FOUS30 KWBC 042014
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
414 PM EDT Fri Apr 4 2025

Day 1
Valid 16Z 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...

...16Z Update...

The forecast remains on track as model guidance from the CAMs and
global models show good agreement on placement of the highest
rainfall totals today through tonight. Overall, fairly minimal
changes were needed for the Day 1 ERO outside of some small
adjustments. Based on latest high-res model and radar trends,
adjustments were made to expand the Slight Risk area in Kentucky
and West Virginia where precipitation has lingered through the
morning hours and in western and northern Ohio where precipitation
is expected later this afternoon and tonight. Adjustments were
also made to the High and Moderate Risk areas, extending them a
bit north and northeast based on latest high-res deterministic and
ensemble guidance. The largest expansion was to bring the Moderate
area further into Kentucky where heavy rain fell this morning and
additional rainfall is expected tonight. The HRRR was trending
more south than other guidance with the higher rainfall totals in
Arkansas, but it seems to be an outlier at this point and doesn't
seem to have a good handle on the current conditions either as
compared to the other CAMs. However, if precipitation trends south
this afternoon, further adjustments may be needed to the High Risk area.

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

...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
ARKANSAS, THE MID-SOUTH, AND THE LOWER OHIO VALLEY...

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

Saturday is forecast to be the final day in the multi-day high-
impact flood event that is unfolding across the Mid-South, Lower
and Mid-Mississippi Valleys, and Lower Ohio Valley. Deep slow-
moving convection is anticipated to be ongoing across the Mid-South
at the beginning of the Day 2 period, and another wave of
widespread convection should develop by late morning/early
afternoon. Saturday night, veering flow aloft and increasing
eastward momentum of the frontal boundary draped from Texas to the
Ohio Valley will result in faster forward storm motion as
convection pushes into the South.

The ingredients will be in place and the synoptic setup will
support widespread, deep convection with efficient/high rain rates
across the Lower and Mid-Mississippi Valleys. Deep southerly flow
across the region will continue to push warm, moist, unstable air
into the region. Anomalous precipitable water values 2-3 standard
deviations above normal, and high MUCAPE values of 2000+ J/Kg will
support rainfall rates in excess of 1-2 inches per hour.
Additionally, areas from Arkansas to the Lower Ohio Valley will be
in the favorable right entrance region of the upper level jet with
a slow-moving frontal boundary across the region. The combination
of slow storm motion and efficient/high rainfall rates is expected
to result in widespread significant additional rainfall totals of
3-6 inches, with locally higher amounts.

The High Risk of excessive rainfall encapsulates areas from
Arkansas through southeastern Missouri to western Tennessee and
Kentucky. All major deterministic models depict an expansive
footprint of 3-6 inches of rainfall, with locally up to 7-9 inches.
These areas have already received significant rainfall since
Wednesday, and any additional rainfall could result in a very
dangerous flash flooding scenario with considerable to potentially
catastrophic impacts.

For the afternoon forecast, the High Risk area remained in place
with small adjustments based on latest high-res model guidance and
WPC QPF. A broad Moderate Risk extends around the High, stretching
from the Ark-La-Tex to southern Ohio, which encapsulates areas that
could also be prone to significant flash flooding. This remains a life-threatening situation for areas of the Lower and Mid-
Mississippi Valley regions.

Dolan/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 ON SUNDAY...

On Sunday, upper level southern stream energy that had been
supporting the heavy rainfall event over the past 4 days will begin
to interact with a northern stream trough and accelerate
northeastward. This will push the surface cold front across the
southeast, with an axis of high moisture ahead of and along the
front. There should be enough instability and shear to support
thunderstorms with locally heavy rainfall from the Central Gulf
Coast through Central Alabama and Northern Georgia. Rainfall totals
are expected to reach 2-3+ inches in these areas, and rainfall
rates could challenge FFGs of 2-2.5 inches per hour (although the
3+ inches per 3 hour FFG may be be less likely to be met/exceeded).

Dolan/Bann

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