
   into Monday with low RH and moderate onshore winds each
   afternoon

 - Hazardous coastal conditions through Monday night due to long
   period southerly swell

&&

.UPDATE...
Issued at 925 PM PDT Sun Aug 31 2025

Meteorological summer (Jun-Aug) ends at midnight and Sep 1, 2025
begins meteorological fall (Sep-Nov). High temperatures climbed
above to well above normal inland again today, it was hottest inland
far from the coastline and bays. Afternoon sea breezes did develop,
but the breezes were stuck beneath a compressed marine layer of
several hundred feet at best, limiting inland intrusion. Three
record highs were set today: San Rafael 100F (old 93F latest in
1998), Redwood City 96F (old 95F in 1943) and Oakland Museum 88F
(old 87F in 2005). However, even with recently hot temps, largely
because of robust cooling sea breezes and marine layer (cool PDO
phase through July) mainly earlier in meteorological summer much
of our forecast area has averaged a little below Jun-Aug 30 year
normals.

We continue to be located on the western side of a strong high
pressure system centered over the Desert Southwest. The larger
scale synoptic pattern over the northeastern Pacific, western
CONUS and Canada is sluggish with an amplified ridge extending
northward from the Desert Southwest. More recent positive (warm)
sea surface temperature anomalies to our northwest-west are likely
assisting in the slowing of the current synoptic pattern. The
northern hemispheric pattern is active with long wave troughs and
will eventually break through the pattern, but it`ll take time. 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.

We are watching the eventual arrival of an upper level low from
the southwest, expected to pass across our forecast area later
Monday and Tuesday. 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. Stay
tuned to the latest updates.

&&

.SHORT TERM...
Issued at 144 PM PDT Sun Aug 31 2025
(This evening through Monday)

Mostly clear skies across the region this afternoon with a few high
level clouds streaming in from the southwest. This is as high
pressure over the Desert Southwest continues to nudge into
California resulting in warm to hot temperatures across the
interior. Meanwhile, mid/upper level low pressure persists of off
the Pacific Northwest coast.

For tonight, looking like only coastal areas have a chance at seeing
low clouds developing early Monday morning as the warm/dry air mass
aloft continues to keep the marine layer compressed to between 250-
500 feet. Thus, another mild night is expected with temperatures
dropping into the 50s to lower 60s (interior valleys). However,
warmer and drier conditions will prevail above the shallow marine
layer and only cool into the upper 60s to near 80 degrees F.

On Monday afternoon, expected similar temperatures as today with the
interior reaching Moderate HeatRisk while coastal areas remain
cooler.

Some tips to avoid heat-related illnesses include:

* Stay hydrated and drink plenty of fluids.
* Wear lightweight, light-colored clothing.
* Reduce time spent outdoors or stay in the shade.
* Never leave people or pets unattended in vehicles.
* Use sunscreen if going to the coast or the pool.

&&

.LONG TERM...
Issued at 144 PM PDT Sun Aug 31 2025
(Monday night through next Saturday)

Monday night into Tuesday a weak cut-off low pressure will approach
from the southwest and become absorbed in the trough of off the
Pacific Northwest coast. This is when we have a non-zero chance of
high-based convection across the North Bay, East Bay and San
Francisco Bay Area. MUCAPE values range between 250-500 J/kg during
this timeframe. However, it appears the limiting factor will be the
available moisture in the 700-500 MB profile aloft. There is the
potential for this trough to result in coastal drizzle late Monday
night into Tuesday morning and again the following night.

A cooling trend will then persist from Wednesday into next weekend
as the marine layer deepens resulting in more widespread night and
morning low clouds.

&&

.AVIATION...
(06Z TAFS)
Issued at 936 PM PDT Sun Aug 31 2025

Currently VFR at all terminals with stratus lurking off the coast.
Highest confidence (albeit still only moderate) in coastal terminals
developing sub-VFR ceilings tonight as the marine layer will likely
be too shallow/compressed to reach interior terminals thanks to the
upper-level shortwave ridge. The marine layer will likely deepen
towards the end of the TAF period as an upper-level low approaches
the region, likely threatening sub-VFR conditions to at least
coastal and bayshore terminals.

Vicinity of SFO...Currently VFR with northwesterly flow. High
confidence in VFR and westerly flow prevailing through the TAF
period.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Currently VFR and calm at MRY and VFR with
northerly flow at SNS. Moderate confidence on the return of LIFR
ceilings to the terminals tonight. Stratus will stick close to the
coast tomorrow afternoon with an early return tomorrow night, likely
on the cusp of LIFR/IFR.

&&

.MARINE...
(Tonight through next Friday)
Issued at 238 PM PDT Sun Aug 31 2025

Fresh to strong northwesterly breezes will prevail through Monday,
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 will prevail
through Thursday.

&&

.FIRE WEATHER...
Issued at 731 PM PDT Sun Aug 31 2025

Warm to hot and dry conditions will continue to Labor Day,
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 Monday
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.

&&

.BEACHES...
Issued at 238 PM PDT Sun Aug 31 2025

Hazardous beach conditions will continue through tomorrow 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 Monday evening for CAZ505-509-
     529.

PZ...Small Craft Advisory until 3 AM PDT Tuesday for Pt Arena to Pt
     Reyes 0-10 nm-Pt Arena to Pt Reyes 10-60 NM.

     Small Craft Advisory until 3 AM PDT Monday for Pt Pinos to Pt
     Piedras Blancas 0-10 nm-Pt Reyes to Pigeon Pt 0-10 nm.

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

&&

$$

SHORT TERM...RGass
LONG TERM....RGass
AVIATION...Sarment
MARINE...Sarment

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