Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
856 PM EDT Tue Sep 8 2026
Day 1
Valid 01Z Wed Sep 09 2026 - 12Z Wed Sep 09 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS IOWA...

01Z Update...
Made only minor modifications to ensure the Slight Risk covered the
WPC Mesoscale Convective Discussion #1225 for the area. That
brought a few counties of Nebraska into the Slight and a bit of
Illinois into the Marginal risk area. Overall forecast reasoning
remains similar to what was described in previous Excessive
Rainfall Discussions. Given trends in radar...removed the Marginal
Risk area over Florida. For the Western US...expectation is that
convection should be waning with the loss of daytime heating but
felt it was too early to remove the Marginal Risk areas at this
point.

Bann

16Z Update..

Only a minor expansion to the inherited SLGT Risk area was needed
into northeast Iowa and through far southwest Wisconsin. The
previously mentioned meteorological setup remains valid, and
adjustments were made based on the latest suite of global and CAMs
guidance. The surface front forecast to move through Iowa will
support a line of a progressive and efficient line of
thunderstorms. PWAT values are forecast to exceed 2", and in some
scenarios approach 2.25", and will support heavy rain rates, with
HREF and RRFS probs suggesting high chances (65% and 85%
respectively) of 1"/hr rates. SREF Spaghetti Plots also suggested
greater consensus of 2" amounts over the Day 1 period in northeast
Iowa, which further supported the expansion.

Otherwise, only cosmetic adjustments were needed for the inherited
MRGL Risk area to match the latest run of global and CAMs
guidance.

Blanco-Alcala

...Previous Discussion...

...Iowa...
The surface front currently pushing east into the Northern Plains
will remain very progressive day 1 as it moves into the Upper
Mississippi Valley/Upper Lakes. The progressive nature of the front
will continue to be a detriment to widespread runoff issues.
However, the southwestern portion of this front will slow somewhat
compared to the farther northern portion in the vicinity of eastern
NE /northeast Kansas, east into Iowa. Across these regions there
is a better chance of a period of training of cells late
Tuesday/early hours of Wednesday. A slight risk area was maintained
where current HREF and RRFS neighborhood probabilities are high
for 1, 2 and 3"+ amounts this period.

Marginal Risk areas

..Eastern Arizona into western to central New Mexico...
No significant changes day 1 to the axis of above average PW
values extending from eastern portions of Arizona into western to
central New Mexico. Continued low confidence with qpf details, but
potential for another round of widespread scattered convection
across this area. Locally heavy rain and isolated runoff issues
again possible. Anomalous PW values also expected to push farther
north from Southern CA into Central CA. At the moment, models are
not showing precip developing day 1 in this anomalous axis.

...Central to South Florida...
No changes made to the previous marginal risk across central to
south Florida. PW values are expected to remain anomalous day 1,
2.00-2.25"+ to the south of the frontal boundary across northern to
central FL. HREF and RRFS neighborhood probabilities are fairly
high for 1,2 and 3"+ totals. Areas of heavy rain likely with
isolated runoff issues possible, especially in urbanized regions.

Oravec


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