658
FXUS66 KMTR 082016
AFDMTR

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

...New SHORT TERM, LONG TERM...

.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

&&

.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...
(18Z TAFS)
Issued at 1042 AM PDT Wed Oct 8 2025

A southerly surge is supporting extensive stratus this morning
with high MVFR ceilings. These clouds are taking longer to clear
than expected, but signs of erosion are starting to show on the
satellite loop, and most terminals should be clear in the next
couple hours. Boundary layer averaged winds are from the south,
though the surface winds have a multiple forcing mechanisms that
will result in some erratic, gusty conditions through the
afternoon. The marine layer will expand considerably over the next
24 hours, bringing another round of stratus with higher ceilings
and a chance for drizzle across the Bay Area terminals Thursday
morning.

Vicinity of SFO...When will the ceilings break at SFO? That is the
pressing question for the 18Z TAFs. The trend in the high
resolution model guidance has been later and later and the
satellite and camera haven`t shown any improvement lately. There
is a decent chance that even 19Z is too early, and an amendment
may be needed. The other challenge today is wind direction. Winds
have been weak for the last 12 hours or so with aircraft soundings
reporting southerly winds just above the terminal. These winds
should filter down to the surface as the boundary layer becomes a
more unstable this afternoon. As the marine layer expands this
evening, returning ceilings will likely be VFR, though some
drizzle is possible particularly around 10-14Z.

SFO Bridge Approach...Similar to SFO

Monterey Bay Terminals...A sliver of dry air has weakened the
cloud deck and will contribute to an earlier clearing time today.
The wind direction forecast is low confidence with the southerly
surge and shifting surface winds today. The deepening marine layer
won`t be as saturated for these terminals, bringing less chance
for drizzle Thursday morning.

&&

.MARINE...
(Today through Monday)
Issued at 855 AM PDT Wed Oct 8 2025

Gentle to moderate SW winds will gradually shift back to a NW
breeze Thursday. Winds will increase to a strong NW breeze late
Saturday, building rough seas by Sunday. Disturbed conditions
with periods of rain and gusty winds are expected early next week.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory until 3 PM PDT this afternoon for SF Bay N
     of Bay Bridge.

&&

$$

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

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