FOUS30 KWBC 270832
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
432 AM EDT Sun Sep 27 2026

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

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PARTS OF THE
COASTAL NORTHEAST...

...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...

...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...

...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 IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE
SOUTHWEST, SOUTHERN AND CENTRAL PLAINS, AND PORTIONS OF THE
MISSISSIPPI VALLEY WEDNESDAY...

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS THE
SOUTHERN CENTRAL PLAINS AND LOWER MISSISSIPPI VALLEY THURSDAY...

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
$$
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