FOUS30 KWBC 281558
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1158 AM EDT Tue Jul 28 2026

Day 1
Valid 16Z Tue Jul 28 2026 - 12Z Wed Jul 29 2026

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PARTS OF
EASTERN PENNSYLVANIA, NEW JERSEY, SOUTHERN NEW YORK, AND FAR
WESTERN CONNECTICUT...

...Mid Atlantic and Northeast...
A significant flash flooding day is expected across portions of the
Mid Atlantic and Northeast. Convection is rapidly initiating
already between 15-16Z across much of the region, and should become
more widespread by early-mid afternoon. Growing instability in
areas of clearing to the southeast of ongoing rain, and increasing
PWs (likely to approach 2 inches in the Moderate Risk area by 21Z)
should support increasingly intense rain rates by this afternoon.
As convection becomes more widespread, there should also be larger
clusters that develop. The combination of these factors should
yield corridors where intense rain rates can persist in certain
areas for 2-3 hours, and may yield locally extreme rainfall. Almost
every available hi-res model shows at least one localized max of 5
or more inches of rainfall in 6 hours or less. Hourly rain rates
appear likely to exceed 2 inches in the most organized convective
clusters, and could even reach 3 inches. Therefore, the expectation
is for numerous cases of flash flooding associated with the high
rain rates from this afternoon into the early evening, with the
potential for locally significant flash flooding. Given the early
initiation of convection, there is considerable concern for impacts
during the evening commute in the populous urban centers of the
Northeast corridor.

There will be a transition in the convective pattern as we reach
the overnight hours. A strong mid-level vort max, very evident in
GOES East satellite channels across north-central Pennsylvania, is
likely to induce the development of a surface low somewhere over
the region. The position of this surface low will be critical to
where the threat of flash flooding maximizes during the overnight
period and as we approach 12Z (and the morning commute). Our best
estimate is that this would be favored just southeast of the
current trajectory of the surface low, and near the largest
concentration of afternoon convection. For now that looks like it
will be somewhere from SE PA to C NJ by the 00-06Z time frame, and
lifting northeast toward S NY and CT by 06-12Z. This is where the
latest 14Z HRRR places it, and is consistent with the ECMWF
Ensemble and AIFS/AIFS Ensemble. However, there is still
considerable uncertainty. The reason the position of the low is
important is that instability should decrease while PWs stay quite
high, favoring low-topped and very efficient convective rain bands,
and these would tend to concentrate near and just north/east of the
surface low in an area of maximum mesoscale forcing and low-level
convergence. There are also signs that rain bands will extend north
along an inverted trough, perhaps well up the Hudson Valley and
into Vermont. While the maximum rain rates may slightly decrease
during the overnight period, more into the 1-2 inch per hour range
in the strongest convective bands, there would be greater potential
for persistence, and thus locally extreme rainfall will still be
possible -- the threat will just shift more into far southern and
eastern New York, Long Island, Connecticut, western Massachusetts,
and southern Vermont. It's possible there will be an ongoing rain
band with active flash flooding around the end of the period and in
the transition to Day 2, around the time of the Wednesday morning
commute, particularly from Long Island north into Connecticut and
western Massachusetts.

...Central and Southern Appalachians and Virginia...
A broader Marginal Risk area was maintained from southern and
western Maryland south into Virginia, northern North Carolina, and
the central and southern Appalachians. A more conditional threat of
localized heavy rainfall and flash flooding exists in these areas,
largely due to stronger deep layer mean winds (over 30 knots),
which should keep storm motions relatively fast. Precipitable water
values remaining in the 1.6 inch (at higher elevations) to 2.0 inch
(at lower elevations) range should support high instantaneous rain
rates, which could yield localized heavy rain where any training
could occur. However, the overall pattern does not favor widespread
training, and thus the threat is more conditional.

...Four Corners States...
The primary adjustment was to expand the Slight Risk to include
much of northern Arizona, where probabilities of localized heavy
rainfall have increased. The Marginal Risk was also pushed west in
Utah to account for some signal for slow-moving thunderstorms.
Overall, the previous reasoning still applies. The strongest signal
for heavy rain is in northeast Colorado where precipitable water
values are above the 90th percentile and deep layer mean wind and
Corfidi vector magnitudes are lower (generally 10 knots or less).
Therefore, the greatest concern would be around the time of
convective initiation over higher terrain, and for the several
hours afterward. With bulk shear of 30-40 knots, however, as the
storms become more numerous they should begin to organize into
clusters or lines and begin to establish cold pools and accelerate
eastward onto the Plains, which would eventually reduce the threat
of flash flooding. Further to the southwest (NM, AZ), storm motions
should remain relatively slow and should promote the threat of
isolated flash flooding.

...Western Florida Peninsula and the Keys...
No changes. PREVIOUS DISCUSSION: A shortwave diving along the
Florida Peninsula/Western Gulf will interact with broad diffluence
aloft to maintain deep layer ascent for convection across the
eastern Gulf and into parts of South Florida and the keys. Mean
0-6km winds of just 5-10 kts will be aligned SW to NW, anti-
parallel to collapsed Corfidi vectors, and parallel to the Keys.
This suggests that convection will repeatedly build across the Gulf
where PWs will be pooled at 2-2.25 inches within impressive
2000-3000 J/kg MUCAPE leading to 2-4" of rainfall with locally
higher amounts.

...Missouri Valley...
A Marginal Risk was maintained but the Slight Risk was removed
across this region, given a reduced signal in the hi-res ensemble
guidance for heavy rainfall.

Lamers/Bann


Day 2
Valid 12Z Wed Jul 29 2026 - 12Z Thu Jul 30 2026

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

...Northeast...
Rainfall rates and additional rainfall amounts will initially be
heavy early in the period over the eastern portion of the Northeast
before they begin to diminish on Wednesday as an anomalously deep
low pressure system begins to occlude. There should be rainfall
rates around one inch per hour leading to widespread 1-3" of rain
with locally higher amounts.

The latest guidance still depicts a sharp gradient of instability
which may limit the rates in parts of Vermont, New Hampshire and
Maine. This part of the Northeast US should have received less
rainfall with Flash Flood Guidance less likely to have been
compromised. The previous outlook largely had this covered and
changes were fairly minor with this outlook.

...Four Corners States into the Central/Northern Plains...
Monsoonal flow persists with isolated to widely scattered
convection extending from the Four Corners region north and
eastward...perhaps extending as far east as the plains of Colorado.
Some of this rain could fall atop soils saturated from heavy rain
on Tuesday and Tuesday night. It remains possible that a Slight
Risk area may be needed for parts of the High Plains but there was
not enough confidence to do so in this outlook.

...Central Gulf Coast Region into parts of the Southeast...
A cold front stalling along the Gulf Coast will be a focus for slow
moving and training thunderstorms on Wednesday. Guidance continues
to indicate a low-probability threat for at least 3" of rain due to
training of 1-2"/hr rain rates within favorable thermodynamics.
This region can generally tolerate significant rain rates and this
amount of rainfall, but any more pronounced training could result
in isolated flash flood related impacts.

Bann/Weiss


Day 3
Valid 12Z Thu Jul 30 2026 - 12Z Fri Jul 31 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE LOWER MISSOURI AND MID TO UPPER MISSISSIPPI VALLEY...

...Lower Missouri Valley into the Mid-Upper Mississippi Valley...
Shortwave energy will be progressing east southeast as it tracks
across parts of the central and northern plains on Thursday into
Thursday night/early Friday morning. The shortwave energy will be
encountering an increasingly moist and unstable airmass with
precipitable water values in excess of 2 inches being drawn into
the region ahead of the shortwave. Given the MCS/upscale growth
potential with this system, concentrated areas of heavy rain are
expected, especially across the Corn Belt. The Slight risk area
already issued for Thursday still covers the risk....especially
where the global ensemble probabilities of >3" in 12/24 hours are
highest.

...Four Corners States into the Central/Northern Plains...
Monsoonal flow and anomalous deep-layer moisture will maintain an
isolated excessive rainfall risk extending from eastern Arizona
northeastward across much of western and northern New Mexico into
parts of Colorado. The expanding mid/upper level ridge will likely
limit the coverage compared to recent days.

...Central Gulf Coast Region into parts of the Southeast...
Heavy rainfall is expected along a front associated with a low
pressure system which will be pulling away from New England.
Heavy rainfall with some potential of flooding will be focused
along and near the front...especially over parts of Florida and
Georgia line.

Bann


Day 4 and Day 5

Valid 12Z Fri Jul 31 2026 - 12Z Sun Aug 02 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FRIDAY ACROSS
PARTS OF THE MID AND UPPER MISSISSIPPI, AND ON SATURDAY OVER
PORTIONS OF THE LOWER OHIO VALLEY, TENNESSEE VALLEY, AND DEEP
SOUTH...

...Mississippi Valley-Ohio Valley-Tennessee Valley-Deep South...
On Day 4 (Friday-Friday night), minor changes were made to
yesterday's Day 5 ERO, including refinements to the Slight Risk
area over parts of the Mid-Upper MS Valley, based on the latest
(00Z) global deterministic and probabilistic QPFs. The operational
models continue to show max 24hr totals in the 3-5" range,
particularly over IA-southeast MN-southwest WI, though again with
some spatial spread in the guidance. 00Z Operational ECMWF has a
bit more areal coverage of the 3+ inch totals, while the ECENS QPF
exceedance probabilities have once again come up from the previous
run. Based on the latest guidance, along with the most recent ECMWF
EFI output (EFI values aoa 0.6), have nudged the southern portion
of the Slight Risk area a little farther north across the Mid MS
Valley.

On Day 5 (Saturday-Saturday night), the globals continue to show a
fairly vigorous 500 mb trough/embedded shortwave energy slowly pivoting
into the Mid MS Valley. There isn't as strong of a signal in the
QPF guidance compared to the previous couple of days; however, the
amplifying upper level trough (favorable Qs forcing and increasing
effective bulk shear) will combine with a favorable thermodynamic
profile (ML CAPEs 1000-2000 J/Kg along a swath of 1.8-2.1" PWs) to
generate organized heavy rainfall threat across western portions
of the Lower OH Valley into the TN Valley and perhaps as far south
as southern AL, depending on how far south the upper shortwave and
favorable 0-6KM bulk shear can get. Concern within this area will
be heavy rainfall rates (2+ in/hr) given the thermodynamic profile,
along with the potential for cell training of convection as the
upper shortwave begins to stall and the low-level inflow out ahead
of it increases up and aligns more parallel to the mean 850-300 mb
flow. Because of these factors, we felt that a Slight Risk was
warranted.

...Four Corners States into the Southern-Central High Plains...
Monsoonal flow and anomalous deep-layer moisture will maintain a
Marginal (isolated) excessive rainfall risk over eastern AZ and
much of NM into much of CO. The expanding mid/upper level ridge
(600dm heights at 500 mb) will likely limit the coverage compared
to recent days, especially on Friday.

...Central Gulf Coast Region into parts of the Southeast...
Heavy rainfall is expected along the New England cyclone's trailing
front near the FL/GA border, the front that will become more
quasi-stationary Thu-Fri before lifting lifting back north on Sat
ahead of the approaching upper trough. Deep-layer moisture and
instability should be more than sufficient and the guidance shows
sufficient effective bulk shear (0-6km magnitudes generally over 20
kts) for more organized/persistent convective organization. As a
result, have maintained the inherited Marginal Risk on Day 4 (Fri-
Fri night), with a similar Marginal Risk area on Day 5 (Sat-Sat
night).

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



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