384
FXUS66 KMTR 300319
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
819 PM PDT Mon Sep 29 2025

...New UPDATE...

.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

&&

.UPDATE...
Issued at 811 PM PDT Mon Sep 29 2025

Minor changes were made to the forecast this evening as the front
remains unimpressive and the rain chances are not looking as good
for some areas. The best chances remain in the North Bay
counties, which is where we`ve seen the most persistent light
rain and reflectivity echoes. To the south chances have tapered,
likely sticking to the San Francisco Peninsula and perhaps some of
the higher terrain for Santa Cruz Co. East and South may see a
couple of sprinkles or a rogue shower, but not much rain
accumulation is expected to occur over night.

&&

.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...
(00Z TAFS)
Issued at 441 PM PDT Mon Sep 29 2025

Tail end of a weak front will sweep through the terminals this
evening bringing a mix of VFR, MVFR, and IFR. The chance for VCTS
has trended down and will not mention in any terminal forecast.
That being said, N Bay terminals will have precip first followed
by SFO/OAK and then the rest later tonight and early tomorrow.
Confidence is lower on all terminals receiving precip, but higher
that all terminals will see a drop in CAT to MVFR/IFR. Early
tomorrow conditions improve with mainly VFR in the afternoon
before another system approaches.

Vicinity of SFO...VFR with some light rain beginning 04-06Z and
then VCSH lingering into the Tuesday AM rush. Winds will be SW to
WSW. VFR Tuesday afternoon.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR conditions, but SNS will hold on to
borderline MVFR cigs this evening. Precip later tonight and early
Tuesday with MVFR cigs. VFR by tomorrow afternoon.

&&

.MARINE...
(Tonight through next Saturday)
Issued at 441 PM PDT Mon Sep 29 2025

A passing cold front will bring rain to the coastal waters this
evening, especially north of Pigeon Point. 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...MM
MARINE...MM

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