FOUS30 KWBC 260842
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
442 AM EDT Sat Jul 26 2025

Day 1
Valid 12Z Sat Jul 26 2025 - 12Z Sun Jul 27 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF 
THE SOUTHERN GREAT LAKES AND PENNSYLVANIA...

Heavy to excessive rainfall is possible along a corridor associated
with vort energy embedded within mid-level flow which was interacting
with a quasi- stationary front and a modestly unstable, but very 
moist airmass pooled along it.The continuous influx of high PW 
values over this boundary will aid in locally enhanced rainfall. 
In addition...the winds aloft will be favorable for training and 
backbuilding of storms over recently wet soils. This will elevate 
the risk for localized flash flooding, especially for the flash 
prone areas within the terrain of Pennsylvania and surrounding 
locations. Given the amount of rain that fell in parts of Illinois
on Friday and in the overnight period from Friday night into early
this morning prior to the start of the Day 1 period...introduced a
Slight risk area in addition to the one that was already issued.

...Gulf Coast...

The disorganized area of showers and thunderstorms associated with
a trough of low pressure situated across the western half of the 
Gulf will make its way inland over southeast Texas today. Deep
moisture/high precipitable water values and mid- and upper level 
forcing continue producing downpours across the central and 
western Gulf coast with an emphasis on the immediate coastal plain.

...Northern Plains...

The next in a series of potent shortwaves will eject eastward
across the noerhern tier of states of the Northern Plains...resulting
in another round of heavy precip potential with more organized 
convective clusters migrating through the Dakotas. Isolated 2 to 3 
inch rainfall maxima are possible and embedded within broader areas
of lesser rainfall amounts. The pattern is fairly progressive so 
see little reason to move from the Marginal risk
category...although minor nudges were made based on the latest guidance.

...Northeastern California and Northwest Nevada...

Almost a carbon copy of the prior period for convective impacts
lingering across the Northern Sierra up through northeastern
California and northwest Oregon. The upper pattern is pretty slow 
to breakdown with the diffluent area in the longwave setup still 
situated over northern California into Nevada. Instability is 
expected to be modest and slightly above normal moisture in the 
terrain will offer the capability for some isolated flash flood 
concerns within any complex terrain and burn scar remnants in the above area.

Bann


Day 2
Valid 12Z Sun Jul 27 2025 - 12Z Mon Jul 28 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE UPPER MIDWEST...

...The eastern Dakotas and Upper Midwest...

The effects of a shortwave trough interacting with a surface front
across the Upper Midwest will continue into Sunday as does 
uncertainty with placement and timing of any convection capable of 
supporting heavy rainfall. Convection early in the period looks to
be progressive enough to preclude much thereat of excessive
rainfall. However...there should be a weak boundary to the
southwest of the initial convection track with stronger 
instability to the south. The instability and the shear set up a 
scenario where additional convection will be more capable of 
producing heavy to potentially excessive rainfall. Opted to 
maintain a the previously issued Slight risk area after shifting 
the area eastward. 

...The Ohio Valley and Mid Atlantic to southern New England...
Mid-level flow flattens over the Great Lakes and Ohio
Valley...allowing embedded shortwave energy to start propagating
eastward. With the center of the upper high expected to shift just
far enough eastward...some weak height falls will allow shortwave
energy to dig over parts of the Mid-Atlantic. With an anomalously 
moist air mass in place...convection with localized downpours have
a low-end threat to result in localized flooding.

...Southwest US...
Deeper moisture that made its way across parts of the Gulf and
inland across Mexico should start to be drawn northwest into parts
of the Southwest United States...resulting in scattered late day
and evening convection across parts of New Mexico and a small
portion of Arizona.

Bann


Day 3
Valid 12Z Mon Jul 28 2025 - 12Z Tue Jul 29 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE NORTHERN
HIGH PLAINS INTO THE UPPER MIDWEST AND ACROSS PARTS OF THE
SOUTHWEST UNITED STATES...

...Plains of Eastern Montana to parts of the Upper Midwest...
Showers and thunderstorms capable of producing excessive rainfall
are possible on Monday and Monday night from the plains of Montana
southeastward into parts of the Upper Midwest as surface/low level
flow accelerates between low pressure to the south and high
pressure strengthening over southern Canada. Development of surface
low pressure over southwest South Dakota/northwest Nebraska 
results in moisture transport up and over the boundary...leading to
a better chance for enhanced rainfall rates from the Dakotas into 
Minnesota or Iowa. Farther west...there should be a feed of 
moisture and some instability in the easterly flow into eastern 
Montana. Given considerable uncertainty...not incline for more than
a Marginal risk area.

...Southwest US...
A continuation of deeper moisture transport into the Southwest US
will linger into Monday...with a continued chance for scattered 
late day and evening convection across parts of New Mexico and a 
small portion of Arizona that extends northward along the southern
and central Colorado front range.

Bann
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