De Anza College California Weather Forecast Discussion

307
FXUS66 KMTR 100445
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
945 PM PDT Thu Oct 9 2025
...New AVIATION, MARINE...
.KEY MESSAGES...
Updated at 106 PM PDT Thu Oct 9 2025
- Cool conditions continue
- Chances for light rain in the North Bay on Friday
- Increasing confidence for widespread significant rainfall early
   next week
&&
.UPDATE...
Issued at 555 PM PDT Thu Oct 9 2025
Very light rain is being reported across the North Bay and these
passing showers will continue through the next 24-36 hours. When
all is said and done the North Bay should have received somewhere
from a couple hundredths to around 1/10th of an inch from this
weak boundary. The rest of the Bay Area will be lucky to get more
than a trace as the moisture plume mixes with drier air as it
slides south through the day Friday. The associated mid-level
trough axis will move through Saturday, bringing some stronger
afternoon gusts than normal. Sunday will be a good day to clean
the gutters before much more widespread rain arrives early next
week. The details are still unclear, but unless the forecast
changes dramatically, it looks likely that from Mon-Tue much of
the cwa will receive somewhere around the normal amount of rain
for the entire month of October. That`s mostly a testament to the
time of year this rain storm is arriving. If this was mid-January
it would feel more routine. Overall it looks like a mostly
beneficial event that will put a significant damper on fire season
while bringing mostly minor impacts, and no real chance for main
stem river flooding.
&&
.SHORT TERM...
Issued at 106 PM PDT Thu Oct 9 2025
(This evening through Friday)
Satellite imagery depicts extensive cloudiness over the North Bay,
some cumulus clouds over the higher elevations of the Bay Area and
Central Coast, and lingering clouds over the Big Sur coastal region.
Radar returns show a narrow stationary boundary extending south
from Marin County to the inner waters off the San Mateo Peninsula
and the outer waters off the Central Coast. Our colleagues at the
WPC are analyzing this as a stationary front on land associated with
a surface low pressure system in the North Coast and a weakening
cold front offshore. Any remaining rainfall totals from this system
will be very light, a few hundredths of an inch at most. Another
shot of light rain comes through Friday, generally limited to Sonoma
County where a few hundredths of an inch will fall in the valleys,
and a tenth to two tenths of an inch in the coastal ranges.
Otherwise, the weather remains remains dominated by an upper level
low currently meandering off the Pacific Northwest, leading to cool
conditions with breezy and gusty winds each afternoon and evening
along with periods of clouds as weak cold fronts pass through the
region. High temperatures today and Friday range from the lower to
middle 70s in the interior valleys of the central bay Area, the
South Bay, and the Central Coast, along with the Bayside regions, to
the upper 60s in the North Bay valleys and the lower to middle 60s
along the Pacific coast.
&&
.LONG TERM...
Issued at 106 PM PDT Thu Oct 9 2025
(Friday night through next Wednesday)
The chance for rain clears out Friday night as the low pressure
system moves eastward into the Pacific Northwest, and leads into a
dry, but cool, weekend with highs broadly similar to the previous
couple of days, except in the North Bay where temperatures warm to
the lower to middle 70s on Sunday. Incidentally, around this time,
the remnants of Typhoon Halong in the northwestern Pacific will
begin to dissipate into the jet stream circulation to the south of
the western Aleutians and lose any sense of identity through the
weekend as its remnants make their way towards Alaska. Sometimes,
the remnants of western Pacific typhoons will recurve and impact
the West Coast, but in this specific instance, it looks like any
impacts will be well to our north.
Confidence is increasing in some form of rainfall, particularly
significant rainfall, for the early part of the next work week. As
the current low pressure system begins to dissipate heading into the
Northern Rockies, a second trough will start to develop over British
Columbia, gravitate offshore, and deepen in to a new low pressure
system that descends parallel to the West Coast, eventually reaching
the coast off central California. The result will be a rather fast
moving storm, but one which will likely be our first significant
rainmaker since the previous winter season. Probabilities of
precipitation in the Monday to Tuesday timeframe top out around 60-
80% across the forecast area. A preliminary estimate of potential
rainfall totals suggests that 1 inch of rain is possible in the
lower elevations with rainfall totals in the coastal ranges reaching
the 2-3 inch range. Please note that there is still a fair bit of
uncertainty with the rainfall totals at present. Taking the National
Blend of Models as a baseline, from Monday at 5 AM to Wednesday
at 5 AM, there is an 80% chance that the rainfall total in
downtown San Francisco falls between 0.0" and 2.4", and a 50%
chance that the total falls between 0.09" and 1.65". For Monterey
Airport, those ranges are 0.01"-2.2" and 0.58"-1.68" respectively,
while at downtown Santa Rosa, the respective ranges are 0.0"-2.1"
and 0.01"- 1.50". Right now, a significant rain event is more
likely than not, but the forecast does have time to evolve.
Wind threats will also arrive as the storm system comes through the
region, with strong gusts potentially developing on Monday and
Tuesday as the descending low causes a strong pressure gradient and
a jet streak comes through the region with 40-50 kt winds at 500 mb.
It is a little far out to mention any specifics, but gusts reaching
or exceeding 30 mph can not be ruled out. In addition, model data is
also suggesting a slight (10-20% probability) chance for
thunderstorms Monday night and Tuesday. This is when the region
would sit on the left exit region of the jet streak, a favored
region for divergent flow that would support a chance for
convection. Again, too early to say what the specifics will be. We
will continue to follow the system as it evolves and refine the
forecast as more model data arrives into the weekend.
&&
.AVIATION...
(06Z TAFS)
Issued at 928 PM PDT Thu Oct 9 2025
Patchy stratus along a very slow-moving frontal boundary currently
bisecting the Bay Area. Not anticipating stratus to hang around
long into the morning hours, with the exception of North Bay
terminals where cloud cover may linger most of the day Friday.
Slight chance of isolated showers invof North Bay terminals by
late morning. Otherwise VFR expected to prevail beyond about
sunrise Friday.
Vicinity of SFO...Slow-moving frontal boundary currently resulting
in MVFR cigs at the terminal will move off this evening, leading
to moderate confidence in VFR through the rest of the night.
Mostly clear through the day Friday with increasing cloudiness
during the evening associated with a weak front aloft. Friday
morning, winds are expected to remain in the 4-7 kt range but will
likely be from the S/SW throughout the morning hours before
onshore flow kicks in during the afternoon.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...Stratus currently building across the
region will linger through tonight but clear early, likely just
before or right around sunrise. Clear and quiet through the day
Friday.
&&
.MARINE...
(Tonight through next Wednesday)
Issued at 928 PM PDT Thu Oct 9 2025
Light winds persist over the waters through the next couple of
days. Steadier rainfall returns early Friday over the northern
waters as another weak front moves in from the northwest.
Northwesterly winds arrive Saturday and increase to a strong
breeze that night, 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...Behringer
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NWS MTR Office Area Forecast Discussion