FOUS30 KWBC 291538
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1138 AM EDT Tue Sep 29 2026

Day 1
Valid 16Z Tue Sep 29 2026 - 12Z Wed Sep 30 2026

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FROM THE
SOUTHWEST INTO THE CENTRAL HIGH PLAINS...

16Z Update: The main premise of the forecast remains unchanged with
a significant, widespread heavy rainfall threat across much of
AZ/NM into portions of west TX as the tropical remnants of Polo
make their way northward, contributing to a prominent flash flood
prospect over the Desert Southwest into the Southern High Plains.
12z CAMs continue to pinpoint a significant heavy rain threat
across southeastern AZ, including the Tuscon area where 2"/6-hr
probabilities running >70% for much of the area, a threshold that
typically verifies more substantial flash flood prospects in that
area, historically. The environment across the Southwest remains
ripe for heavy rainfall rates between 1-2"/hr and potential for
widespread 2-4" totals, locally higher given the deep moisture
profiles stemming from advection of Polo's moisture being pulled
poleward. Latest 500mb analysis indicates the evolution we have
been anticipating the last several forecast cycles as the mid-level
energy from Hurricane Polo begins to interact and couple with the
upper level low (ULL) currently positioned over the Colorado River
basin out between AZ/CA/NV. Expectation is for the main energy from
Polo to become fully incorporated into the ULL leading to the
addition of stronger vorticity advection into the overall setup
that will materialize over the course of the D1 and beyond.

Time frames of note will include now through 00z across southeast
AZ as the initial surge of convection will occur over the area with
a quasi-linear band of heavy rainfall will initiate over eastern
Pima county and slide east-northeast through the course of the
afternoon. This will lead to widespread flash flood concerns over
the I-19 corridor from the border to north of Tuscon with the
complex of thunderstorms likely expanding east into impact areas
along and north of I-10 into western NM. This area is where the
signal is very prevalent for rainfall totals >3" with the latest
12z HREF blended mean QPF between 3-5" over the area just northeast
of Tuscon into western NM with the maximum located within the
Apache and Gila National Forests. Latest probabilities for >3" are
substantial in that same area with the extension of 60-90+%
probabilities all the way into Tuscon proper which enhances the
threat of the locally significant flash flood concerns in the
region.

As we expand further east, the rest of NM will become inundated
with widespread heavy rainfall late-afternoon through the overnight
time frame with the heaviest rainfall positioned over the southern
and eastern area of the state. Remnant moisture and vorticity
advection being absorbed into the upper pattern will co-locate
within the area from far west TX to points northeast leading to
substantial convective coverage, as well as some organization
trends that are conducive for more appreciable heavy rain
footprints. The areas of concern will lie within the Upper Trans
Pecos from Clines Corner down to the southeastern reaches of NM,
including the more flash flood prone areas along the Pecos river
(Roswell-Artesia-Carlsbad) where the combination of small stream
and arroyo flooding and urbanization will likely provide an avenue
for more locally significant flash flood prospects this evening as
a prolific line of thunderstorms moves through the area. Latest 12z
KEPZ RAOB this morning came in with PWATs of 1.55", a daily
maximum already established for the date, and well beyond the 95th
percentile for seasonal climatology. Current forecast is for PWATs
to rise further, putting the region within historical thresholds
when it comes to some of the higher PWAT readings in general. This
type of environment typically precludes more substantial flash
flood concerns, and this setup is no different. Expect the Eastern
NM High Plains, Upper Trans Pecos, Sacramento Mountains, and
adjacent West TX High Plains from the TX Panhandle into the Permian
Basin to be under the threat of enhanced flash flood chances
through the back half of D1, bleeding into D2.

A targeted MDT was added over the AZ/UT for the central portions of
the bordering area due to another round of convection occurring
this afternoon under the influence of the ULL. Ongoing flooding
over the area enhances sensitivities for the area with local FFG's
basically as low as they could go before more flooding ensues. In
coordination with the local SLC and FGZ WFO's, a MDT risk was added
to target this specific area with an additional 1-2" of precipitation
possible in any convection that occurs this afternoon. This threat
will wane with the loss of diurnal heating around sunset.

Broad SLGT risk continues over the Central Plains with a bit more
of an isentropic pattern materializing across KS into NE/IA which
will likely include some convective schemes, but less instability
and a less robust deep moisture presence compared to areas of the
Southwest will limit the threat to more scattered flash flood
concerns this evening. 12z HREF EAS probabilities for >1" are still
fairly robust (50-80%) over the area from nort-central KS up
through the Missouri River Valley and adjacent Corn Belt areas of
IA, but the >2" signal drops off considerably which notes a limit
to the upper threshold of the forecast which curbs the threat
closer to a SLGT for this period.

Kleebauer


Day 2
Valid 12Z Wed Sep 30 2026 - 12Z Thu Oct 01 2026

...THERE ARE MODERATE RISKS OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE CENTRAL/SOUTHERN HIGH PLAINS AND SOUTHERN PLAINS...

During this period the developing low over the High Plains, which
have absorbed the remnant energy and moisture from Tropical
Cyclone Polo, will strengthen and take on a much more classic comma
shape, with the warm front extending towards the Upper Great
Lakes. Tropically enhanced heavy rainfall will continue to spread
from the Southwest to the Southern/Central Plains and into the
northern Mississippi Valley. Multiple days with overlapping heavy
rain footprints will keep elevated threats for excessive rainfall
and flash flooding. Locations particularly sensitive will be dry
washes, burn scars, slot canyons, and areas of higher terrain.

Focusing on topographic effects, the Colorado/New Mexico Moderate
Risk area was maintained for the upslope flow into the southern
Front Ranges. From the High Plains to Wisconsin/Illinois the
greatest QPF is expected to fall along and north of frontal
boundary. With the instability being offset to the south, rainfall
rates may not support widespread flash flooding, instead favoring scattered
instances consistent with a Slight. However, further monitoring
will be needed as the expected amounts are flirting with a possible
Moderate Risk.

To the south an organized line of convection is expected to setup
across central Oklahoma and Texas. The Moderate Risk in
Texas/Oklahoma was nudged a bit to the west, with growing consensus
that a line of convection will be slow- moving as well as expanded
southward to the Mexican border. Interactions with the dry slot to
the west will ultimately determine where the storms form and how
much flash flooding results. Finally, speaking of the dry slot,
growing consensus that it will track through eastern New Mexico and
the Panhandles, (which makes sense given the track of Polo), means
that fortunately many of these very hard hit areas from Tuesday
will finally get to dry out under the dry slot. Thus, the no-risk
area and Marginals were expanded northeast to include these areas
as the risk of flooding rainfall is very low.

Campbell/Hurley/Wegman


Day 3
Valid 12Z Thu Oct 01 2026 - 12Z Fri Oct 02 2026

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHERN PLAINS...

During this period a postively-titled upper level trough is
expected to weaken, then stall over the Central and Southern
Plains. Along the surface frontal boundaries heavy rainfall will
persist with potentially numerous instances of flash flooding for
the Southern Plains. A Moderate Risk was maintained southeast
Oklahoma to the Texas Hill Country and South Texas where large-
scale ascent afforded in part by favorable upper jet forcing will
continue to interact with an axis of anomalous moisture (2-2.25+
inch PWs). This multiple day event will have possible overlap, thus
keeping an elevated risk for flooding.

A broad Slight Risk extending north from Texas to Wisconsin and
northern Indiana and then west to eastern Colorado. While areal
average rainfall amounts are expected to be less in this region,
moisture drawn north and focusing ahead of a cold front settling
south across the region is likely to fuel the potential for
widespread moderate to localized heavy rainfall, resulting in at
least isolated runoff concerns.

Campbell/Pereira


Day 4 and Day 5

Valid 12Z Fri Oct 02 2026 - 12Z Sun Oct 04 2026

...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
SOUTHERN TEXAS ON DAY 4 AND FROM THE CENTRAL GULF COAST INTO THE
TENNESSEE VALLEY ON DAY 5...

Day 4...
Mid-level energy associated with the trough moving into
the central and southern Plains on Thursday will remain over the
region into at least the early part of the weekend. Deep moisture
will remain as well over parts of the southern Plains --
maintained by backing low level winds tapping Gulf moisture and
drawing it westward into a lingering low level boundary. As this
moisture interacts with the slow-moving mid-level energy that is
forecast to remain over the area, there will continue to be some
potential for heavy rainfall producing storms, impacting some areas
that are likely to be left vulnerable to excessive runoff by the
previous day(s) heavy rains. While forecast confidence has
diminished some, there remained enough of a signal in the
overnight guidance that showed additional heavy accumulations
centered across parts of South to South-Cental Texas to maintain a
Slight Risk over the region. Farther north and east, a cold front
will continue to settle southeast through the Ohio Valley, with
some potential for heavy rain along and ahead of the boundary.
Supported by deep southerly flow along the western extent of the
subtropical ridge, guidance continues to show a broadening pool of
anomalous moisture spreading from the Gulf Coast into this
boundary. Mid-level energy embedded within the flow may help
produce areas of heavy rain, with some guidance indicating heavy
amounts along the Gulf Coast from Mississippi to the Florida
Panhandle this period.

Day 5...
Moisture will continue to deepen from the Gulf Coast northward into
the Tennessee Valley -- with PW anomalies increasing to 2-3 std
dev above normal across much of the region during this period. A
Slight Risk was drawn along the axis of deepest moisture (PWs at
or above 2.25 inches) which is expected to extend from the central
Gulf Coast northward into an area of enhanced ascent centered in
the vicinity of the Tennessee Valley. The previously noted front
dropping south into the area along with mid-level energy emanating
from the Plains are expected to help focus heavy rainfall
development over this region. Forecast confidence was limited in
part by model spread regarding the placement of the front, which is
likely to play a role in where the heaviest amounts occur.
Therefore, notable adjustments to at least the northern extent of
the Slight Risk may be required in future updates.

Pereira


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



$$