Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
338 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

The Day 3 outlook will be updated by 2030Z.


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