Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1156 AM EDT Sat Apr 01 2023

Day 1
Valid 16Z Sat Apr 01 2023 - 12Z Sun Apr 02 2023

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

Locally heavy rainfall remains probable across portions of AL/GA
into the FL panhandle. However amounts exceeding flash flood
guidance appear unlikely, and thus we opted to remove the Marginal
risk with this update.

A line of strong to severe thunderstorms is expected to move
across portions of PA/NY this afternoon. While these storms will
produce briefly heavy rain, the quick movement of the cells should
generally prevent a flood threat. There is some chance we could
see localized urban high water across portions of eastern PA
towards NYC as storms enter a slightly more unstable environment
with higher lower level dewpoints. However, even here storms will
be moving so quick that the flood risk should be low. The overall
environment certainly favors severe weather as the main impact
with this activity, and thus while the urban minor flood threat is
non-zero, we will continue to go with no areas in the ERO.

Chenard


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