Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
855 PM EDT Fri Mar 31 2023

Day 1
Valid 01Z Sat Apr 01 2023 - 12Z Sat Apr 01 2023

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE LOWER MISSISSIPPI/OHIO/TENNESSEE VALLEYS... 

Continued to shrink portions of the Slight and Marginal Risk areas
largely based on trends in late afternoon and early evening
radar/satellite imagery. Surface reports and MRMS tended to show
higher rainfall rates around the western/southern portion of the
Great Lakes...but the storms with the heaviest rates have remained
progressive and had not had too much repeat convection or training
given the orientation of the storms to the mean flow.  Even
so...observed rainfall amounts in and around Chicago were close to
an inch in the 6-hours ending at 01/00Z suggesting that
occurrences of flooding were possible given the flash flood
guidance in the area so changes to the on-going Marginal Risk area
was minimal. 

Farther south, changes to the size and placement of the Slight and
Marginal Risk areas were based on short-term radar and satellite
trend.  Convection was more scattered compared with coverage close
to the Great Lakes, but locally heavy rainfall was still
occurring.  And convection tended to be oriented closer to the
mean wind which was setting up the potential for some
training...but the changes made at 01/01Z were fairly minor over
the Tennessee and Ohio Valleys compared with the previous outlook.

Bann



Day 2
Valid 12Z Sat Apr 01 2023 - 12Z Sun Apr 02 2023

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHEAST... 

No notable changes were needed to the existing Marginal Risk. This
still appears to be a low-end flash flooding threat for convection
in the first half of the period that could cause brief high
rainfall rates before the front clears offshore. See the previous
discussion for more.

Tate


...Previous Discussion...

Deep convective activity is expected to be most robust but
generally progressive Saturday morning along the southern edge of
a departing deep layer cyclone moving from the Great Lakes into
New England.  The best overlap of available moisture and
instability looks to be from south-central AL into GA/SC and the
northern FL Panhandle.  Some 1-2" totals may occur over a
relatively short period, with the FL Panhandle and surroundings
most likely to experience localized training along the tail end of
a cold front. The rapid movement of the convection will likely
limit the potential for flash flooding to isolated/localized areas
(though relatively wet antecedent conditions are noted, per NASA
SPoRT-LIS 0-100 cm moisture anomalies as high as the 90th
percentile across a portions of the region). 
 
Roth

Day 3
Valid 12Z Sun Apr 02 2023 - 12Z Mon Apr 03 2023

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FROM THE ARKLATEX
THROUGH THE ARKLAMISS...

The Slight Risk was shifted a bit southward from the previous
issuance, which follows the updated WPC QPF. There remains some
spread in the model guidance for this time period on the axis of
heaviest QPF. GFS/UKMET/GEFS mean amounts and GEFS probabilities
for QPF of 1" are farther north than the 12Z CMC and 00Z EC/ECens
suite--though the 12Z deterministic ECMWF came back north a bit to
a middle ground position that matches the WPC QPF well. Most
models indicate convective initiation in northeastern Texas but
then have the storms propagating various degrees from straight
east. Propagation with some southerly component would make the
most sense along the instability gradient, and the 12Z CMC GEM
regional seemed to have a reasonable handle on this. In any case,
the risk area may shift around with time as the models hopefully
converge. See the previous discussion for more details on the
pattern.

Tate


...Previous Discussion...

A southern stream shortwave interacting with a Gulf Coast warm
front along with sufficient moisture/instability is expected to
produce locally heavy rainfall from eastern Texas into the Lower
Mississippi Valley.  Its general progression should keep amounts
from becoming particularly extreme.  Outside of the 00z NAM, there
was pretty good agreement on a zone of low- to mid-level
frontogenesis tracking roughly between the ArkLaTex and through
the ArkLaMiss into central MS.  Instability and low-level
inflow/effective bulk shear should be sufficient for convective
organization, though the 00z NAM and 21z SREF mean/CAPE spread
were not sold on this area.  Hourly rain totals up to 2" with
local amounts in the 2-4" range are expected in this region, which
should have received some heavy rainfall Friday and Friday night
and saturated soils to some degree by the time this round occurs,
hence the upgrade to the Slight Risk.

Roth


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