Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1157 AM EDT Thu Mar 23 2023

Day 1
Valid 16Z Thu Mar 23 2023 - 12Z Fri Mar 24 2023

...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL OVER A PORTION
OF THE MID/SOUTHERN MISSISSIPPI VALLEY..

An abundant Gulf moisture will be streaming northward ahead of a
deep-layer trough that ejects from the Southern Rockies and
results in strengthening low level flow.  At about the same time,
upper level divergence from an upper level jet aids in surface low
pressure forming along a quasi-stationary front extending from
Oklahoma northeastward into the Ohio/Tennessee Valley. A quick
look at the 12Z NAM and the latest runs of the HRRR still
fundamentally show precipitation breaking out along the same axis
as earlier runs although the timing of heavy rainfall may be just
a tad quicker to develop than shown in the past couple of runs. 
12Z soundings showed an axis of stronger flow (30+ kts) from the
Gulf coast region towards the ArkLaTex.  Models still point to the
axis of an anticyclonic upper jet approaching the region that
increases upper level divergence by late afternoon and early
evening...so the  potential for 1 to 2 inch/hour rain rates still
looks to be on-track.  While this area is not as saturated as it
was a couple of weeks ago...there is local concern given that
ground conditions are still fairly moist. Overall...there were a
few minor adjustments to the on-going outlook areas based on the
latest guidance but those changes do not reflect a major shift in
forecast reasoning.

Bann



Day 2
Valid 12Z Fri Mar 24 2023 - 12Z Sat Mar 25 2023

...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL FROM THE OZARKS
TO THE OHIO VALLEY...

Showers and thunderstorms will continue from the day 1 period in
proximity to the sprawling frontal boundary with moisture flux
convergence increasing across parts of the Mississippi Valley and
into the Ohio Valley as a surface low pressure with trailing cold
front moves east across the Southern Plains. By 25/00Z a PW fetch
of 1.5 inches will span from the Gulf Coast to the MO boot heel,
feeding right into the frontal boundary, which is some (+2 to +2.5
standard deviations greater than climatology for late March. 
Repeating storms will track across areas soaked from recent rains
and will likely surpass local FFGs early into the Day 2 period.
The latest model runs have been persistent with areal averages of
2 to 4+ inches with above average confidence. This part of the
nation will be sensitive to additional rainfall, let alone a large
area with several inches of new accumulations. The Moderate Risk
area was adjusted a little to the southwest.The Slight Risk was
expanded a smidgen toward the Oklahoma/Arkansas border while
trimmed northward across northern Mississippi and Alabama. The
Marginal Risk area was also trimmed northward across the Lower
Mississippi Valley.

Campbell

Day 3
Valid 12Z Sat Mar 25 2023 - 12Z Sun Mar 26 2023

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Campbell


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt