Rio Del Mar, California Weather Forecast Discussion

824
FXUS66 KMTR 300542
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
1042 PM PDT Mon Sep 29 2025
...New AVIATION, MARINE...
.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...
(06Z TAFS)
Issued at 1033 PM PDT Mon Sep 29 2025
The weak frontal boundary is draped along the coast, although the
impacts are petering out. Showers are now isolated and weak, and
continue on a drying trend. Ceilings will be a mix of IFR, MVFR
and VFR, with a good chance for fog at Santa Rosa after a rainy
day with the mid level clouds now clearing.
Vicinity of SFO...VFR ceilings expected through the night with
some a possible light shower or two. Tuesday will be drier with
high confidence in VFR conditions. Winds will likely remain out
of the WSW through the afternoon ahead of a second cold front that
will arrive on Wednesday.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...VFR ceilings should persist through the
night with a slight chance for a light shower before scattering
out Wednesday morning. Winds will pick up in the afternoon from
the SW at MRY and NW at SNS before more high ceilings roll in
associated with Wednesday`s front.
&&
.MARINE...
(Tonight through next Sunday)
Issued at 1033 PM PDT Mon Sep 29 2025
In the wake of a dissipating cold front a few scattered showers
will persist Monday night. Another 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. Occasional 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...Flynn
MARINE...MM
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NWS MTR Office Area Forecast Discussion