759
FOUS30 KWBC 041221
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
821 AM EDT Fri Apr 4 2025

Day 1
Valid 1213Z Fri Apr 04 2025 - 12Z Sat Apr 05 2025

...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE OZARKS AND MID-SOUTH...

...LIFE-THREATENING FLASH FLOODING EXPECTED FRIDAY AFTERNOON
THROUGH EARLY SATURDAY...

...12Z Special Update...

Convection lingering past 12Z has prompted the issuance of a
special update for the Day 1 ERO. The Marginal and Slight Risk
areas were extended east across Kentucky into portions of West
Virginia where heavy showers and thunderstorms are expected to
produce widespread areas of areal flooding and flash flooding, with
locally considerable impacts possible, through the early and mid-
morning hours. Some improvement is expected by late morning.

Dolan

...Previous Discussion...

Both global models and CAMs have come into better agreement over
the past 24 hours and have largely maintained that agreement in
the 04/00Z model production cycle. The development of at least one
north- northeast to south- southwest oriented band of convection
located generally from the ArkLaTex into southeastern Missouri was
still depicted...with convection being oriented generally parallel
to fast, meridionally oriented flow aloft, with abundant moisture,
low- level shear, and instability profiles all favoring abundant
rainfall (potentially exceeding 10 inches on a localized basis).
These totals will fall on areas that will have experienced 2-6
inches of rainfall leading up to the forecast period, with wet
soils/terrain variations all suggestive of a potentially widespread
and significant flash flood event.

The overall outlook was on track with an eventual broad axis of
heavy precipitation expected from north/central Texas northeastward
to Ohio/Pennsylvania.

The previous synoptic scale forecast was relatively unchanged.
What should be the final surface wave had already started to
enhance convection across over southwestern Texas in the predawn
hours...and it should continue to make its way across the Southern
Plains to Mississippi Valley by later this morning through the
remainder of the period. This wave will likely be the strongest of
the 3 allowing a much more amplified surface evolution that will
shift the focus of heavier precip further northwest while also
providing some vigor in the overall QPF distribution. The trend the
past 48 hrs is for a zone of significant (3-6+") totals to be
recognized across portions of the Southern Plains up through the
Ozarks and Mid-Mississippi Valley of Missouri, spanning as far
north as south- central Illinois.The heaviest precipitation axis
based off the latest NBM probabilities still has a significant
footprint of at least 3" located near the Dallas metro and points
just east (40-60%) to as high as 60-80% in a large area spanning
the Red River of northeast TX up through much of western and
northern AR, through the Ozarks of MO, finally reaching the
Mississippi River south of St.Louis. Ensemble mean QPF output
signals a massive area of at least 2" with urban centers like
Dallas, Little Rock, St. Louis, and Springfield, MO all within a
core of the heavy precip footprint.

Considering the neighborhood probabilities from the 04/00Z
HREF...maintained the Moderate Risk area with only minor
adjustments based on the model and WPC QPF. Also made a slight
eastward expansion of the Marginal and Slight Risk areas in the
Ohio Valley and the Mid-South where Flash Flood Guidance was
largely reduced to less than half an inch per 3 hours.

Bann

Day 2
Valid 12Z Sat Apr 05 2025 - 12Z Sun Apr 06 2025

..LIFE-THREATENING FLASH FLOODING IS EXPECTED ON SATURDAY AFTERNOON
THROUGH SUNDAY MORNING..

Deep, slow-moving and intense convection is expected to be on-going
across parts of the Mid-South as the Day 2 period begins at 12Z
Saturday. Many of the adjustments made previously were still
supported by the 04/00Z model runs...with south to southwesterly
flow aloft at the start of the period expected to continue
fostering a very slow eastward progression of convection, while
1.75+ PW values,, 1500+ J/kg MLCAPE, and convective
training/mergers continue to foster high to locally extreme rain
rates and FFG exceedance on a widespread basis. Additional 3-5
inch rainfall totals...with locally higher amounts...are expected
across parts of the region which will be increasingly water-
logged.

Eventually, veering flow aloft will result in a faster forward
speed of any convective complex(es) toward the Ohio and Tennessee
River Valleys during the latter half of the forecast period. As
this occurs, low FFGs are expected to remain from abundant
antecedent rainfall, and it is very likely that ground conditions
will not have completely recovered from abundant antecedent
rainfall. In this scenario, widespread significant impacts are
expected with both ongoing and newer instances of flash flooding.

The synoptic forcing for enhanced deep layer lift will be strong
given the magnitude of an intense right-entrance region jet
dynamics in an area of highly anomalous atmospheric moisture and
very saturated soils. This spells another high impact flash flood
forecast for Saturday afternoon through Sunday morning across the
Lower to Mid- Mississippi Valley. This threat will extend farther
east as a cold front begins to finally sweep the stagnant pattern
out of the region, but not before another round of heavy rainfall
inundates areas that will be sensitive to any appreciable rainfall.

All major deterministic models depict an expansive footprint of 3
to 6 inch rainfall...with locally up to 8" of rainfall across a
large portion of Arkansas extending east- northeast through far
southeastern Missouri, all of western and central Tennessee and
Kentucky, to as far north as the Ohio/Kentucky border along the
Ohio River. Northwest Mississippi will also lie within the axis of
heaviest precipitation leading to quite a large area of not only
heavy precip, but significant overlap of areas that will have super
saturated grounds that will struggle to maintain the ability to
absorb more rainfall. This setup will yield a very dangerous
scenario where widespread flash flooding with considerable and
potentially catastrophic impacts would be favored.

As a result...the previously issued High Risk remained largely in
place with only modest adjustments based on the latest model and
WPC QPF with a broad Moderate Risk area that spans the outer
perimeter of the High Risk leading to several more areas prone to
significant flash flood concerns due to the setup. This remains a life-
threatening situation for areas of the Lower and Mid- Mississippi
Valley region.

Bann

Day 3
Valid 12Z Sun Apr 06 2025 - 12Z Mon Apr 07 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHEAST US ON SUNDAY|...

There should finally be enough of a shift in the upper flow pattern
such that moisture gets drawn northward across portions of Alabama
an Georgia by southerly low level flow ahead of an approaching
cold front and an upper level trough. Given the timing. There looks
to be enough instability and a shear profile that thunderstorms
should be able to produce locally heavy rainfall rates and maximum
rainfall amounts from 2 to 3+ inches over from southeast Alabama
into central/northern Georgia. As a result...rainfall rates may be
high enough to challenge current FFG in the area on the order of
2.5 inches per hour (although the 3+ inches per 3 hour FFG may be
be less likely to be met/exceeded).

Bann


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt