Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
323 AM EDT Sat Apr 01 2023

Day 1
Valid 12Z Sat Apr 01 2023 - 12Z Sun Apr 02 2023

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 that shows persistence is in the vicinity of the
AL/GA/FL border junction.  There's a bit of dispersion in the 00z
QPF guidance in this region, but an inspection of one of the
instability fields from the 00z GFS suggests that the mesoscale
guidance/HREF are on to something regarding the highest chances
(which are low) of 2"+ being across portions of the FL Panhandle,
FL Big Bend, and portions of southern AL and southern GA.  The
0-100 cm moisture is telling, showing a dry patch across
south-central GA which allowed their removal from the Marginal
Risk area.  To the north across the Carolinas, the front is more
progressive and moves through right as the instability is
increasing.  The moisture isn't overly stellar in the Carolinas
either, generally below 1.5".  Coordination with CHS, CAE, and ILM
led to the removal of the Marginal Risk area in those regions. 
Some 1-2" totals may occur over a relatively short period, with
the FL Panhandle and surroundings most likely to experience
isolated/ localized training along the tail end of a cold front. 
Cell mergers and mesocyclones cannot be ruled out.  The rapid
movement of the convection will likely limit the potential for
flash flooding to isolated areas.  

Roth


Day 2
Valid 12Z Sun Apr 02 2023 - 12Z Mon Apr 03 2023

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.  Some low- to mid-level
frontogenesis tracking roughly between the ArkLaTex, near the
AR/LA border, and through the ArkLaMiss into MS, but even amongst
the GFS and NAM guidance the axis of such is different.  As the
placement remains a question mark due to the north-south spread in
the guidance, with the 00z Canadian Regional unusually on the
southern side and the 00z NAM remaining up north.  The ECMWF has
been steadily shifting northward.  Instability and low-level
inflow/effective bulk shear should be sufficient for convective
organization.  Hourly rain totals up to 2" with local amounts in
the 2-4" range are expected in this region.  A couple narrow
swaths of 1-2" of rainfall fell Friday into Friday Night across
southern AR and from northeast TX across northern LA and should
have saturated soils to some degree.  The Slight Risk remains in
place and like the QPF splits the difference in the guidance. 
 
Roth


Day 3
Valid 12Z Mon Apr 03 2023 - 12Z Tue Apr 04 2023

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL IN AND AROUND
SOUTHERN AL & THE FLORIDA PANHANDLE...

While there are differences amongst the guidance mass fields,
there is a general signal for a moisture plume with precipitable
water values at or above 1.5" to become stuck vaguely near the FL
border with AL and GA.  Instability, low-level inflow, and
effective bulk shear appear sufficient for convective
organization.  The flow should be fairly unidirectional with
height from the west-southwest, leading to concerns for cell
training.  With low-level inflow slightly convergent, cell mergers
can't be ruled out either.  The low- to mid-level frontogenesis
seen on Sunday should be exiting to the east and northeast.  The
guidance spread in amounts is fairly wide, with the 00z NAM the
driest and the 00z ECMWF the wettest, showing local amounts of 5".
 Soil moisture across portions of southern AL and the FL Panhandle
and Big Bend is elevated, and rainfall during the day Saturday
(and possibly Sunday) could lead to further saturation.  The model
guidance has a general signal for 1-3" in this area, which could
fall as quickly as an hour or two.  Considering the above, a
Marginal Risk area was added for this region.

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