Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
345 PM EDT Fri Oct 17 2025
Day 2
Valid 12Z Sat Oct 18 2025 - 12Z Sun Oct 19 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR SOUTHERN
MISSOURI...

...2030Z Update...

The most significant change this update was to shrink the Slight to
just the Ozarks of southern Missouri. In coordination with LSX/St.
Louis, MO forecast office, the Slight was removed from the St.
Louis metro area. Much of the latest guidance suggests the heaviest
rainfall will be concentrated in the Ozarks, where there is likely
an upslope component allowing the rain to concentrate across the
Ozarks. From St. Louis on north and east into Michigan, instability
will be lesser, and the front organizing all of the rainfall will
be more progressive, which will limit the flash flooding potential.

Perhaps a greater factor than even the potential for heavy rainfall
will be the antecedent extremely dry/drought conditions in place
across all of the Midwest. While in the Ozarks, the terrain helps
focus the rainfall, which in combination with leaf debris should
increase the instances of flash flooding, given the peak in
forecast rainfall in the Ozarks, once into Illinois, Indiana, and
Michigan, even more extreme drought and the lack of topography
should allow any heavy rain to be readily absorbed by the parched
earth. Thus, the area remains in a Marginal, with a focus on any
urban areas (Indiana suburbs of Chicago for example) or any flood-
prone, low-lying areas.

Wegman

...Previous Discussion...

The latest models remain in good agreement on the overall
sharpening of the mid to upper level trof across the mid section of
the nation Saturday into early Sunday. Well defined upper difluence
likely between the northern stream vort moving east southeast
across the Central Plains and the southern stream vort moving east
northeast across the Southern Plains. PW values will increase to 1
to 2+ standard deviations above the mean in this broad upper
diffluent axis from eastern portions of the Southern Plains,
northeastward into the Mid MS Valley and Great Lakes, along and
ahead of the associated eastward moving frontal boundary. Model
consensus is for a broad area of moderate to heavy rainfall amounts
in this axis. All of this region has been very dry with relative
soil moisture low, low stream flows and subsequently FFG values
very high, which will be a detriment to widespread runoff issues.
The previous slight risk area was shrunk considerably, focusing it
where the HREF and RRFS means show higher probabilities for 1 and
2"+ amounts in the 12 hour period from 12Z Sat Oct 18 to 00Z Sun
Oct 19 from northwest AR, across southern MO into southern IL. This
region also corresponds to where the relative soil moisture is not
as low as points farther to the northeast and southwest, although
overall, still low. There may be a period of training of cells in
the slight risk area during the first half of the day 2 period,
before the convection begins to accelerate to the east and
southeast during the second half of day 2. The previous marginal
risk area was also decreased significantly, narrowing the eastern
extent through the OH and TN Valleys.

Oravec


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