FOUS30 KWBC 280817
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
417 AM EDT Mon Sep 28 2026

Day 1
Valid 12Z Mon Sep 28 2026 - 12Z Tue Sep 29 2026

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHWEST/FOUR CORNERS...

...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 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...

...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 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...

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 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 moisture 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
$$
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