Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
426 PM EDT Sat Apr 27 2024
Day 2
Valid 12Z Sun Apr 28 2024 - 12Z Mon Apr 29 2024

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS EASTERN PORTIONS
OF THE SOUTHERN PLAINS INTO THE LOWER AND MIDDLE MISSISSIPPI
VALLEY...

Convection will likely be ongoing at 12Z Sunday across portions of
eastern OK and TX, into western AR/MO. Current expectations are
that convection will have enough of a cold pool by this time to
result in a decent eastward propagation, while also running into a
less unstable airmass with time. While some lingering flash flood
risk into Sunday morning is likely over these areas, the risk
should be on a decreasing trend.

The bigger question becomes what happens by later in the day into
the overnight hours. Synoptically, the mid to upper-level trough
over the southern/central High Plains on Saturday will lift NNE
toward Minnesota through Monday morning. As this occurs, a lack of
height falls across the Gulf Coast region will coincide with a
retreating dryline in Texas, but locations from Arkansas down the
Sabine River Valley will remain beneath diffluent and divergent
flow aloft for much of the period (though jet-induced divergence
may be focused better across northern locations than those closer
to the Gulf Coast).

Current thinking is for new convective development along the
front/dryline or perhaps remnant outflow (from the early morning
convection) in the 18-00Z window over northeastern Texas into
southeastern Oklahoma and western Arkansas. The leading edge of the
cold pool from the afternoon/evening thunderstorms is expected to
be intercepted by 25-45 kt of southerly flow at 850 mb, forcing the
development of additional thunderstorms within the unstable
environment. Orientation of convection is likely to match mean
steering flow from the southwest at times, supporting training and
rainfall rates of 1 to 3 in/hr (although localized rates in excess
of 3 in/hr will also be possible).

Overall this event remains as a higher end Slight risk from
east and northeastern Texas into southeastern Oklahoma and western
Arkansas. A more focused and concentrated area of numerous and
significant flash flooding is still a possibility with higher
neighborhood probabilities of exceeding 5 inches in 24 hours
(ending 12Z Monday) showing 40-60 percent from near the Arklatex
down to a Huntsville to Jasper to Fort Polk line and 20-40 percent
for EAS probabilities for 3+ inches along the northern
Texas/Louisiana border. Along with the typical convective
uncertainties, the 12Z global and hires guidance (FV3, NAM_nest)
showed some southward displacement compared to their previous runs,
into drier antecedent conditions where flash flood guidance is
about 4+ inches in 3 hours. Due to these factors, confidence in
placement of a Moderate is not there yet, although Moderate Risk
coverage of flash flooding (locally significant possible) is
expected somewhere within the Slight Risk. 24 hour QPF from the 12Z
model suite ranged from 4 to 7+ inches ending Monday morning.

Across the Midwest into Wisconsin and Michigan, a broad Marginal
Risk remained in place with minor adjustments from continuity.
With northern extent, there remains uncertainty with the
availability of instability but a slow moving front with steering
flow parallel to the boundary will be in place across northern
Illinois/southern Wisconsin into Lower Michigan. While potential
for higher coverage of rates over 1 in/hr is not as high for these
northern locations, flash flood guidance is low in pockets across
the northern half of the outlook area, especially for portions of
Nebraska/Iowa due to recent heavy rain.

Otto/Chenard