Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
405 AM EDT Sun Aug 9 2026
Day 1
Valid 12Z Sun Aug 09 2026 - 12Z Mon Aug 10 2026

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

...Upper Midwest...

A series of thunderstorms currently ongoing from northern South
Dakota and in parts of Minnesota this morning are expected to
congeal into an MCS by late this morning, moving into the Slight
risk area of Wisconsin around midday. This first bout will begin to
prime the soils, but due to its fast eastward motion, is unlikely
to cause too many flash flooding issues. The storms traverse the
L.P. of Michigan late this afternoon and evening. Behind it, with
southerly return flow still in place, the cold pool behind this MCS
will act as a frontal boundary. The southerly flow will uplift
moist and incredibly unstable air across the Chicagoland region. A
renewed round of showers and thunderstorms is likely to form in
that area as soon as this evening. Between the continued
southwesterly flow of instability and moisture and the new cold
pools from the storms, this second round of storms will have a
struggle moving much of anywhere, unlike its predecessor. While it
appears most likely this will occur near Chicago, which would be a
big problem as far as flooding concerns go, there remains some
uncertainty. Regardless, the storms continue to grow upscale and
downwind, which in this case is towards the east. This will lead to
additional training thunderstorms extending along the
Michigan/Indiana border, potentially stretching into far
northwestern Ohio. It's this overnight convection, which will
likely persist to some extent right through the night that the
Slight Risk is for.

A higher end Slight is in place from Milwaukee and Chicago east
along all of the Michigan/Indiana border to the corner of Ohio.
Considering the aridity of the soils in this region for this first
day of rain, it appears unlikely at this time that a Moderate Risk
upgrade will be needed. However, prolonged heavy rainfall almost
anywhere can overcome even the driest of soils, so such an upgrade
can't be ruled out. Much of the shower and thunderstorm activity
should be east of the Slight Risk area into Ohio and Ontario before
sunrise on Monday morning.

...Southeast...

Abundant deep tropical moisture will remain in place across much of
the Southeast, as it has been for several days already, through the
day today. Unlike further north, at these latitudes, a
comprehensive forcing mechanism, especially from the upper levels
will be lacking. Thus, with plentiful daytime heating, widespread
disorganized convection is likely once again this afternoon over a
large swath of country. The inherited Marginal was expanded down to
the Gulf coast, as storms in the vicinity of New Orleans and
Houston could cause localized and isolated flash flooding since any
storms in this environment will be capable of short periods of
multiple inch per hour rainfall rates given the abundance of
moisture for the storms to work with. If flash flooding were to
occur, it appears most likely in this region in the southern
Appalachian region of far north Georgia and western North Carolina,
where terrain effects could locally increase the duration of heavy
rain from any storms that form.

...Southwest...

Of the next 3 days, the monsoonal storms impacting the Southwest
will be at a minimum today, generally focused on the larger terrain
features such as the Mogollon Rim, and portions of Utah. The
greatest moisture, as usual, will be closest to the Gulf of
California in southwestern Arizona. So any storms that form along
the Mexican border will likely have a higher chance of producing
flash flooding compared to most other areas of the Southwest.
Regardless, with a lack of real forcing, the storms should still be
disorganized and therefore only produce isolated flash flooding in
their localized areas today.

Wegman


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt