881
FXUS66 KMTR 090437
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
937 PM PDT Wed Oct 8 2025

...New AVIATION...

.KEY MESSAGES...
Updated at 115 PM PDT Wed Oct 8 2025

 - Cooler and cloudier weather begins today, drizzle tonight and
   Thursday morning

 - Light rain chances increase Friday through Saturday morning for
   the North Bay

 - More significant rainfall possible early next week

&&

.UPDATE...
Issued at 803 PM PDT Wed Oct 8 2025

The picture from space this evening shows an area of low pressure
west of southern California. For the Bay Area and Central Coast,
we have a mix of clear to mostly cloudy skies, with drier air
currently keeping the clouds at bay for much of the Central Coast
and portions of the Bay Area. The marine layer is still slated to
deepen tonight, allowing for low clouds to infiltrate much of the
Bay Area. For areas around the Central Coast, it`ll depend how
dry air plays out and where the southerly winds hold their
influence. Santa Cruz is currently cloudy and should hold. Clouds
may return late tonight, perhaps closer to near or around sunrise
for the southern rest of the Monterey Bay area.

&&

.SHORT TERM...
Issued at 115 PM PDT Wed Oct 8 2025
(This evening through Thursday)

Satellite imagery shows extensive stratus coverage across the North
Bay, barring interior Napa County, parts of the East Bay from
Concord and Mount Diablo west to the Bay, extending as far south as
Hayward, the city of San Francisco, western San Mateo County, Santa
Cruz County, and most of the southern Monterey Bay region. The
clouds will retreat towards the coast through the day, but most of
Sonoma and Marin Counties, in addition to western San Mateo County
and Santa Cruz County, are expected to remain socked in through the
rest of the day.

A pattern change today is causing cooler temperatures across the
region, as an upper level low off the Central Coast that contributed
to mild offshore flow yesterday was absorbed into a developing upper
level trough off the Pacific Northwest. A precipitous drop in
temperatures has been observed throughout all but the immediate
coastal region, with the RTMA analysis showing Bayside areas seeing
drops of 5 to 10 degrees from this time yesterday, while the inland
valleys seeing temperatures drop 15 degrees or more over the same
period. High temperatures top out in the middle to upper 70s for the
inland valleys, the lower to middle 70s for the Bayside areas, and
the middle to upper 60s for the Pacific coast. Tonight into Thursday
morning, a slight chance for drizzle arrives in the region with the
arrival of a weak surface low, which at the time of writing is
around 250 miles west of Point Conception, expected to continue
traveling towards the east. Any accumulating rainfall will be very
light, around a few hundredths of an inch at most. Otherwise,
Thursday`s conditions will be very similar to today`s.

&&

.LONG TERM...
Issued at 115 PM PDT Wed Oct 8 2025
(Thursday night through next Tuesday)

Temperatures remain remarkably stable into the upcoming weekend as a
couple of upper level trough axes marches through the West Coast.
Chances for light rain accompany one such trough axis on Friday
through Saturday morning, with rainfall totals once again remaining
light and non-impactful. The latest forecast shows wetting rains
(totals above 0.1") confined to the Sonoma Coastal range and the
Mayacamas, with a few hundredths of an inch in the Sonoma County
valleys and totals for the rest of the Bay Area and Central Coast at
0.01" at most.

As that trough axis moves onshore into the Pacific Northwest,
another axis develops from the same trough and reinforces the cooler
conditions through the weekend into the next week. That`s about as
much as can be confidently said about next week`s forecast. Ensemble
model clusters continue to depict a forecast where the impact levels
remain heavily sensitive to how the upper level pattern evolves.
Taking Day 6 (next Tuesday) as an example, potential scenarios
involve a stronger upper level low off the Pacific Northwest or
northern California, or a weaker low off Central California, perhaps
even an open trough over the West Coast. The scenarios that involve
the stronger low pressure systems are generally the ones that bring
more rain to the region than the ones with weaker lows or open
troughs. To give some numbers here: For the period from Monday the
13th at 5 AM, to Thursday the 16th at 5 AM, the National Blend of
Models gives an 80 percent chance that the rainfall total in
downtown San Francisco falls between 0 inches and 2.5 inches, while
there`s an 50 percent chance that the downtown San Francisco
rainfall total falls between 0.02 inches and 1.9 inches. Spare a
thought for our colleagues at Sacramento, Hanford, and Reno, who
have to deal with similar levels of uncertainty on top of the
possibility for significant snowfall in the Sierra Nevada. Needless
to say, the forecast will evolve and get refined over the next few
days.

&&

.AVIATION...
(06Z TAFS)
Issued at 936 PM PDT Wed Oct 8 2025

Very deep marine layer evident on the 00Z OAK sounding promoting
cloud cover largely unphased by terrain in the North Bay. Elsewhere,
some low level drying due to winds off of terrain allowing cloud
layers to mix out. This will likely change over the next few hours
as the trough moves through and promotes widespread cloud cover and
-DZ generally north of KHAF. Did introduce TEMPO -SHRA to account
for isolated pockets of "heavier" precip with a more intense line
of ascent progged to move through before sunrise. Otherwise
terminals outside the North Bay likely to clear to VFR by late
morning, lasting the rest of the period.

Vicinity of SFO...Cloud cover increasing by the midnight hour and
high confidence of -DZ through the morning hours. As mentioned
above, slight chance of heavier precip (-SHRA) just before sunrise,
but lower confidence in timing due to spotty nature. Relatively
light southerly winds through sunrise will give way to onshore flow
by the early afternoon. VFR through the rest of the period once the
-DZ ends mid-morning.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR. Lingering southerly component of the
winds will help keep stratus at bay, literally speaking, through
tonight. Slight chance of cloud cover increasing as temps cool
around sunrise, but moderate confidence in remaining VFR.
Otherwise, high confidence VFR forecast through the rest of
Thursday.


&&

.MARINE...
(Tonight through next Tuesday)
Issued at 840 PM PDT Wed Oct 8 2025

Gentle to moderate southwest winds will gradually shift back to a
northwest breeze Thursday. Northwesterly winds increase to a
strong breeze late Saturday, building rough seas by Sunday.
Disturbed conditions with periods of rain and gusty winds are
expected across the waters early next week.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...None.
&&

$$

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

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