324
FXUS66 KMTR 051748
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
1048 AM PDT Sun Oct 5 2025

...New AVIATION...

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

 - Warm and dry weather through mid-week.

 - Disturbed weather returns late week.

&&

.UPDATE...
Issued at 750 AM PDT Sun Oct 5 2025

Areas of fog persist early this morning across the Monterey Bay
region and into the adjacent valleys. However, fog will begin to
clear out shortly as temperatures warm throughout the remainder of
the morning. Expecting sunny sky conditions this afternoon as
temperatures warm into the 60s near the immediate coast, lower 70s
just inland away from the coast, and the low 70s to lower 80s
across the interior. Offshore flow prevails across the higher
elevations as troughing persist over the Intermountain West and
high pressure continues offshore, this will allow for the
afternoon warming trend. The ongoing forecast remains on track
with no updates anticipated at this time.

RGass

&&

.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...
(18Z TAFS)
Issued at 1048 AM PDT Sun Oct 5 2025

Generally light winds are most terminals today under VFR, with
exception of Half Moon Bay where the cigs are hanging on a longer
than anticipated and the vsby is lower than expected at this time.
Winds will turn onshore and up valley at all locations by the
early afternoon hours, which will bring up the humidity and give
the chance for some low elevation clouds to pop up here and there.
As such, you`ll see "FEW" for cigs in some of the coastal TAFs,
like MRY. Winds will ease in the early evening then make a shift
overnight to become offshore. At this time the expectation is the
offshore winds will bring a drier air mass in and clear out cigs.
This pattern should remain into mid morning Monday. With the
offshore winds downsloping at HAF, opted to add in some gusts
there for a time too.

Vicinity of SFO...Just a few clouds lingering around this morning
due to residual moisture from overnight. However, with winds
backing to the WNW, moisture content will increase causing
humidity to plateau for the day. As such, opted to keep a "few"
deck in through the day and into the evening. Offshore flow will
develop overnight, bringing in a drying air mass. This will clear
out clouds and allow for full SKC in the overnight and early
morning hours. Look for onshore winds try and come back Monday
late afternoon into early evening. This could bring back a few
clouds.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...SNS will benefit from being inland today
with SKC and generally light to moderate winds. MRY is the
challenge as higher humidity is lingering just offshore. With
onshore winds developing this afternoon, the question is whether
or not it will push clouds into the terminal. For now have hedged
with a "SCT" deck this afternoon. If we get a stronger push, this
could become BKN or OVC. That said, models indicate an Otter Eddy
developing in the Bay which could bring SW winds to the terminal,
causing some downsloping winds off of del monte forest helping to
dry the air out and dissipating the clouds. Either way, by sunrise
Monday, offshore winds will develop, dry the air mass out, and
bring full P6SM SKC for a portion of Monday morning.

&&

.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.

&&

$$

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

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