805
FXUS66 KMTR 051140
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
440 AM PDT Sun Oct 5 2025

...New AVIATION, MARINE...

.KEY MESSAGES...
Updated at 1250 AM PDT Sun Oct 5 2025

 - Warm and dry weather through mid-week.

 - Disturbed weather returns late week.

&&

.SHORT TERM...
Issued at 1250 AM PDT Sun Oct 5 2025
(Today and tonight)

The SFO-WMC gradient is -3.9 mb and strengthening. N to NE winds
prevail above the boundary layer. This offshore flow is bringing
drier air to the cwa. The marine layer is less than 1,000 feet
deep in the North Bay, and around 1,500 feet deep throughout the
Central Coast. The driest air today was actually measured on the
western slopes of Mt. Tam with an minimum RH of 23% due to a
combination of a drier air mass and adiabatic warming from the
downslope flow. With a deeper marine layer further south, the dry
air isn`t quite making it all the way to the coast south of the
Golden Gate, where there is still a blanket of marine layer
coastal stratus. Today will be a couple degrees warmer and
slightly drier than yesterday as the offshore flow becomes more
widespread and a bit stronger. Bottom line, expect another nice
day with comfortable temperatures and plenty of sunshine.

&&

.LONG TERM...
Issued at 1250 AM PDT Sun Oct 5 2025
(Monday through Saturday)

The offshore gradient between troughing over the coast and high
pressure over the Intermountain West will continue to strengthen,
eventually bottoming out around -9 mb Monday morning. That`s
not quite reaching Diablo wind territory (-12 mb, -15 mb for
strong event). The 06Z HRRR shows the strongest 925 mb winds over
NE Napa County reaching 25 knots from the NE Monday morning. While
weaker, the entire cwa will have some easterly component to the
925 mb winds by this point. The marine layer will also compress
and start to entrain some of this dry air Monday morning. High
pressure over the Rockies will hold through Tuesday, but the
troughing over California will weaken, relaxing the gradient and
calming the offshore winds. By Wednesday the surface high over the
Rockies will also start to weaken and move east into the Plains,
further reducing the offshore gradient, which will likely flip
back to neutral or onshore by midday.

The forecast becomes chaotic and highly uncertain late next week.
Ensemble clusters agree that a mid level deep trough or cut-off
low will move offshore of the Pacific Northwest by Thursday.
That`s where the agreement ends. Some solutions bring the low
onshore, dragging a pretty standard cold front and some light to
moderate rain across the cwa by the weekend. Others have the low
stall or even retrograde back west. Another complicating factor
is Tropical Storm Priscilla in the Eastern Pacific. This storm is
currently quite large and will very likely strengthen into a
hurricane by Monday morning as it drifts slowly NW parallel to the
west coast of Mexico. This system will begin to decay as it
encounters much colder water and a drier air mass by Wednesday.
However, the forward speed will increase and the residual moisture
plume will reach Southern California as early as Thursday. The
biggest question in the forecast is if this moisture will reach
the Bay Area before the cold front arrives and stops its forward
progress. It it does, the front will have a very juicy atmosphere
to lift, potentially bringing up to an inch or rain to parts of
the Bay Area. If it doesn`t, the offshore winds all week will
leave a drier than normal air mass, severely limiting rain
potential. To demonstrate the uncertainty, the NBM 72 hour
precipitation spread (10th-90th percentile) paints a stark
contrast. The wet scenario would bring 1.7" of rain to NW Sonoma
County through the weekend, while the dry scenario wouldn`t bring
a drop. Those outcomes are both equally likely at the moment, so
don`t put too much faith in the forecast details. All we can say
now is the next chance for rain arrives late next week, but we
can`t yet say how much will fall.

&&

.AVIATION...
(12Z TAFS)
Issued at 428 AM PDT Sun Oct 5 2025

Light offshore pressure gradient contributing to dry air in place
across much of the landmass. This will keep stratus contained
mostly to the coastline across the Bay Area. Patchy development is
likely through sunrise this morning, but overall anticipated quick
mixing out of any cloud layers and VFR through the day today.

Vicinity of SFO...Patchy stratus developing around the SF Bay this
morning, potentially affecting the terminal through sunrise.
Should remain in the vicinity, overall NOT anticipating solid
stratus coverage over the terminal and along approach. Any clouds
that do develop will have a hard time sticking around after
sunrise thanks to dry air in place from light offshore flow.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Less influence from the aforementioned
dry air as stratus is working its way well into the Salinas
Valley. Still dealing with at least some influence of dry air as
stratus is shallow and patchy at times, highly sensitive to light
airflow invof terrain. Can expect some fluctuations along the
coast, particularly KMRY. Therefore, low confidence in prevailing
stratus, but high confidence in VFR into this afternoon.

&&

.MARINE...
(Today through Friday)
Issued at 428 AM PDT Sun Oct 5 2025

Winds continue to diminish across the waters. Gentle breeze
today, becoming light through the rest of the week. Seas abate
with the wind through the week. Next upper level disturbance
anticipated by the end of the week with winds increasing across
the outer waters by Thursday.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory until 3 PM PDT this afternoon for Pt Arena
     to Pt Reyes 10-60 NM.

     Small Craft Advisory until 9 AM PDT this morning for Pigeon Pt
     to Pt Pinos 10-60 NM.

&&

$$

SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...Behringer
MARINE...Behringer

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