FOUS30 KWBC 260829
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
429 AM EDT Sat Sep 26 2026

Day 1
Valid 12Z Sat Sep 26 2026 - 12Z Sun Sep 27 2026

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

...Northern Mid-Atlantic Coast into Southern New England...

The latest guidance is favoring a deeper nor'easter to approach the
Northeast and then stall while spreading heavy rain generally
along the I-95 Corridor from Philadelphia to Boston. Prevailing
southeast to easterly flow will support a substantial influx of
moisture into parts of southern New England, southern New York, and
New Jersey. Increasing hydrologic concerns are expected. In
addition to an increasing threat for flooding in urbanized and poor
drainage areas, this heavy rain may also exacerbate ongoing
coastal flooding. The Slight Risk highlights the areas where the
general model consensus shows widespread amounts of 1-3 inches
during the period. A higher-end Slight Risk encompasses much of
Long Island, Rhode Island and eastern Massachuesetts.

...Southern Plains to the Upper Mississippi Valley...

A Slight Risk was maintained for portions of eastern Nebraska,
central/southern Iowa, and far northern Missouri where a corridor
of cell-training is possible along a NW-SE oriented stationary
front. Hi-res guidance is suggesting 24-hour accumulations may
reach or exceed 2 inches, which could support scattered flash
flooding as this rainfall overlaps with heavy rainfall from the
previous day. The Marginal Risk was also expanded further south
where CAMS signal the potential for locally heavy rainfall near
sensitive soil, although coverage of storms is expected to be lower
than previous days. Rainfall developing late in the Day 1 period,
may continue into late Saturday morning across parts of Iowa and
southern Minnesota. Expect a diminishing threat for heavy to
excessive rain by late morning, followed a potential return later
in the day centered farther to the southwest. While confidence is
limited, there remains some signal in the guidance for storms
developing back across across parts of the central Plains along a
strengthening low level jet and axis of increasing instability and moisture.

...West Texas...

Weak mid-level energy interacting with an uptick in low level
moisture may produce some additional localized heavy amounts,
impacting the more sensitive areas of West Texas around the Davis
Mountains and Stockton Plateau. A Marginal Risk was maintained.

Campbell/Pereira/Asherman
 

Day 2
Valid 12Z Sun Sep 27 2026 - 12Z Mon Sep 28 2026

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

...Northeast...

The potent nor'easter will begin to weaken as it lingers in the
vicinity of the northern Mid-Atlantic coast. How far inland, if at
all, the storm drifts will help dictate how far beyond the coastal
regions its associated heavy rains will reach. Heavy rainfall is
expected to linger overhead, leading to an additional 1-2.5 inches
of rainfall over much of the same area. These rains will overlap
areas impacted during the Day 1 period thus maintaining an elevated
threat for excessive rainfall and flash flooding concerns. A Slight
Risk remains in effect from coastal New Jersey to eastern
Massachuesetts with a Marginal from southern New Jersey to southern Maine.

...Southwest...

Increasing southwesterly flow ahead of an amplifying trough
digging south across California will begin to tap moisture and mid-
to-upper level energy, supporting an increase in shower and
thunderstorm activity and the potential for localized areas of
flash flooding from Southern California into the Southwest.

...Northern Plains...

A Marginal Risk area was raised for portions of the Northern Plains
as a northern stream trough exits the Rockies and begins to tap
into moisture over the plains. Rainfall rates exceeding 0.50
inch/hour may lead to isolated instances of flash flooding across the area.

Campbell/Pereira
 

Day 3
Valid 12Z Mon Sep 28 2026 - 12Z Tue Sep 29 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE FOUR CORNERS REGION...

...Southwest, Southern Rockies, and adjacent High Plains...

There will be a strong influx of tropical moisture from tropical
cyclones Odalys and Polo directed into the Desert Southwest. This
abundant moisture when coupled with the afternoon diurnal
instability max will trigger scattered to widespread convection
across the region. An interaction between the eventual closed upper
low over California and what will be Polo as it begins moving
northward towards Baja over the course of the weekend into early next week.

The ensemble means for this time period continue to depict a broad
expanse of heavier rainfall increasingly likely for the Southwest
/Four Corners area due to the ascent pattern downstream of the
upper low swinging into the region. PW anomalies during this period
will be between 2-3.5 standard deviations above seasonal normal
for late September. A broad Slight Risk for excessive rainfall and
flash flooding remains in effect while a higher-end Slight Risk
was raised for east-central Arizona and western New Mexico.,

...Northern and Central Plains...

Progressive northern stream trough coupled with prevalent moisture
advection from the Southwest will lead to a period of convective
concerns and isolated flash flood potential. There is a broad
Marginal Risk in effect from northern Kansas to North
Dakota/northwest Minnesota. Remnant energy from what is currently
tropical cyclone Odalys will get drawn northeastward out of the
Pacific prior to the period reaching the Northern and Central
Plains Monday into early Tuesday leading to quite a larger coverage
of convection over the above area. Areal coverage of convection
will increase Tuesday as there will be a stronger PW surge with the
remnant energy from Odalys. Mean QPF is currently between 1-2.5
inches with locally higher at 5-day leads, a testament to the
threat in question as there will likely be more significant
rainfall potential with this setup.

...New England...

Lingering rainfall expected as the coastal low begins to drift
away from the region. The cumulative effect of rainfall over the
long period into the urban corridor of eastern Massachusetts will
cause hydrologic concerns, including some flooding in poor drainage
locations. A Marginal Risk area is in effect from eastern
Connecticut up to coast to southern Maine.

Campbell/Santorelli
 

Day 4 and Day 5
Valid 12Z Tue Sep 29 2026 - 12Z Thu Oct 01 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER MUCH OF THE
SOUTHWEST, FOUR CORNERS REGION AND INTO THE SOUTHERN AND CENTRAL PLAINS...

The eventual absorption of tropical cyclone Polo's remnants into
an amplifying Southwest U.S. upper trough have led to a slower and
wetter trend across the Southwest-Four Corners and into the High
Plains by midweek. The latest suite of guidance persists with the
spreading of tropically enhanced heavy rainfall from the Southwest
into the Plains overlapping some areas with heavy rainfall in prior
days increasing the threat for flash flooding particularly over
sensitive washes, burn scars, slot canyons, and areas of higher
terrain. A broad Slight Risk for Tuesday spans from Arizona to
Texas and northeast to Iowa/Illinois and from Texas to
Iowa/Illinois on Wednesday. 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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