Concord, California Weather Forecast Discussion

475
FXUS66 KMTR 301224 CCA
AFDMTR
Area Forecast Discussion...CORRECTED
National Weather Service San Francisco CA
524 AM PDT Tue Sep 30 2025
...New AVIATION, MARINE...
.KEY MESSAGES...
Updated at 1251 AM PDT Tue Sep 30 2025
- Scattered showers possible along the coast and the higher
   elevations through the day
- Another cold front brings renewed chances for rain tonight into
   Wednesday
- Slight warming and drying trend begins Thursday with offshore
   flow possible this weekend
&&
.SHORT TERM...
Issued at 1251 AM PDT Tue Sep 30 2025
(Today and Wednesday)
The short term forecast is dominated by a pair of cold fronts
stemming from a developing low off the coast of the Alaska Panhandle
and British Columbia, which is expected to pull in moisture from the
remnants of Typhoon Neoguri as it develops in the North Pacific. As
of midnight a band of showers associated with a weak cold front was
located off the coast of Monterey County, coming onshore in western
Santa Cruz and extreme southern San Mateo counties with the band
falling apart beyond the crest of the Santa Cruz Mountains. This
band is expected to continue diminishing as it travels southward
with any rain in the Central Coast limited to the coastal regions.
Any accumulations will be light and generally below a tenth of an
inch. To the north, scattered post-frontal showers linger through
the day across topographically favored locations across the North
Bay and into the Bay Area, again with generally light accumulations
expected through the evening.
A second weak cold front will then approach the region tonight
through Wednesday, bringing a new chance for rainfall. Pre-frontal
showers will begin tonight across the North Bay before the main rain
band comes through the region on Wednesday. Rainfall totals for this
system remain rather similar to the previous forecast. The mountains
of Sonoma and Marin counties should expect 0.5-1" with this system,
with 0.25-0.5" in the North Bay Valleys, 0.1-0.25" in the San Mateo
Peninsula, the western part of Santa Cruz County, and the East Bay
west of the Berkeley Hills and north of Oakland, and less than 0.1"
in the interior East Bay, South Bay, and the Central Coast.
High temperatures today remain around 5 to 10 degrees below
seasonal averages, with the inland valleys reaching the lower to
middle 70s and the coastal regions seeing the lower 60s to the
lower 70s.
&&
.LONG TERM...
Issued at 1251 AM PDT Tue Sep 30 2025
(Thursday through Monday)
Upper level troughing remains off the coast through Thursday. By
Friday, the trough moves inland with an upper level ridge building
in the eastern Pacific, with a possible inside slider developing
over the weekend. The ensemble model clusters continue to show
significant uncertainty in the location and strength of a potential
inside slider. This, in turn, keeps the uncertainty over the
potential impacts rather high. A weaker inside slider, or one that
develops closer to the northern part of California, may keep onshore
flow in place over the region, while a stronger inside slider that
develops over Nevada may turn the winds into a northerly/offshore
pattern. Wetting rains across the North Bay, and to a lesser extent,
the rest of the Bay Area, will likely alleviate fire weather
concerns, but not eliminate them.
Beyond the weekend, ensemble model clusters continue to hint at some
form of troughing persisting over the western United States through
the early part of next week. While conditions are expected to remain
on the drier side, the troughing will limit the expected warming,
perhaps leaving temperatures close to the seasonal averages to start
the next work week (middle 70s to lower 80s in the inland
valleys, middle 60s to lower 70s along the coast).
&&
.AVIATION...
(12Z TAFS)
Issued at 439 AM PDT Tue Sep 30 2025
It`s a complex set of TAFs this morning. Yesterday`s cold front
disrupted the marine layer, and there are still lingering isolated
showers this morning. Today will feature a recovering marine
layer and patchy low clouds this morning, before a second cold
front approaches Wednesday. Ahead of this next system, the
gradient will favor SW winds through the day, although local sea
breeze and diurnal valley effects may overcome this forcing at
times in the afternoon. Ceilings are also tricky as the morning
stratocumulus erodes in the late morning before frontal clouds
arrive in the afternoon. These frontal clouds will initially
arrive as broken high clouds before filling in and lowering
through the evening. Recent forecast trends have delayed the onset
of rain until late tonight in the North Bay and Wednesday morning
further south.
Vicinity of SFO...There are still a few isolated showers left over
from yesterday`s frontal passage, but the coverage and intensity
is too low to include explicitly in this set of TAFs. MVFR
ceilings should persist through 18Z, although since the clouds are
stratocumulus rather than the typical stratus blanket, the
confidence in the exact clearing time is lower. There is fairly
high confidence in VFR conditions under some high clouds through
the afternoon. These clouds will lower overnight before light rain
showers arrive Wednesday morning. Winds have remained S-SW over
the last 24 hours, and that trend is expected to continue ahead of
Wednesday`s cold front.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...MVFR-VFR ceilings are expected through
the morning before breaking up around 18Z. Ceilings will likely be
lower tonight as low level stability briefly increases before the
next cold front arrives Wednesday evening and disrupts the marine
layer again. These terminals will be a little further removed
from the SW gradient and the wind direction may be a little more
variable through the TAF period as a result.
&&
.MARINE...
(Today through Sunday)
Issued at 439 AM PDT Tue Sep 30 2025
A storm force low pressure system located 10 degrees west of the
Washington is generating a moderate-rough NW swell that will
arrive as 10-12 footers to the outer coastal waters Wednesday. A
cold front associated with this storm will also move through
Wednesday, bringing a wind shift from S to NW. This system will
bring gusty conditions and some rain showers as it moves through.
The swell will gradually subside Thursday before a strong NW
breeze develops Friday. This will bring another round of rough
seas and small craft conditions to the coastal waters through
Saturday.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory from 9 AM Wednesday to 3 AM PDT Thursday
     for Pt Arena to Pt Reyes 10-60 NM.
Small Craft Advisory from 3 PM Wednesday to 3 AM PDT Thursday
     for Pigeon Pt to Pt Pinos 10-60 NM.
&&
$$
SHORT TERM...DialH
LONG TERM....DialH
AVIATION...Flynn
MARINE...Flynn
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NWS MTR Office Area Forecast Discussion