Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
858 PM EDT Fri Aug 8 2025
Day 2
Valid 12Z Sat Aug 09 2025 - 12Z Sun Aug 10 2025
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR EASTERN IOWA,
SOUTHERN WISCONSIN, AND FAR NORTHWESTERN ILLINOIS...
...20Z Update...
...Upper Midwest...
In coordination with DVN/Quad Cities, IA; MKX/Milwaukee, WI;
ARX/LaCrosse, WI; and DMX/Des Moines, IA forecast offices, a
Moderate Risk upgrade was introduced with this update for much of
eastern Iowa, southern Wisconsin, and far northwestern Illinois. A
cold front will move eastward into the upper Midwest from the
northern Plains tonight. This front will stall out as both its
parent surface low tracks northeast, and the upper level low
supporting the cold air pushing the front east also retreats
towards the north. This will leave the front stalled out over
central Iowa northeast into far western Wisconsin. Meanwhile,
abundant Gulf moisture is already in place across the upper
Midwest, and will further increase overnight as the cold front both
pushes it eastward, and allows the moisture to pool along the front
in the form of showers and thunderstorms. The initial time the
front stalls will be when it is strongest, and able to support the
greatest coverage of storms. Instability will be incredibly high
south and east of the front in the warm and very humid air mass,
with the high resolution guidance on average showing MUCAPE values
north of 4,000 J/kg. This will support explosive development of the
storms as they form, with the cold pools of those storms supporting
additional storm development.
Guidance is in decent agreement that the storms will initiate to
the north in Wisconsin, then like a chain reaction, pop down the
front towards the southwest. The predominant flow will be
southwesterly, parallel to the front, with Corfidi vectors
perpendicular to the flow, out of the northwest. Thus, expect some
training of storms towards the northeast as any lines of storms
track towards the southeast. Since the forcing front will be
stalling out, the steady advection of additional Gulf moisture and
instability on a 25 kt southwesterly LLJ at 850 mb will support
additional convective development towards the south and west, with
those storms then tracking southeast. This will result in areas
where there are multiple hours of heavy rain over individual areas.
While the exact location of those areas is very difficult to
pinpoint much before the event begins, the Moderate Risk area
highlights where the greatest density of locations subject to
multiple hours of heavy rain will be. Previous heavy rainfall over
Iowa have led to rivers in the area already higher than normal, so
this significant addition of rain will greatly increase stream,
creek, and river levels all across the area. Considerable flash and
urban flooding is likely, resulting from 1-3 inch per hour rainfall
rates with the stronger storms, widespread rainfall totals of 4-6
inches, and isolated higher rainfall amounts possible. As is
typical of these events, the greatest coverage and intensity storms
are likely to occur overnight Saturday night into Sunday, as the
heaviest rains have pushed into eastern Iowa. For the storms in
Wisconsin, the storms will be through the day, but with a similar
setup for movement, coverage, and intensity of storms.
Expect 2-3 rounds of storms over the heaviest impacted spots, which
appears likeliest to occur over eastern and southeastern Iowa. ARIs
are near 25 year recurrence intervals along the Mississippi River,
and up to a 70% chance of exceeding flash flood guidance over much
of eastern Iowa. The flash flood guidance numbers are likely to
decrease as a result of the heavy rain on D2/Saturday.
For the surrounding Slight Risk area, much of it factors in
uncertainty with how far away from the main front the heaviest rain
can get, and stays largely equidistant from the Moderate Risk area.
That said, lesser amounts of training and storms are still
expected, and could cause widely scattered instances of flash
flooding.
...Southwest...
The Marginal Risk across the Southwest was greatly trimmed in
coordination with ABQ/Albuquerque, NM and EPZ/El Paso, TX forecast
offices. The monsoon will be waning through the period, which
should keep any wet thunderstorms confined to mostly southeastern
Arizona on Saturday, elsewhere, abundant dry air should keep most
convection as dry thunderstorms. Guidance has greatly increased the
precipitation footprint across that area, which too contributed to
the downgrade.
...Southeast...
No changes were made to the Marginal Risk from southern South
Carolina through much of the Florida Peninsula. Scattered to
numerous storms are likely to combat very high FFGs in the area,
resulting in isolated instances of flash flooding. The storms could
organize into local clusters, but heavy rain is unlikely to remain
over any one area for an extended period of time, despite the
abundance of moisture (PWATs well over 2 inches), so for now the
Marginal Risk remains in place with little agreement on where any
embedded higher risk areas will be.
...High Plains of Colorado and Western Kansas...
Widely scattered convection across eastern Colorado could pose an
isolated flash flooding risk over the area on Saturday. While a few
storms may initiate over the area during the afternoon, the greater
coverage of storms will occur overnight Saturday night. Storm
motions should be fast enough to the southeast to only pose an
isolated flash flooding risk.
Wegman
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
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