Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
420 PM EDT Mon Sep 28 2026
Day 1
Valid 16Z Mon Sep 28 2026 - 12Z Tue Sep 29 2026
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHWEST...
...16Z Update...
A few smaller changes were made to the risk areas with this update.
The Moderate Risk was expanded southwest down the Colorado River to
include the CA/AZ/NV tripoint region in coordination with VEF/Las
Vegas, NV forecast office. Numerous pieces of guidance suggest a
small cluster or line of slow-moving thunderstorms will develop
there later this afternoon during peak heating, driven by clearing
skies there now allowing maximum heating there by this afternoon,
with plentiful moisture available.
The Marginal and Slight risk areas were expanded northward for many
of the same reasons along the NV/UT border. Showers developing
there now could develop into nearly stationary lines by this
afternoon, leading to a higher flooding threat.
Fortunately, much of the rainfall ongoing currently across Arizona
has been stratiform in nature, due to the lack of instability. Well
above normal levels of moisture and the flash-flood-prone nature of
the area due to wet soils still favor flooding occurring, though
it's on the low end of the MDT risk category. The very flashy
nature of southern UT, with plentiful slot canyons and burn scars,
the San Juan Mountains of Colorado oriented perpendicular to the
southwest flow, and much of NM due to saturated antecedent
conditions still appear on track, with no changes made.
No changes were made to the Slight Risk area across ND/MN, as that
event is ongoing.
Wegman
...Previous Discussion...
...Southwest...
The predecessor rainfall event (PRE), from the combination of
Odalys and the approach of Polo is still expected to produce a
widespread 1-3 inches of rain across the Four Corners Region, where
most soils are already at or near saturation from recent heavy
rainfall. As PWs increase to 2-3.5 std dev above normal across much of the
region, strong upper level divergence combined with amplified
southwesterly flow orthogonal to the terrain will support
significant synoptic and orographic forcing. The Moderate Risk
forks a bit further south in New Mexico to include the Lordsburg,
NM area, where numerous burn scars are, and to include the rest of
the Sacramento Mountains in south- central Colorado. The flow will
be uniformly be out of the southwest, south and west facing slopes
along the Mogollon Rim, and the San Juans are likely to see the
most rainfall. Rocky soils, burn scars, and slot canyons will all
locally exacerbate any flooding from the widespread rainfall.
...Dakotas/Northwestern Minnesota...
Coupled upper jet forcing along with deepening moisture (PWs ~1.5
inches) on the nose of a southerly low level jet will promote
training showers and storms across portions of the eastern Dakotas
into northwestern Minnesota. The latest model guidance and WPC
forecast increased the amounts across central North Dakota. A
Slight Risk was maintained with some expansions further southwest
toward southern North Dakota. A Marginal Risk extends from northern
South Dakota to the North Dakota and far northwest Minnesota.
Campbell/Pereira
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
420 PM EDT Mon Sep 28 2026
Day 2
Valid 12Z Tue Sep 29 2026 - 12Z Wed Sep 30 2026
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FROM THE
SOUTHWEST INTO THE SOUTHERN HIGH PLAINS...
...2030Z Update...
Not too many major changes were needed for Tuesday's forecast. The
remnants of Hurricane Polo will make their way out of Mexico, near
El Paso, then track generally northward, roughly following the
NM/TX border. The heaviest rains of the event are likely to follow
along the track of the center. However, a much broader rainfall
shield will continue to envelop the Southwest. Thus, the large
Moderate Risk from southeast Arizona through southeast CO and
extreme southwest KS remains largely unchanged. A higher-end
Moderate Risk remains from about the middle of New Mexico to the
northwest corner of the TX Panhandle; Cimarron County, Oklahoma;
and the southeast corner of Colorado, including the Sangre de
Cristos. The combination of nearly saturated soils, low FFGs,
instability associated with Polo's center, as well as moisture
influx from the south will all work to highlight this region as the
area most likely to experience locally catastrophic flooding from
the 2-4 inches (locally higher) of rain expected through the
period.
Changes with this update generally included trimming the eastern
end of all the risk areas due to both late dry slotting, and the
track of the center of Polo remaining west. Easterly flow out ahead
of Polo continues to paint a risk of locally catastrophic flooding
into the east facing slopes of the Sangre de Cristos in far
southern Colorado.
Elsewhere, changes were relatively minor. Guidance generally
shifted back south and east across the Plains from Nebraska through
Iowa, with a lack of instability along that warm front still
suggesting that most rain that occurs here will be stratiform, and
therefore limit the rates enough to avoid numerous and/or
widespread cases of flash flooding.
Wegman
...Previous Discussion...
...Southwest to the Upper Mississippi Valley...
Widespread heavy rain and potential numerous instances of flash
flooding are expected to continue across portions of the Southwest
while spreading into the southern High Plains this period. Model
consensus depicts the remnant mid-to-upper level energy associated
with Tropical Cyclone Polo becoming incorporated within the upper
trough moving across the Southwest. As the trough moves east, this
will bring the better forcing and focus for heavier rains farther
southeast into southeastern Arizona and New Mexico, including the
extremely vulnerable southeastern parts of New Mexico. A strong
low level jet is forecast to develop from eastern Mexico to the
central Great Plains as the trough shifts eastward, contributing
an additional influx of deep moisture to parts of the southern and
Central Plains. The heaviest amounts of the period will focus over
the juncture of the Texas Panhandle, northeast New Mexico and
southeast Colorado as this moisture interacts with increasing
ascent afforded by strong mid- to-upper level forcing. A broad
Moderate Risk was maintained from south-central Arizona to the
Panhandle Region and north into western Kansas.
A Slight Risk was also maintained farther to the northeast across
parts of the Central Plains into southern Minnesota, where
moisture from the south pooling ahead of a surface-to-low level
cold front and ahead of the remnant tropical system will support
the growing potential for heavy rainfall.
Campbell/Pereira
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
420 PM EDT Mon Sep 28 2026
Day 3
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...
...2030Z Update...
The developing Plains low that by Wednesday will have effectively
absorbed the remnants of Polo will strengthen over the High Plains.
The low will take on a much more classic comma shape, with the warm
front extending towards the Upper Great Lakes. Thus, the rainfall
pattern by Wednesday will take on a backwards Greek letter tau
shape. The Moderate Risks were generally little changed. Focusing
on topographic effects, the Colorado/New Mexico Moderate risk area
was expanded on its western side to highlight the upslope flow into
the southern Front Ranges. The top of the tau extends from south-
central Colorado through to southern Wisconsin. The heaviest
rainfall along the northern end of the precipitation shield extends
from southeastern Nebraska through much of central Iowa.
Instability will be the limiting factor here as well, with most of
it stuck further to the south. Thus, despite near saturated soils
in this area, the rates should stay muted enough to prevent
widespread flash flooding, instead favoring scattered instances
consistent with a Slight. However, amounts are close to maybe
needing a Moderate Risk in this area as well, so the area will need
to be evaluated for a Moderate with future updates.
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. Thus, the storms may form a bit west than previous
forecasts had, generally from Wichita Falls southwest through San
Angelo and the Hill Country. 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.
Wegman
...Previous Discussion...
Tropically enhanced heavy rainfall will continue to spread from
the Southwest to the Southern/Central Plains and into the northern
Mississippi Valley. Guidance continues to show a fairly persistent
theta-e/PW axis across this corridor (PWs aoa 2.25"), with the
confluence of anomalous mid- upper level moisture (subtropical
connection, including Polo's remnant upper level moisture plume) as
well as the uptick in low- level moisture transport from the
western Gulf with the strengthening LLJ (850 mb flow +4 to +5
standard deviations above normal) ahead of the deepening/amplifying
upper trough. 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.
Some persistent model spread present, however hinting the location
and (especially) timing of heaviest rain as Polo's remnant
moisture plume becomes absorbed by the deepening SW US upper
trough. Not withstanding the timing uncertainties, the ECMWF EFI
continues to signal a potentially significant rainfall event,
particularly on Tuesday across eastern Arizona and western-
central New Mexico.
A Moderate Risk was maintained over southeast Colorado, northeast
New Mexico, extreme portions of the Texas panhandle and southwest
Kansas. Another Moderate Risk was maintained yet expanded N-S
across central Texas and eastern Oklahoma. A broad Slight Risk
encompasses New Mexico/Texas to southern Minnesota/Wisconsin.
Campbell/Hurley
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
420 PM EDT Mon Sep 28 2026
Day 4 and Day 5
Valid 12Z Thu Oct 01 2026 - 12Z Sat Oct 03 2026
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHERN PLAINS ON DAY 4...
A positively-tilted upper trough is forecast to gradually weaken as
it drifts out into and then stalls over the central and southern
Plains. However, additional heavy rains and potentially numerous
instances of flash flooding are expected, especially on Thursday
(Day 4) across portions of the southern Plains. A Moderate Risk was
maintained from south-central Texas to southeastern Oklahoma, where
larage-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). Raising the threat for flash flooding
will be the possible overlap of heavy rains spreading into this
region the previous day. Also on Day 4, a broad surrounding Slight
Risk extending north into parts of the central Plains and through
the mid Mississippi Valley was also maintained. 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.
By Friday (Day 5), with the system continuing to weaken, so too
is the potential for additional widespread heavy rains over the
southern Plains. However, there is likely to remain some areas
with an elevated chance for heavy rain and flash flooding
concerns. A Slight Risk was placed over parts of the southern
Plains -- from South Texas to southeastern Oklahoma -- where models
show a lingering axis of deeper moisturure supported by increasing
low level convergence. This moisture may interact with slow-moving
mid-level energy remaining over the region to support organized
storms. Once again, the possible overlap of heavy rainfall from
previous day(s), may elevate the threat for flash flooding. Farther
north and east, the previously noted front will continue to settle
southeast through the Ohio Valley, with some potential for heavy
rain along and ahead of the boundary. Guidance shows a broadening
pool of deep moisture developing over the southeastern U.S. as
southeasterly flow increases along the western edge of the
subtropical ridge. 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.
Pereira
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
420 PM EDT Mon Sep 28 2026
Day 4 and Day 5
Valid 12Z Thu Oct 01 2026 - 12Z Sat Oct 03 2026
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHERN PLAINS ON DAY 4...
A positively-tilted upper trough is forecast to gradually weaken as
it drifts out into and then stalls over the central and southern
Plains. However, additional heavy rains and potentially numerous
instances of flash flooding are expected, especially on Thursday
(Day 4) across portions of the southern Plains. A Moderate Risk was
maintained from south-central Texas to southeastern Oklahoma, where
larage-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). Raising the threat for flash flooding
will be the possible overlap of heavy rains spreading into this
region the previous day. Also on Day 4, a broad surrounding Slight
Risk extending north into parts of the central Plains and through
the mid Mississippi Valley was also maintained. 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.
By Friday (Day 5), with the system continuing to weaken, so too
is the potential for additional widespread heavy rains over the
southern Plains. However, there is likely to remain some areas
with an elevated chance for heavy rain and flash flooding
concerns. A Slight Risk was placed over parts of the southern
Plains -- from South Texas to southeastern Oklahoma -- where models
show a lingering axis of deeper moisturure supported by increasing
low level convergence. This moisture may interact with slow-moving
mid-level energy remaining over the region to support organized
storms. Once again, the possible overlap of heavy rainfall from
previous day(s), may elevate the threat for flash flooding. Farther
north and east, the previously noted front will continue to settle
southeast through the Ohio Valley, with some potential for heavy
rain along and ahead of the boundary. Guidance shows a broadening
pool of deep moisture developing over the southeastern U.S. as
southeasterly flow increases along the western edge of the
subtropical ridge. 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.
Pereira
