Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1152 AM EDT Mon Apr 27 2026
Day 1
Valid 16Z Mon Apr 27 2026 - 12Z Tue Apr 28 2026

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

16Z Update...
Few changes were made to the overall outlook. There western
boundary of the Slight Risk area was trimmed based on the latest
radar and satellite imagery but made few changes to the Marginal in
deference to additional showers bubbling up over northeast Kansas.
Those showers are not likely to be a major rainfall producer but
they would move over area soaked overnight/early this morning and
where flash flood guidance has lowered considerably. The HRRR
continued to be tracking reasonably well with the evolution...but
along with other CAMs...seemed to be running too slow with the
forward motion of the cells. Elsewhere...the changes to the
previous outlook were minimal and much of the previous discussion
remains valid.

0830Z Excessive Rainfall Discussion...

Multiple rounds of convection are expected today ahead of a cold
front extending from an anomalously deep spring low. Significant
northward moisture transport will result in a plume of PWs above 1
inch as far north as Lake Superior, and above 1.5 inches as far
north as Chicago. Along the entire length of the cold front, these
values will generally approach the 99th percentile for late April
and, combined with moderate to strong instability, should support
hourly rain rates in the 1 to 2 inch range. A limiting factor for
flash flooding will be the dominant convective mode -- very likely
to be relatively narrow squall lines, forced by a steadily
advancing cold front, that will limit the overall duration of the
heavy rain rates.

However, an area from northern and central Missouri, into much of
Illinois and Indiana, will have a greater potential for training
convection. This will be due to a significant upstream complex of
thunderstorms over northeast Kansas as of 07Z, which should shift
east over the remainder of the morning. The orientation of the
convective line is closer to parallel with the deep layer shear
vector, which will favor training over northern Missouri at the
start of the period. Eventually, this activity may produce an
outflow boundary oriented in a west-east fashion that would focus
subsequent convective development in a more favorable configuration
for additional training. The broad Slight Risk was maintained and
adjusted to better fit the latest guidance where probabilities of 1
inch per hour rain rates are highest.

Further south, a Marginal Risk was extended from Arkansas and
Tennessee southwest into central Texas to account for a conditional
threat of localized heavy rainfall and flash flooding. The
environment in these areas will be characterized by strong
instability (CAPE in excess of 3000 j/kg) and abundant deep
moisture (PW between 1.5 and 1.8 inches). The QPF signal in
guidance is inconsistent, but enough to indicate a threat for
isolated to scattered thunderstorms, primarily in the afternoon and
evening. The combination of the aforementioned instability with
strong deep layer shear could lead to a few slow-moving
supercells, which has been a theme in recent days. It is a classic
conditional threat -- while much of this region further south along
the front could remain dry, any storms that become organized and
rooted in the very unstable boundary layer could produce localized
very heavy rainfall.

Lamers

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