Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
848 PM EDT Sat May 4 2024
Day 3
Valid 12Z Mon May 06 2024 - 12Z Tue May 07 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL AND NORTHERN PLAINS ON MONDAY AND MONDAY NIGHT...

21Z update... There is a growing consensus for much of eastern
Montana and surrounding areas to get a soaking rain for this
period. Much of this region has nearly saturated soils which has
reduced the 3-hr FFG to as low as 1 inch. Portions of the Bear Paw
Mountains in north-central Montana still have liquid equivalent of
1 to 2.5 inches in the snowpack. Rain over this snowpack will
likely speed up snowmelt while increasing the amount of water
available for runoff. Areas of local flash flooding will be
possible.

For the Central Plains, the latest guidance and WPC forecast
shifted the QPF to the east/northeast about 100-150 miles. This
resulted in trimming the back edge of the Marginal to central
South Dakota to northeast Oklahoma and expanding the eastern
boundary into southern Minnesota and eastern Iowa. The Slight Risk
was shifted eastward and now encompasses eastern Kansas/Nebraska,
southeast South Dakota, western Iowa and western Missouri.

Campbell

Heavy rainfall that results in flash flooding is possible from
convection that is expected to develop over the Plains on Sunday as
an upper trough ejects northeastward across the northern/central
Plains on Monday. Precipitable water values of 1.2 to 1.6 inches
will be drawn northward on 850 mb flow of 30 to 45 kts and modest
height falls ahead of the trough aloft and a surface dry line.
Rainfall rates over 1.5 inches per hour appear reasonable given the
amount of instability to aid updraft strength. The broadly
diffluent aloft that forms could result in repeated rounds of
heavy rainfall in some cases but broader/larger scale organization
may keep individual cell motion progressive enough to mitigate at
least some of the excessive rainfall potential.

Bann