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FOUS30 KWBC 142004
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
404 PM EDT Tue Oct 14 2025

Day 1
Valid 16Z Tue Oct 14 2025 - 12Z Wed Oct 15 2025

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL OVER PORTIONS OF
SOUTHERN CALIFORNIA AND NEW MEXICO...

...16Z Update...

...California & Nevada...

Very small edits on the national scale were made, but significant
ones on the local scale with this update. In coordination with
LOX/Oxnard, CA forecast office, A higher-end Slight has been
introduced for the San Bernardino Mountains with this update. This
is for continued upslope and convective rainfall that is ongoing
this morning across much of the Los Angeles Basin and adjacent San
Bernardino Mountains. Forecasted rainfall in this area remains
over 3 inches through this afternoon, which when combined with
urban and especially burn scar effects locally lowering FFGs,
chances for impacts due to flash flooding, debris flows, and land
slides have been increasing. The Slight Risk area was expanded to
the coast near Malibu with the Palisades burn scar, as in this area
as well impacts will be outsized from the continued rainfall, which
will persist steadily for the next several hours, followed by the
possibility of off-and-on convection into the afternoon hours.

Outside of the L.A. Basin, the surrounding Marginal Risk was
expanded will north and east due to spill-over rainfall expected to
impact the lower elevations of western Nevada. While flooding
impacts will not be nearly as widespread or severe into Nevada as
with areas further south, persistent rainfall could cause isolated
flash, urban, and small stream flooding once the plume of heaviest
rainfall clears the Sierras. The higher elevations of the Sierras
were kept out of the Marginal Risk as snow levels tumble to 5,500
feet, meaning even at pass levels the primary threat will be
wintry, and not caused by flooding.

Wegman

...Previous Discussion...

...Central to Southern California...

Current UA analysis and WV satellite imagery pins the powerful ULL
off the northern CA coast, situated just west of San Francisco Bay.
The forecast trajectory will place the low center off the the
central CA coast by tomorrow morning with a reprieve in lost
latitude as the low center will mostly come to a stop and begin a
slow pivot inland. The proxy of the ULL will create a strong
south-southwest fetch off the Pacific around the base of the ULL
creating a prolonged period of orthogonal upslope flow in-of the
Transverse and Peninsular Ranges of southern CA. The threat will be
ongoing at the very beginning of the forecast cycle as the effects
of heavy rainfall will bleed from the current D1 into this new D1
time period. Consensus in pretty much all guidance maintains a high
confidence posture of locally significant rainfall across those
coastal terrain areas, especially over the eastern Transverse and
northern Peninsular Ranges to the north and northeast of Los
Angeles. The predominant flow is textbook for upslope enhancement
within the confines of those areas which historically can lead to
scattered bouts of flash flooding and localized debris flows. PWATs
will hover around 2 deviations above normal with a small axis of
MUCAPE located right near the coast to just inland over the
terrain. Despite the signal reflected being between 200-500 J/kg of
CAPE, this is likely sufficient to push rates over the terrain to
~1"/hr with perhaps a few stronger cores exhibiting higher rates
for short periods of time.

Considering the multitude of burn scar remnants and just complex
terrain funneling over the Transverse and Peninsular Ranges, this
setup is more than conducive for scattered flash flood concerns, at
a minimum. 00z HREF neighborhood probs for >3" of total precip in
the D1 is between 60-90% over that areas in the eastern Transverse
and northern Peninsular Ranges, overlapping some of the more prone
burn scar areas that remain over these locations. The good news in
the setup is the progression of the heavy rainfall will not be an
all day affair, but still will cover the period between 12-21z
Tuesday with some rainfall encountered just prior to the new D1.
This allowed for a general maintenance of the SLGT risk in place
over the region with a minor adjustment to expand on the southern
flank into the Peninsular Range as models hint at a secondary
maxima situated just northeast of San Diego within the terrain
aligned over western Riverside county.

...New Mexico and West Texas...

Persistence in the general longwave pattern closer to the divide
will create yet another active period for scattered to widespread
convective impacts during the day on Tuesday. As of this time,
elevated moisture on the order of 2-3 deviations above normal,
according to the latest NAEFS signal an environment conducive for
locally heavy rainfall, especially in-of terrain elements that can
provide local enhancement given the persistence of southern flow
overhead. A stout mid-level ridge sits over the Southern Plains
allowing for a channel on the western flank of the ridge to
coincide with the prevailing easterly flank of the broad upper
trough centered over the Western U.S. The latest trend within
guidance is to advect a relatively stout mid-level shortwave out of
Chihuahua late-Tuesday afternoon and evening with a steady south to
north progression as we move through the second half of the
forecast period. The signal is reflected in both the 500mb
vorticity panels, and is defined in the 700mb wind field leading to
higher confidence in the potential of this shortwave advection
scheme over the region.

Recent HREF blended mean QPF has jumped from run to run with a
relative max centered over the Sangre de Cristos of northern NM
with some smaller maxima positioned over the Sacramento Mtns. These
two areas are still fairly sensitive considering multiple burn
scar remnants, including a few very sensitive grounds based within
both terrain features. Totals of 1-2" with locally higher are
forecast within individual deterministic leading to an uptick in
neighborhood probs for >2" now upwards of 60-80% in the
Sacramento`s and 35-50% over the Sangre de Cristos. This is more
than enough to spur issues over these two specific areas leading to
an upgrade to a SLGT risk now centered over south-central to
north-central NM. The threat is a little less daunting over areas
outside the terrain, but there could still be issues outside the
higher elevations due to drainage and heavier cells drifting off
the terrain into the adjacent valleys. This is the reasoning for
maintaining the SLGT over the valleys between the two ranges.

Kleebauer


Day 2
Valid 12Z Wed Oct 15 2025 - 12Z Thu Oct 16 2025

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL OVER PORTIONS
OF THE CENTRAL AND SOUTHERN ROCKIES...

...2030Z Update...

No changes were made to the Marginal Risk area which includes
portions of southern Colorado and northern New Mexico. The axis of
heaviest rainfall associated with the right entrance region of a
120 kt jet and approach of a deep cutoff low remains along the San
Juans and Sangre de Cristo mountains. Deep south/southwesterly flow
is keeping the expected heaviest precipitation on the south and
west facing slopes of those ranges, and has not moved from previous
forecasts. Thus, this supports keeping the highlighted area the
same with few changes.

An expansive precipitation shield will extend from northern Nevada
northeast to the northern Plains. Locally heavy rain and snow may
impact the higher elevations around Yellowstone/the Tetons, but
overall precipitation amounts will be low enough in these areas
that a flash flooding risk is not supported.

Wegman

...Rockies...

The evolution of the longwave pattern across the Western CONUS will
lead to the development of a surface low over the Inter-Mountain
West with a trailing cold front anticipated to move west to east
over the Four Corners into the Continental Divide. The combination
of continued ascent within the RER of a vacating jet streak to the
north and the frontal approach will lead to one last instance of
organized precip within the San Juans down into the Sangre de
Cristos. Considering the nature of the pattern bookending quite an
active stretch within the aforementioned areas, the threat for
flash flooding will lie within the MRGL risk threshold despite the
lack of stronger instability that tends to necessitate a greater
flash flood potential. Antecedent conditions will trend very wet
overall leading to lower FFG`s across the terrain of northern NM
into southwestern CO. Totals within guidance are currently running
between 0.5-1" with some locally higher amounts based in the
individual deterministic. The greatest threat for higher totals
lies within the San Juans, an area that has been the benefactor of
significant rainfall totals the past 4 days. This will likely be
the greatest area of focus for the setup, albeit a threat that will
be closer to a 6-8 hr. window as the front approaches. The previous
MRGL risk was generally maintained, but trimmed up the southern
edge as model trends point to limited potential for heavier
rainfall south of the Sangre de Cristos in northern NM.

Kleebauer


Day 3
Valid 12Z Thu Oct 16 2025 - 12Z Fri Oct 17 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG MUCH OF THE
MONTANA/NORTH DAKOTA BORDER...

...2030Z Update...

...Northern Plains...

The wraparound/comma-head region of a deepening low that will track
north up the Plains will concentrate its longest duration rainfall
along the Montana-North Dakota border Thursday through Thursday
night. Instability will be minimal, however as with most developing
lows, there will still be some on the dry slot-comma-head interface
that will locally increase rainfall rates across this otherwise
typically dry area. Thus, think there could be some isolated flash
flooding impacts, particularly in any poor-drainage areas across
portions of eastern Montana and western North Dakota. Nonetheless,
expect these instances to be quite isolated, and this a low-end and
low-confidence Marginal Risk.

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