Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
340 PM 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

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
NORTHEAST TEXAS INTO THE LOWER MISSISSIPPI VALLEY...

The closed low over southern CA (as of 18z Sat) will track into
the Southern Plains on Sunday and be responsible for an area of
organized convection Sunday afternoon and night. Model agreement
in the overall synoptic setup is pretty high. The aforementioned
mid level shortwave combined with an upper jet to the north will
provide the large scale ascent. This forcing will interact with a
lifting warm front, with upwards of 40 kts of 850mb flow resulting
in pretty good moisture transport/convergence and PWs around the
climatological 90th percentile. QPF differences amongst the models
are most likely mainly driven by different handling of the
convective evolution. Cells will form Sunday afternoon over
portions of northeast TX into southeast OK, and should evolve into
an organized area of convection fairly quickly. Mean wind fields
would suggest an east northeastward progression of this
convection. However as convection organizes it should tend to
build into the low level jet and track along the instability
gradient. This would support more of a east southeast and eventual
southeast progression of the most intense convection. Corfidi
vectors and thickness lines also support this east southeast to
southeast movement. For this reason we continue to lean away from
the guidance that has more of a east to northeast progression of
the heaviest rainfall. Guidance such as the 12z HRRR, ARW and Gem
Reg seem to have the more realistic evolution based on these
thoughts.

The system as a whole will be progressive, limiting the higher end
QPF/flood potential. However some east to west training seems
reasonable given the broad moisture transport along the warm
front, and this scenario is supported by some of the 12z HREF
members. It seems likely that we will see a swath of 2-4" of rain
somewhere from northeast TX towards the AR/LA border and into
central MS, although at the moment 5"+ probs from the HREF are
zero (indicative of the upper bound cap due to the progressiveness
of the system). Still think this supports an isolated to scattered
flash flood risk, so the Slight risk will be maintained. The area
was adjusted a bit north over northeast TX, and then south over MS
to account for the aforementioned expected east southeast movement
of stronger convection.

Chenard

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, SOUTHWEST GA AND THE FLORIDA PANHANDLE...

19z Update: No changes made to the Marginal risk. Organized
convection should be ongoing at 12z Monday across the area. What
happens the rest of the day remains a bit uncertain, but some
chance we see continued convective activity with potential
training/backbuilding. The better forcing leaves the area, but
850mb moisture transport persists and some guidance does have a
training signal. The GFS remains too dry, with something closer to
the 12z GEM Reg, FV3CAM and ECMWF preferred...which all depict
2-3"+ totals in spots. We may eventually need to give
consideration to a Slight risk upgrade across portions of southern
AL into southwest GA, but given relatively high FFG and some
lingering uncertainty with convective details, thought leaving the
Marginal was good for now and we can continue to reassess with
future updates.

Chenard

...Previous Discussion...

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