Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
845 PM EDT Mon May 6 2024
Day 2
Valid 12Z Tue May 07 2024 - 12Z Wed May 08 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE GREAT LAKES TO OHIO VALLEY AS WELL AS NORTHERN HIGH
PLAINS/ROCKIES...

...20Z Update...
Only small adjustments were needed in the
afternoon update to account for trends in model guidance. The
strong low pressure system that impacted the Plains on Monday will
push northeast on Tuesday while weakening. At this time, there
still was not enough confidence to introduce a Slight Risk for this
area, but one may be needed with the overnight update. Hi-res CAMs
continue to show there is potential for isolated to scattered
showers/storms with heavy rainfall, but there is still uncertainty
in the location and coverage. Storm motions will also be fast,
which should help to limit flash flooding, but repeat activity
could be a concern.

For the northern Plains and Rockies, the prolonged heavy rainfall
event will continue as a slow moving low remains positioned over
the region. Rainfall rates look like they will be lower than the
previous day (Monday), but additional rainfall falling on top of
moist soils could lead to isolated instances of flash flooding.
The Marginal Risk area was extended to the southeast to include
more of southeastern Montana and far southwestern North Dakota
where rainfall totals could reach 1-3 inches.

Dolan


...Previous Discussion...

...Great Lakes to Ohio Valley...
Aloft, the strong storm system from Day 1 will evolve into a closed
mid/upper level low over the Northern High Plains with the leading
warm air advection/isentropic lift precipitation advancing through
the Upper Midwest and Great Lakes during the peak of daytime
heating. Locally heavy rainfall will be possible with marginal
values of instability expected and an increasingly moist
environment aided by strong southerly low level flow. Storm
motions should be rather quick though, at least from the Upper
Midwest to Great Lakes, however further south across the Ohio
Valley, the flow becomes more parallel to the storm motions and may
lead to a greater threat of training/repeating rounds (as well as
deeper instability to support more intense rain rates). Some
guidance points to a greater rainfall threat over the Ohio Valley
that could support the introduction of a Slight Risk in future
updates but uncertainty with how morning convection may influence
outflow boundaries and later thunderstorm development in the
afternoon kept confidence for an upgrade low at this point.

...Northern Plains/Rockies...
The closing mid/upper level low over the Northern High Plains will
linger through the period, with embedded vort energy
pivoting/retrograding westward across the region. This should
continue the forcing/lift across the region and with the anomalous
moisture in place and orographic enhancement, an additional 0.5-1"
is expected and this could cause additional flooding concerns.

Taylor