Chualar, California Weather Forecast Discussion

003
FXUS66 KMTR 291829
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
1129 AM PDT Mon Sep 29 2025
...New AVIATION, MARINE...
.KEY MESSAGES...
Updated at 119 AM PDT Mon Sep 29 2025
- A cold front will bring rain and a slight chance for
   thunderstorms today
- A second cold front will bring renewed rain chances Wednesday
&&
.SHORT TERM...
Issued at 119 AM PDT Mon Sep 29 2025
(Today and tonight)
Fall is here and we have our first wet cold front of the season.
According to the OPC/WPC 06Z surface analysis, the front is
located about 300 miles west of Bodega Bay and will be heading
our way today. We are already seeing widespread southerly wind and
extensive cloud cover ahead of the front. As the cool, dense air
behind the front lifts the warm moist air mass, many of these
clouds will grow dark and bring some rain later today,
particularly for the North Bay. After a recent drying trend in the
forecast, the lastest update has stabilized with around 1/4"
expected in the North Bay and 1/10" along the Peninsula this
afternoon and evening. The rest of the area (East Bay, South Bay,
Central Coast) should only see a few hundredths if anything from
this first cold front. It now looks like the associated surface
low will make landfall around 12Z Tuesday. These landfalling lows
tend to serve as a good trigger for stronger convection if the
environment is unstable. The 06Z HRRR model soundings show surface
CAPE peaking around 200 J/kg this afternoon at Santa Rosa, with
0-6 km shear reaching as high as 50-60 kts ahead of FROPA around
18Z. The ingredients are all there, but they don`t line up
perfectly in time (first high shear, then CAPE, then trigger) and
while the chance for thunderstorms has increased, it remains
slight at around 15% in the North Bay from the late afternoon
through the evening.
&&
.LONG TERM...
Issued at 119 AM PDT Mon Sep 29 2025
(Tuesday through Sunday)
Brief ridging will build in after the first front on Tuesday.
This will bring drier, more stable weather, and perhaps allow the
sun to peak back out. Temperatures will remain suppressed as the
850 mb temperature drops below 10C for just the 2nd time since
June 22 (9.8C 00Z 9/10). This calmer weather won`t last long as a
second cold front arrives Wednesday. This system looks a little
more robust than the Monday system, with the West WRF ensemble
mean IVT peaking around 400 kg/ms and remaining above 250 for
around 30 hours from 30/18Z to 02/00Z. This moisture has the
potential to bring around 1/2" of rain to the North Bay and
between a few hundredths to 1/4" to the rest of the Bay Area.
Again, the Central Coast will be lucky to get more than a few
novelty drops. While the moisture looks pretty robust and the deep
layer shear persists, the instability looks a little more anemic
on Wednesday with 700-500 mb lapse rates around -5 C/km compared
to around -7 C/km on Monday. This should keep the chance for
thunderstorms lower than 10%, but still higher than zero. When all
is said and done, we should be off to a good start for the first
day of the 2026 water year. By Thursday, most of the interesting
weather will be behind us, but the details of the more subtle
pattern become uncertain. The ensemble cluster analysis reveals
significant uncertainty by Saturday with either weak ridging or
weak troughing possible. While there isn`t any real threat of
more rain, the exact temperatures are hard to pin down with this
uncertainty. Our official forecast shows a warming trend, while
the current WPC progs actually bring a dry cold front through on
Saturday.
&&
.AVIATION...
(18Z TAFS)
Issued at 1129 AM PDT Mon Sep 29 2025
Waves of light showers are moving into the region from the west.
Shower activity spreads across the region through the day. The main
rain band associated with the cold front arrives in the North Bay
this afternoon, spreads south and east into the evening, and pushes
into the Monterey Bay in the late night. Spotty lower CIGs will be
possible, while most cloud heights remain mid-level. Expect some
reductions in visibilities within the main rain band. As the front
pushes into the more interior areas, rain rates are expected to
reduce, limiting visibility reduction. As the rain band exits, cloud
cover and showers become more scattered with rain exiting the region
into the late morning and early afternoon on Tuesday. However
additional showers arrive that night.
Vicinity of SFO...Scattered light showers are moving through the
area with breezy southwest winds building this afternoon. The main
rain band arrives in the late evening, bringing slight reductions in
visibilities, and more consistent rain rates. Expect the rain band
to exit into the late night with spotty light showers lingering in
its wake. Shower activity exits into late Tuesday morning.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...Passing light showers are expected into the
night with mostly mid-level cloud cover and spotty lower clouds. The
main rain band from this cold front arrives in the late night, but
breaks apart as it builds into the Monterey Bay. This will lead to
more consistent rain chances, but will mostly be light. After the
front exits, light and spotty showers linger until the late morning
and early afternoon on Tuesday.
&&
.MARINE...
(Today through Saturday)
Issued at 1129 AM PDT Mon Sep 29 2025
Expect showers to continue to build across the waters through
early Tuesday. Embedded thunderstorms are possible across the
northern outer waters through early this evening. A second system
will bring additional rain showers to the waters late Tuesday
into Wednesday. Gentle to moderate southerly winds generally
prevail across the coastal waters through late this week. Isolated
to scattered fresh wind gusts are possible across the northern
waters as each system passes. Seas become moderate to rough and
build to 10 to 12 feet mid to late week and winds are expected to
strengthen again late week into next weekend.
&&
.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.
&&
$$
SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...Murdock
MARINE...Murdock
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NWS MTR Office Area Forecast Discussion