FOUS30 KWBC 270901
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
501 AM EDT Sun Jul 27 2025

Day 1
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, NORTHERN APPALACHIANS AND PARTS OF THE NORTHEAST...

...The eastern Dakotas and Upper Midwest...
Continue to make minor adjustments to the placement of a Slight
Risk area mainly confined to Minnesota as a shortwave trough
continues to propagate eastward along the northern tier of states.
There has been an upward trend in the guidance...both in the
coarser/global guidance and with the CAMs and related ensemble
forecast systems. With guidance now showing 2000 to 3000 J per kg
being possible and precipitable water values approaching 1.5
inches,,,another expansion southward of the Slight and Marginal 
risk areas was made compared with the previous outlook. 

The Marginal Risk area on the northern/western periphery of the
Slight risk area was expanded into North Dakota as a well developed
line of convection was moving across the state and not likely to
diminish prior to the start of the Day 1 period at 12Z.

...The Ohio Valley and Mid Atlantic to southern New England...

The Slight Risks which were raised over the Ohio Valley was
expanded to the west as a complex from overnight moves eastward.
The expectation is that the convection should weaken for a period
before reigniting later. Made a few changes to the outlook area
from the DelMarVa northward based on latest guidance.  Locally 
enhanced rainfall will persist in the very moist environment near 
the west- east frontal boundary.

...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 2
Valid 12Z Mon Jul 28 2025 - 12Z Tue Jul 29 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF 
THE NORTHERN PLAINS...

...Plains of Eastern Montana to parts of the Upper Midwest...
A surface cold front and mid-level shortwave trough will be 
approaching an area rich in moisture and instability over parts of
South Dakota and Minnesota/Iowa on Monday afternoon. With low 
level flow likely to increase the amount of moisture transport into
the region during the evening with a corresponding increase in 
moisture flux convergence along the front...the potential exists 
for excessive rainfall especially in the event of cell training or 
repeat convection. Uncertainty remained with respect to placement
in the numerical guidance...but the RRFS seemed too far north. The
WPC deterministic QPF was closer to a GEFS/ECMWF ensemble mean
position...which still left some room for excessive rainfall to
build south. Consequently...allowed for a somewhat broader apron of
Slight Risk than shown by guidance.

Surrounding the Slight risk area...only modest changes were made 
to the Marginal risk area extending northwest into the plains of 
eastern Montana where a secondary QPF maximum is expected. The 
expectation is that there will be two discrete QPF area with a risk
of excessive rainfall associated with each...but maintained the 
one area given the range of possible solutions. 

...Southwest US...

A continuation of flow with deeper moisture into the Southwest US 
will linger on Monday and Monday night...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


Day 3
Valid 12Z Tue Jul 29 2025 - 12Z Wed Jul 30 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE WESTERN HIGH PLAINS...

Northern Rockies eastward to Great Lakes and southward to the Southwest US...

Building mid-level ridge over British Columbia and Albert will
result in lowering heights over parts of the northwestern US and
increasing mid- and upper flow. This results in an increasingly 
well defined surface boundary that serves to focus showers and 
thunderstorm on Tuesday. Ingredients for thunderstorms to produce
heavy to potentially excessive rainfall appear to be aligning from 
southeast Montana into northern Nebraska along a corridor of 1000 
to 2000 J per kg instability and 1 to 1.25 inch precipitable 
water values...and even some 1.5 inch PW values in northern 
Nebraska. Showers and thunderstorms are also likely to spread 
eastward toward the Great Lakes where the instability and synoptic 
scale forcing looks to be weaker. In addition...another round of 
late day convection is possible from the Southwest US into the 
Rockies and some of this moisture may approach...if not come into
play,,,across the western High Plains late in the period.

...Southeast US...
Precipitable water values in excess of 2 inches will be in place
with 1000-1500 J per kg of CAPE in place and weakening of the broad
upper level ridge should lead to a somewhat better chance for
showers or thunderstorms on Tuesday. The GFS and ECMWF both show
hints at weak vorticity centers in the vicinity to help
focus/support some activity. The expectation is that isolated but
very intense rainfall rates could lead to isolated instances of
excessive rainfall.

Bann
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