FOUS30 KWBC 281352
QPFERD

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

Day 1  Valid 1345Z 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...

...Southern Appalachians through New England...
1345Z SPECIAL UPDATE: A special update was coordinated with WFO Mt.
Holly and the National Water Center to extend the Moderate Risk
southward through the remainder of eastern Pennsylvania, as well as
the I-95 corridor through Philadelphia and Wilmington, DE.
Increasing hi-res model signal for intense rain rates, particularly
between 18Z and 03Z, prompted the upgrade. The combination of
moderate to strong instability and PW values increasing through the
day to between 1.6 and 1.9 inches will make rain rates reaching 2
inches per hour in the strongest convection likely, and there is a
chance the most organized clusters could see rain rates reach 3
inches per hour. In an area with a lot of flashy streams and
urbanization, this will make flash flooding increasingly likely
this afternoon, potentially impacting the evening rush in parts of the area.

PREVIOUS DISCUSSION: An anomalously strong upper low will produce
impressive dynamics to create the potential for significant flash
flooding across the Tri-State area today and tonight.

A closed low will be forming over Upstate NY this evening and then
continue to amplify and deepen as it curls southeast into New
England. This intense system will help drive increasing divergence
downstream, which when combined with the accompanying height falls
will result in broad deep layer ascent from the Mid- Atlantic into
New England. At the same time, the deepening system will force
acceleration of the 850mb inflow,

Precipitable water values and the magnitude of the surface-based
CAPE will be near climatological maximums (reflected via the SPC
sounding climatology), with precipitable water values around 1.75
inches overlapping most-unstable CAPE of 2000-3000 J/kg. Strong
ascent in the vicinity of this anomalous closed low and near the
accompanying occluded surface low into these strong thermodynamics
sets a situation favorable for heavy, training rainfall, and flash flooding.

The 28/00Z HREF and REFS both maintain probabilities for 2"/hr
rainfall reach 15-25%, and Corfidi vectors aligned anti- parallel
to the mean wind which will be aligned to the approaching front,
which suggests a strong backbuilding and training potential. This
fostered enough confidence for the day-shift to upgrade part of the
area to a Moderate Risk. The Moderate risk was maintained with the
western boundary of the Slight and Moderate expanded a bit
westward--mainly in New York state--due to the model suite slowing
down the timing of convective initiation and propagation.

...Four Corners States across the Central Plains...
The expansive monsoon ridge will become additionally squeezed as
shortwaves rotate anti-cyclonically around it both to the north and
south. This will again support at least scattered monsoon
convection as precipitable water values at or above 1 inch northward
into the Four Corners overlapped with sufficient instability for
afternoon development. Once again the risk will likely initiate in
the higher terrain during the onset of peak heating, and then there
appears to be sufficient shear (20-30 kts) for some organization
off the terrain and into the lower elevations, primarily in New
Mexico and Colorado, although there is potential for storms to be
very slow movers.

...Western Florida Peninsula and the Keys...
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...
Overnight convection leading into the start of the Day 1 period at
29/12Z prompted a Slight Risk area ahead of the storms. The
expectation is that the storms will weaken...followed by the next
round of storms later today into tonight as a weakening shortwave
crests the monsoon ridge before diving southeast. As the shortwave
moves along a front tonight...it will help accelerate the already
intensifying LLJ to draw robust thermodynamics northward. This will
support organizing clusters of convection or an MCS aligning to
the front as both 0-6km mean winds and Corfidi vectors become
parallel. With rainfall rates likely reaching 1-2"/hr at times, any
training has the potential to produce 2-4" of rain (HREF and REFS
probabilities for 3+" 20-25%) leading to instances of flash flooding.

Bann/Weiss
 

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
$$
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