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FOUS30 KWBC 090058
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
858 PM EDT Fri Aug 8 2025

Day 1
Valid 01Z Sat Aug 09 2025 - 12Z Sat Aug 09 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
NORTHERN MINNESOTA INTO FAR NORTHWESTERN WISCONSIN...

01Z Update...
Awaiting the initiation of the next MCS across portions of the
Upper Midwest near or just east of the ND/MN border, and rapidly
progress east in very similar fashion to the behavior of the
MCS earlier this morning. The MCS is likely to take the form of a
squall line, and therefore be a minimal flooding threat, despite
the storms having ample deep-layered moisture (up to 3 sigma above
normal) climatology to work with and precipitable water values
approaching 2 inches. The primary flooding concern remains where
any storms get hung up on the southern end of the line, resulting
in localized training. If this does occur, then the southern
portion of the Slight along the MN/WI border would have the highest
flash flooding risk. The 18Z HREF largely supported placement of
the previously-issued Slight Risk area and the surrounding
Marginal Risk area. The past couple

Farther south, the Marginal risk area was pulled south to cover
much of Iowa and a portion of eastern Nebraska where the latest
runs of the HRRR have been supporting the 18Z NAM nest idea of
convection with potential for some intense rain rates building
farther south and east than suggested by the HREF...although the
NAM nest maximum may be too high.

...Southeast...
Maintained the Marginal risk area over coastal portions of South
Carolina and Georgia into the northeast corner of Florida given
persistence of on-shore flow and the lingering cyclonic energy in
the low levels just off shore. A downward trend should continue
along the west coast of the Florida peninsula so that portion of
the state was removed from the Marginal risk area.

Bann


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 3
Valid 12Z Sun Aug 10 2025 - 12Z Mon Aug 11 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FROM FAR NORTHERN
MISSOURI THROUGH MUCH OF EASTERN IOWA AND INTO SOUTHWESTERN
WISCONSIN...

...20Z Update...

In coordination with DVN/Quad Cities, IA; MKX/Milwaukee, WI;
ARX/LaCrosse, WI; and DMX/Des Moines, IA; and EAX/Pleasant Hill, MO
forecast offices, a Moderate Risk upgrade was introduced with this
update for far northern Missouri, much of eastern Iowa, southern
Wisconsin, and far northwestern Illinois. This area is very similar
to the Moderate Risk area from D2/Saturday, as the front forcing
all of the storms that will cause the flash flooding will be
ongoing across much of eastern Iowa Sunday morning, weaken during
the day, with more storms moving in from the west on Sunday night.
The storms are likely to develop along the same stalled out front
again Sunday night, but will extend well southwest of the
D2/Saturday period across much of northern Missouri and likely
eastern Kansas as well. The Moderate Risk was kept to mostly Iowa
for now since it will be a continuation of very heavy rainfall from
the D2/Saturday period. However, expect multiple inches of rain to
impact much of northern Missouri, including the Kansas City and St.
Joseph metros. Since relatively little rain is expected in those
metros on Saturday, the areas may take a round of storms before
soils are moistened up, but even there, training and multiple
rounds of storms are likely. Thus, while Kansas City and St. Joseph
are currently in a higher-end Slight, it`s very possible the area
may need to be upgraded to a Moderate as well with future updates.
Regardless, the continuation of very heavy rain in already hard hit
areas will likely result in more impactful and widespread flooding
impacts across Iowa, far western Illinois, and far southwestern
Wisconsin on Sunday as compared with Saturday.

More widespread convection is also likely across much of Kansas and
into far eastern Colorado on Sunday. The storms are unlikely to
result in more flooding on Sunday since the storms on Saturday
should be relatively few and far between, but the area will be
monitored for potential need to upgrade to a Slight Risk, extending
from the current Slight across eastern Kansas.

The Marginals in the Southeast and Southwest were largely
unchanged. See the previous discussion below for more details on
those areas.

Wegman

...Previous Discussion...

...Southern Mid-Atlantic Coast to Northeastern Gulf Coast...

A lingering frontal boundary over northern Florida and high
pressure anchored over the northern Mid-Atlantic will maintain
low level southeasterly winds into the southeastern U.S. on Sunday.
Precipitable water values of 2.0 to 2.3 inches over the Southeast
and Florida will again support intense tropical downpours as
thunderstorms increase with daytime heating. Low level
southeasterly winds and confluent axes will combine with sea breeze
convergence and other mesoscale boundaries to support scattered
thunderstorms with isolated flash flood potential from 2-4 in/hr
rainfall rates.

...Southwest...

Continued advancing of the base of an upper trough from the central
Rockies into the central Plains and an elongated ridge over
California will favor northwesterly mid-level flow into the
Southwest. While this may reduce moisture availability across the
Four Corners region, low level southerly flow is expected to be in
place over the northern Gulf of California into Arizona, with
westerly flow advecting low to mid-level moisture downstream toward
New Mexico. Meanwhile, the front over the High Plains from
Saturday night is forecast to again sink down to the south with an
upslope flow regime setting up over northeastern New Mexico,
helping to increase low level moisture into the region. Moisture
values for much of central/southern Arizona and New Mexico are
expected to be near average and monsoon thunderstorms are expected
during the late afternoon and evening hours with perhaps greater
coverage than previous days. Typical potential for high short term
rain rates (1-2 inches in an hour or less) will set up with
isolated to widely scattered flash flood potential.

Otto


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