Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
402 PM EDT Sun Sep 27 2026

Day 1
Valid 16Z Sun Sep 27 2026 - 12Z Mon Sep 28 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PARTS OF THE
NORTHEAST, PARTS OF THE HILL COUNTRY TO NORTH-CENTRAL TEXAS, AS
WELL AS PARTS OF ARIZONA...

...16Z Update...

...Northeast...

In coordination with OKX/Upton, NY and BOX/Taunton, MA forecast
offices, the Slight Risk was dropped with this update. The
Nor'easter continues across much of the Northeast this morning. The
storm has begun weakening, with any heavier rainfall concentrated
in rather narrow bands. The most significant is the one southeast
of Boston, which is nearly stationary. Even within that band, rates
are between 1/2 and 3/4 inch per hour. More bands are expected to
impact the NYC metro, northern NJ, and southern New England late
this afternoon through tonight, but they too are not expected to
feature much heavy rainfall. Thus, the flash flooding threat is
likely to be isolated at best, leading to the Marginal Risk
downgrade.

...Texas...

A surge of moisture, the leading edge of a multi-day low level jet
which will fuel a multi-day heavy rainfall event in this area, will
begin to advance northward from off the Gulf and into the Southern
Plains today. Upper level ridging and a distinct lack of forcing
will put a cap on any significant rains today. However, along
frontal boundary/interface, a few isolated storms can't be ruled
out, especially along the Balcones Escarpment, where topography
oriented perpendicular to the southeasterly flow may allow for a
stationary storm or two. Into north central Texas, it will be the
warm frontal interface having to do all the work to cause any
storms capable of flash flooding. Soils remain well drier than
normal, so this is a very low-end Marginal, with only an isolated
instance of flash flooding or 2 possible. The inherited Marginal
was left largely unchanged, save for a few very minor tweaks in
southern Oklahoma.

...Arizona...

The beginning of a surge of moisture associated with the upper half
of East Pacific Tropical Storm Odalys will move into the Southwest
today. Ingredients for a multi-day heavy rain and flash flooding
event across the Southwest, which will unfold over the coming days,
will only begin to come together today. Thus, while widely
scattered showers and storms are likely, resultant flash flooding
should be isolated at best. No significant changes were made from
inherited.

Wegman

...Previous Discussion...

...New Jersey, Southern New York, and Southern to Central New
England... A slow-moving nor'easter lingering along the coast near
northern New Jersey and eastern Long Island will continue to funnel
moisture across southern New England, southern New York, and New
Jersey. Persistent northerly flow will keep instability low,
limiting instantaneous rainfall rates; however, saturated soils
from coastal inundation and prior heavy rains will maintain an
elevated risk for flooding, especially in urban and poor drainage
areas. A Slight Risk was maintained from eastern Massachusetts to
the NYC Metro where additional rainfall amounts of 1-3 inches, with
isolated heavier amounts, are expected. A Marginal Risk remains
farther north into parts of Upstate New York and central New
England, where orographic effects could help support some heavier
totals.

...Southern Plains...
The overnight CAMs guidance is showing some signal for localized
heavy amounts developing along a ribbon of higher PWs extending
from the Texas Hill Country into south-central Oklahoma. The HREF
and REFS guidance indicate some potential for localized amounts of
2+ inches and some modest probabilities for amounts exceeding 3-hr
FFGs, especially around the Texas Hill Country. Therefore, a Marginal
Risk was introduced for this area.

...Southwest...
Flow is expected to turn southwest ahead of a trough digging into
California, which will begin to draw moisture from Hurricane Polo
into the region. Overall, conditions are expected to remain
relatively quiet, in what is expected to be last day before
conditions greatly deteriorate across the the region. However,
there is a low-end threat for isolated flooding, especially across
portions of northern Arizona, where favorable mid-to-upper level
dynamics may interact with the deepening moisture.

Pereira
 

Day 2
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...

...2030Z Update...

...Southwest...

No major changes were needed with the afternoon update today. The
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. 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. Some minor tweaks
were made to the surrounding Slight, but since there haven't been
major changes to the track, and the flow will be uniformly 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.

...Northern Plains...

No significant changes were made to the Slight Risk area in eastern
North Dakota and far northwestern Minnesota. The QPF did come up a
bit there, so there is some potential that a higher-end Slight may
be needed should those trends continue.

Wegman

...Previous Discussion...

...Southwest/Four Corners...
A predecessor rainfall event (PRE) is likely to impact much of the
region beginning by midday and continue into the end of the
period. Moisture preceding the arrival of the primary moisture
plume associated with Hurricane Polo will continue to spread across
the region as it is drawn northeastward ahead of a mid-to-upper
level low digging south across California toward the lower Colorado
Basin.

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.

A limiting factor will be instability, hampered by thick cloud
cover and shortening daylight. However, the strong forcing and
anomalous moisture is likely to support very efficient warm-rain
processes that will compensate for the limited CAPE.

A broad Moderate Risk remains in place -- covering the complex
terrain and burn scars of southern Utah, southwestern Colorado, as
well as much of northern and central Arizona and New Mexico. One
smaller adjustment this issuance was to extend the Moderate Risk a
bit farther southeast into the extremely vulnerable areas of
southeastern New Mexico, where soils remain nearly saturated from
recent rounds of heavy rain.

...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. A Slight Risk was maintained from
northeastern North Dakota into northwestern Minnesota, where
there remains a good model signal for widespread totals of 1-2
inches, with locally heavier totals.

Pereira
 

Day 3
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...

A bit more significant changes were needed with this update
compared to Day 2. Guidance is in good agreement on a north and
westward trend to the rainfall associated with the main remnants of
East Pacific Hurricane Polo. This more amplified pattern led to
expansions of the Slight and Moderate Risks north and west with
this update. Further, a slower eastward progression means the
forecast rainfall has come up a bit. Thus, the Moderate Risk was
expanded north to include much of southeastern Colorado in
coordination with PUB/Pueblo, CO forecast office. The surrounding
Slight was adjusted north and west to include more of southern
Utah, much of central Colorado, and additional areas of Nebraska
and southeastern South Dakota. To ensure that this doesn't flip-
flop back, the eastern sides of those risk areas were largely left
unchanged.

A higher-end MDT has also been added including much of the
northeastern half of NM, the southeastern corner of CO, and the
western OK and TX Panhandles. All of these areas have saturated
soils and lowered FFGs. The slight westward shift in the track of
the remnants of Polo have the corridor of the heaviest rains now
tracking up the eastern half of NM. This will spread associated
very heavy rainfall north into the San Juans and Sangre de Cristo
ranges on the NM/CO border, which led to the upgrades there.
Meanwhile, heavy rain on the immediate eastern half of the center
of circulation will keep very heavy rainfall ongoing into the
Oklahoma and Texas Panhandles. For total mean/areal rainfall, the
Panhandles may have the highest rain totals anywhere outside
immediate mountains on Tuesday.

The notable westward shift of the band of heaviest rainfall across
Nebraska increased uncertainty on if a Moderate Risk needed to be
expanded into the central Plains. While that entire area also has
been very wet recently, with surface layer soils also well above
normal for moisture, the large shift precluded any upgrades. Should
the guidance stabilize on where the heaviest rains will occur in
future updates, then a MDT risk expansion there may need to be
considered.

Wegman

...Previous Discussion...

...Southwest to the Central Plains...
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. With
some lingering differences in the details, the majority of the
guidance shows 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. As the
trough moves and east, a strong low level jet is forecast to
develop from eastern Mexico to the central Great Plains,
contributing an additional influx of deep moisture to parts of the
southern and central Plains. The overnight guidance generally
indicated the heaviest amounts of the period will focus over the
Texas Panhandle-Northwest Texas region as this moisture interacts
with increasing ascent afforded by strong mid-to-upper level
forcing. This evolution of events continues to warrant a broad
Moderate Risk that was maintained from southeastern Arizona to the
Panhandle Region.

A Slight Risk was also maintained farther to the northeast across
parts of the central Plains into eastern Iowa, 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.

Pereira
 

Day 4 and Day 5

Valid 12Z Wed Sep 30 2026 - 12Z Fri Oct 02 2026

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

2000Z Update: Considerable, noteworthy changes were made to both
the Days 4-5 EROs. On Day 4, per collaboration with the WFOs, we
introduced a Moderate Risk across SE CO and far NE NM, along with
portions of central to North TX into south-central OK. The guidance
is starting to converge, though still some timing and thus
longitudinal (W-E) differences in terms of the placement of the
heaviest rainfall Wednesday and Wednesday night. Obviously a highly
dynamical system with the upper level energy and moisture profiles
buoyed from Polo's remnants. We are still leaning more towards the
operational ECMWF solution in terms of timing and (thus) eastward
extent, which is slower than the mean of the other operational
global models, but matches the EC AIFS as well as the AIGFS fairly
well. Still thinking that the infusion of Polo and resultant
deepening (amplifying) upper trough will tend to favor the slower
solutions. 00Z-based ECMWF EFI (Extreme Forecast Index) is beginning to hone
in on the event across the Southern Plains, with shift-of-tails
nearing 2 in some spots across central to North TX.

On Day 5 (12Z Thu-12Z Fri), the more notable change made was to
hoist a Moderate Risk across south-central TX through North TX and
south-central to southeast OK. There is some overlap with the Day 4
Moderate, so certainly if there is some retardation to the forward
(eastward) progression of this system, the concern for a higher
excessive rainfall threat will continue to grow. The latest EC and
AI guidance 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.

Hurley

Previous discussion...

Tropically enhanced heavy rainfall will spread from
the Southwest, Southern/Central Plains and into the Mississippi
Valley. The threat for excessive rainfall will remain elevated
given areas of overlapping heavy rain footprints from prior days.
Locations particularly sensitive will be dry washes, burn scars,
slot canyons, and areas of higher terrain. While the guidance is
slowly converging on a solution, there is still spread with the
location and (especially) timing of heaviest rain as Polo's remnant
moisture plume becomes absorbed by the deepening SW US upper
trough. Timing issues are most notable by 12Z Wed-12Z Thu, with
most of the 12Z operational global model runs and both AIGFS and EC
AIFS slower (farther west with the heaviest QPF) than the NBM and
ensemble mean QPFs. Not withstanding the timing uncertainties, the
ECMWF EFI continues to signal a potentially significant rainfall
event, particularly on Tuesday across eastern AZ and western-
central NM.

A broad Slight Risk for Arizona/Texas to Iowa/Illinois on
Wednesday and from Texas to the Arkansas/Missouri state line on
Thursday. This part of the country will continue to be monitored
for QPF/track of remnant energies trends as they may impact future
upgrades of the Risk levels.

Campbell/Hurley
 

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