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FOUS30 KWBC 151600
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1100 AM EST Sat Nov 15 2025

Day 1
Valid 16Z Sat Nov 15 2025 - 12Z Sun Nov 16 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHERN CALIFORNIA...

...16Z update...

No significant changes were made to the outlook given 12Z HREF
guidance showed similar overall to 00Z hires components (some
higher, some lower regarding QPF). Moisture analysis this morning
showed record to near record moisture for November east of a strong
offshore closed low into southern California, southern Nevada and
the lower Colorado River Valley. An initial band of heavy showers
with embedded thunder came across the Channel Islands into the
coast of southern California between 12-15Z with observed
15-minute rainfall totals between 0.5 and 0.75 inches in the Santa
Monica Mountains. Strong ascent ahead of a negatively tilted
shortwave trough is expected to bring another round of high rain
rates into the southern California coast late this morning into the
afternoon with potential for additional rounds of convection
beyond 00Z.

As forcing moves inland through the remainder of the day and
overnight, peak hourly rainfall in excess of 0.5 inches will move
through the desert regions of southern California into southern
Nevada/Utah and portions of Arizona where weak (<500 J/kg) CAPE is
expected to be present. The highest rainfall through 12Z Sunday
is expected within the Transverse Ranges where an additional 3 to 5
inches (locally higher possible) is expected with 1 to 2 inches
(max 3 inches) expected elsewhere.

Otto

...previous discussion follows...


A large and energetic upper level low centered off the coast of
southern California will approach and make its way inland during
this period. As the low approaches this morning, it will draw a
very moisture-rich moisture plume into southern California, due to
its connection with the tropics. This will be in the form of an
atmospheric river. While it has been raining in light to moderate
intensity over much of the overnight period, when this atmospheric
river approaches, it will cause a notable increase in rainfall
intensity. This is due not only to the moisture content of the
atmosphere increasing to nearly 1.5 inches PWAT, but also the
advection of instability. 400 J/kg of MUCAPE is expected at the
peak of the event late this morning. This will support scattered
elevated convection. Thunderstorms with maximum rainfall rates to 1
inch per hour are likely across much of coastal southern
California today.

Rainfall totals yesterday broadly ranged from between 1/2 and 1
inch across the L.A. basin to 1.5 to 2 inches from Santa Barbara
west to Point Conception. Today with multiple rounds of showers and
thunderstorms capable of heavy rain, expect those amounts to at
least double from San Diego north to Point Conception. As usual in
these strong southerly flow setups, the heaviest rains will be in
the Transverse Ranges north of L.A., with likely runoff impacting
the adjacent urban areas. Given the area`s sensitivity to heavy
rain, the Moderate Risk was largely left the same with this update,
albeit with a small southward expansion over all of Orange County
and far northern San Diego County. Otherwise there were no major
changes as the event remains on track with good agreement in the
guidance.

Around midday, the back edge of the heaviest rains will move
across the coast, reducing further rainfall to scattered off-and-on
showers for much of the rest of the forecast period. The vigorous
shortwave forcing the heaviest rain will push north into the
interior of the Southwest, ending the heavy rain threat at the
coast. Thus, the heavy rain threat will continue further inland
across the southeastern California deserts, as well as into
southern Nevada and western Arizona. The greatest forcing inland
will be across the southern Sierra Nevada from Bakersfield north
and east to southwestern Nevada. It`s in this region that the
greatest threat for inland flooding will be present, and where a
higher end Slight Risk remains in effect. Given it`s November, the
higher elevations of the southern Sierra Nevada will see multiple
feet of snow from this event.

For Las Vegas, expect light rain to impact the city and points west
for much of the morning, but the heaviest rains will move through
from midday into the evening. Being further inland with a bit less
moisture to work with, a Slight Risk remains in effect for that
area with few changes to the overall forecast.

Finally, the upper low itself will move ashore in the late
afternoon, then track northeast to near the Lake Tahoe region by
12Z. Wraparound rainfall, while much lighter, will persist from the
Bay Area south to Santa Barbara, which could worsen any ongoing
flooding occurring there late tonight.

Wegman

Day 2
Valid 12Z Sun Nov 16 2025 - 12Z Mon Nov 17 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF
CENTRAL AND NORTHERN CALIFORNIA AS WELL AS FOR SOUTHWEST UTAH...

...Central and Northern California...

The upper level low and its associated energy that brought the
heavy rainfall Day 1/today will rapidly lift northeastward into the
Intermountain West and weaken through the day, meaning its
influence on the sensible weather across California will be limited
to the very early morning. Much of the rainfall associated with
this Marginal Risk will be associated with another deep trough that
will dig southward and cut off by 12Z Monday. As the low tracks
roughly parallel to the coast, additional shortwave energy
embedded within the low will force widespread light to moderate
rainfall across much of northern California on Sunday. The low`s
trailing cold front will cause the precipitation shield associated
therewith spread southward down the coast into late Sunday night.
The Marginal for points south of San Luis Obispo will be mostly due
to heavy rainfall in the area from Saturday, since the duration and
intensity of rainfall during this Day 2/Sunday period will be
rather minimal. Regardless, rainfall amounts overall across all of
northern California will be significantly lower than the heavy rain
expected across southern California today. Since most of the area
can handle the inch or less of rain expected in most areas away
from the favored upslope areas of the northern Sierra Nevada and
the Klamath mountains.

...Southwest Utah...

As the shortwave from today lifts north across Nevada and into
Idaho, there will be a prolonged period of SSW flow into
southwestern Utah on Sunday. Further, weakening but still potent
secondary shortwave energy riding south and east of the first upper
shortwave will cause both shortwaves to turn negatively tilted
during the day. This will increase the lift favorable for heavy
rain. Across the Pine Valley Mountains area of southwest Utah, the
mountains there are the first a south-southwest flow up the
Colorado River Valley encounter. Thus, some guidance is hinting at
outsized rainfall amounts that could impact the area due to the
added upslope and locally greater moisture moving into those
mountains. Flooding potential will also be increased in this area
due to the presence of a burn scar with a history of flash flooding
resulting from heavy rain. A Marginal Risk was added to this area
due to the multiple pieces of guidance showing higher heavy rain
potential.

Wegman

Day 3
Valid 12Z Mon Nov 17 2025 - 12Z Tue Nov 18 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE LOS
ANGELES BASIN OF SOUTHERN CALIFORNIA...

A cutoff low tracking south down the coast of California will move
across the hard-hit areas of the Los Angeles Basin on Monday. As it
moves through, the upper level cold air associated with the low
will once again increase instability around the low. Thus, expect
another 1-3 rounds of showers and embedded elevated thunderstorms
to move across the L.A. Basin through the day and into Monday
night.

A few things to note about this round of heavy rain: 1) Unlike
today`s low, this low will be moving south-south-east with time. It
should have somewhat less tropical moisture to work with as
compared with today`s low. PWATs will struggle to approach 1 inch.
This is because the southward movement will draw cooler and drier
air from the north Pacific with it, instead of having a fetch from
the tropics. However, the cooler air with it would mean a bit more
instability. Thus, while the storms may be a bit stronger than
today in some areas and at times, the much lower moisture levels
will more than offset that. 2) The low will be a bit faster mover,
towards the SSE, parallel to the coast. The faster movement should
allow any storms to also be faster movers, limiting the time any
one area is in heavy-rain-producing storms. 3) The storms will be
moving SE with time on Monday as contrasts the northward storm
movement of the storms today. This too should mean the storms will
have less upslope into the Transverse Ranges, as they`ll be moving
more parallel to the mountains. All of these factors should limit
the rainfall.

Meanwhile, the one thing that Monday`s storms have going for them
above the storms that today will have is antecedent conditions, as
the heavy rain from today will have risen local stream and creek
levels such that the lesser amounts of rain expected Monday will be
starting at a higher starting point than the rain expected today.
This is the rationale for the Slight Risk upgrade.

Wegman


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt