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

Day 1
Valid 16Z Fri Mar 31 2023 - 12Z Sat Apr 01 2023

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

The main change to the Slight Risk was to extend it a bit eastward
into Kentucky, which seemed necessary based on potential for FFG
exceedance there per the HREF and higher totals in the 12Z suite
of hi-res guidance. Considered an extension into West Virginia as
well where FFG is lower and 24 hour QPF totals could be higher
overall--but rain rates are likely to be lower there with lower
instability, with the QPF occurring over multiple rounds. But did
extend the Marginal eastward in West Virginia for the potential of
multiple rounds priming soils that eventually exceed FFG.

Farther north, also saw a good signal for exceedance of FFG in
parts of the Upper Mississippi Valley, around the intersection of
southern MN/northern IA/southern WI. The surface low pressure
system is likely to pivot there, allowing for convection to
perhaps stall somewhat before stratiform rain could come through
and then could change to wintry precipitation. In coordination
with the MPX/DMX/ARX offices, this is not expected to cause
notable flash flooding impacts, as those areas are no longer snow
covered. So kept the risk at Marginal at this point--it did have
to be expanded southwest a bit from the previous issuance.

See the previous discussion for more details on the synoptic
pattern.

Tate


...Previous Discussion...

A deep layer cyclone rapidly strengthens as it moves through Iowa
from Friday afternoon/evening and then weakens as it moves across
portions of WI and MI Friday night into Saturday. The relatively
swift progression of its attendant cold front would limit the
excessive rainfall potential over the northern OH Valley and
western Great Lakes.  Farther south, there will be a better chance
of cell training along outflow-generated effective fronts (as
mid-level capping across the region should be negligible to zero)
oriented quasi-parallel to the deep-layer west-southwest flow.
Where instability is plentiful and the dynamics are most
impressive, deep convection is expected to rapidly develop
probably around local noon with quasi-discrete supercells
initially expected to the preferred storm mode.  Some of the
strongest activity is anticipated to be across the Slight Risk
area, where hourly totals of 2-2.5" and local amounts of 3-5" are
possible (despite the recent downturn in QPF amounts across the
available guidance).  The evolution of the moisture field
indicates the possibility of two skirmishes of showers and
thunderstorms, which could lead to a longer duration heavy rain
threat overall.  Low-level moisture transport is expected to be
quite impressive, as a strong low-level jet (50-70 kts at 850 mb)
ushers in precipitable water values of 1.2-1.8" (above the 90th
percentile for the bulk of the MS/OH/TN Valleys).  Farther to the
north in the Midwest and Great Lakes region, hourly rain totals
will be less impressive (perhaps as high as 1.5" at times Friday
afternoon and evening), but the prolonged nature of the rainfall
may lead to some higher areal average totals (closer to ~2",
though localized totals will likely be higher to the south). 
Farther south of the Slight Risk area (into portions of the Deep
South), flash flood guidance is generally higher, the dynamics of
the system are less impressive, and there's a greater chance for
mid-level capping when looking at 700 hPa temperatures.  The 00z
HREF and coordination with JKL and MRX led to some contraction of
the Slight and Marginal Risk areas across eastern KY and northeast
TN which appear to be relatively far removed from the best
instability.   
 
Roth


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