Onizuka Air Force Base, California Weather Forecast Discussion

882
FXUS66 KMTR 292044
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
144 PM PDT Mon Sep 29 2025
...New SHORT TERM, LONG TERM...
.KEY MESSAGES...
Updated at 141 PM PDT Mon Sep 29 2025
- A cold front will bring rain and a 15% chance for thunderstorms
   today
- A second cold front will bring renewed chances for rain Tuesday
   night into Wednesday
- Warming and drying trend kicks off Thursday with an inside
   slider-like pattern beginning to develop by the weekend
&&
.SHORT TERM...
Issued at 141 PM PDT Mon Sep 29 2025
(This evening through Tuesday)
Pre-frontal rain showers are streaming into the North Bay with the
cold front right on the doorstep of Sonoma County. Rainfall totals
the last 12 hours have not amounted to much from pre-frontal rain
showers and/or drizzle with the highest total of 0.12" being
recorded atop Mt. Toro. Expect rain to increase in coverage and
intensity through the afternoon as the cold front makes landfall.
This first system is expected to bring up to 0.75" in far
northwestern Sonoma County along the North Bay Coastal Range with
totals significantly decreasing inland and south. Since the moisture
fetch from this system is from the subtropics (off to our
southwest), expect the highest rainfall totals along southwest
facing terrain (North Bay Coastal Range, Santa Cruz Mountains) with
northwest/southeast oriented valleys (North Bay Interior Valleys,
Santa Clara) getting rain shadowed. Some lightning has been observed
with the cold front while it was over the Pacific Ocean this
morning. There is a slight (15%) chance for thunderstorms in the
North Bay today with chances diminishing southward. The essential
ingredients for thunderstorms are lift, instability, and moisture.
With plenty of lift and moisture readily available throughout the
atmosphere, instability seems to be the limiting factor with MUCAPE
values forecast to be 200 J/kg or less. If a thunderstorm were to
develop, lightning, locally heavy rainfall, and erratic/gusty winds
would be the primary concerns. The cold front will slowly drift
southward tonight and into tomorrow morning as it gets pushed south
by high pressure building in from the Eastern Pacific Ocean ahead of
the second cold front slated for Wednesday morning. Drizzle/light
rain showers can still be expected tomorrow, but they will likely be
few and far between and favor the aforementioned southwest facing
terrain. A developing storm force low pressure system developing off
the Pacific Northwest Coast will pull in moisture from Typhoon 25W
(Neoguri) in the Northwestern Pacific Ocean. This will be the system
that will bring us Wednesday`s cold front. Pre-frontal rain showers
will begin Tuesday evening as the cold front approaches the North
Bay Coast from the northwest. The cold front will finally push into
the region Wednesday morning through Wednesday evening, bringing
with it increasing rainfall coverage and intensity. There are a lot
of similarities between the two systems, but the biggest differences
seem to be the direction in which the moisture is coming from and
how long it lingers. Today`s IVT forecast according to the GEFS and
ECMWF ensemble mean is expected to peak near 400 kg/ms and remain
over 250 kg/ms for about 18 hours whereas the second system is
expected to peak near 400 kg/ms and remain over 250 kg/ms for about
24 hours. PWAT (precipitable water) values are also forecast to be
higher with this second system (1.50+ inches versus the 1.23 inches
observed this morning). The slow progression of the cold front will
allow for training of rain showers Tuesday night through Wednesday.
Fortunately, nothing looks to be overly concerning, but any training
of rain showers/thunderstorms will pose the risk for flooding. This
second system is expected to bring up to 1.00" in far northwestern
Sonoma County along the North Bay Coastal Range, 0.25"-0.50" for the
rest of the North Bay, 0.10"-0.25" for the San Francisco Peninsula,
and less than 0.10" expected south of San Jose.
&&
.LONG TERM...
Issued at 141 PM PDT Mon Sep 29 2025
(Tuesday night through next Sunday)
Global ensemble clusters are in agreement that upper-level troughing
will continue off the West Coast through Thursday. By Friday, this
troughing will shift inland with upper-level ridging building across
the Eastern Pacific Ocean. This will result in an inside slider-like
pattern beginning to develop over the weekend. There is uncertainty
in the strength, location, and progression of this feature. A
stronger, closer, slower feature will bring more impacts than a
weaker, farther away, quicker feature. The location will really make
all the difference - if the upper-level low/trough can develop
closer to the coast, onshore flow may be able to be preserved
whereas the farther inland it is, the more likely that we will get
northerly/offshore flow. Fortunately, the rainfall that we are
expecting over the next 2-3 days will help at least somewhat ease
fire weather concerns, still it is a pattern worth watching.
&&
.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...Sarment
LONG TERM....Sarment
AVIATION...Murdock
MARINE...Murdock
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NWS MTR Office Area Forecast Discussion