Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
430 AM EDT Fri Sep 18 2026
Day 2
Valid 12Z Sat Sep 19 2026 - 12Z Sun Sep 20 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE UPPER MIDWEST INTO PARTS OF THE SOUTHERN GREAT LAKES AND IN
SOUTHERN MONTANA...

...Midwest and Ohio Valley...

The environment will remain conducive for heavy rainfall with
precipitable water values firmly at or above 1.8" across the entire
region, locally exceeding 2 inches west of Lake Michigan. This
will provide efficient rainfall types, even in convective elements
that could put down 2-4" of rainfall over the entire area leading
to at least scattered flash flood potential. In particular, there
is potential for overlap between the rainfall on Saturday with a
west-northwest to east- southeast stripe of low Flash Flood
Guidance in northern Illinois which resulted from heavy rainfall 36
to 48 hours earlier. The RRFS was most supportive of this scenario
but it has a bias of high QPF and higher probability of
exceedance. Even so...finer scale models also show some overlap
with locally heavy amounts so the Slight Risk was expanded in areal
coverage and deemed a higher-end Slight Risk...especially if there
are multiple rounds of convection.

...Western and Central U.S...

Latest deterministic model and WPC trended upward across the
Northern Rockies on Saturday as an upper low over the Northern
Rockies slowly pivots eastward providing continued/prolonged
diffluent flow.  The flow aloft ahead of the upper low creating a
threat for pockets of heavier convective potential over the Front
Range of Colorado/Wyoming while also still delivering heavier
precip to Montana as ascent remains strong under the closed low. As
a result...there was enough support to upgrade a portion of
southern Montana to a Slight Risk given the expect amounts on
Saturday that would be in addition to what falls today.

Over the Central Plains, scattered thunderstorms will be possible
along and just ahead of an associated cold front as it advances
southeastward leading to perhaps an isolated flash flood threat for
any stronger storms forecast. The MRGL risk extends throughout
much of the Central Plains and Missouri Valley as a result.

...South Florida and Keys...

A Marginal Risk was continued this period as a continuation from
the Day 1 period. High rainfall rates will be possible across the
southern tip of the peninsula southward across the Keys with
flooding a concern due to weak steering flow.

Bann


Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt