Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
852 PM EDT Wed Apr 23 2025

Day 1
Valid 01Z Thu Apr 24 2025 - 12Z Thu Apr 24 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE PLAINS AND THE SOUTHEAST...


...01Z Update...
Broke the large Marginal Risk area into two areas--one area over
the western High Plains in vicinity of a surface dry line and the
second area extending from the Southern Mississippi Valley into
parts of the Tennessee Valley/Southeast US along and south of a
quasi-stationary front which was acting as a focus. The HRRR/HREF
both have convection persisting into the late night/early morning
hours in the Plains...although the 23Z run from the HRRR tended to
focus activity and heaviest rainfall more in western Kansas than
elsewhere. The convection in from the Gulf states into the
southern Appalachians should taper off by late evening and
dissipate...but felt it was too early to remove the Marginal risk
area at this point.

Bann


...16Z Update...

Only minor changes were made to the large Marginal Risk area across
the Plains and Southeast this morning. From a review of an ensemble
of the CAMS to HREF probabilities, the Marginal Risk looks in
really good shape. Ongoing convection across Texas, Kansas, and
individual cells developing in the Southeast as well as expected
dryline storms in the southern Plains and MCS development further
north into NE/KS all remain in generally good agreement among the
guidance. The smaller details of those features, however, are far
from certain, lending to only "Marginal Risk" levels of confidence.
Regardless, for the vast majority of the area, impacts should
remain localized. Cell interactions and localized training may
cause more widely scattered flash flooding in a few areas, but high
uncertainty about where that may happen precludes any Slight Risk
upgrades at this time.

Wegman

...Previous Discussion...

Persistent Gulf moisture advecting northward through the Gulf
states over a stalled frontal boundary draped across the Southeast
will continue to produce scattered to widespread convection capable
of producing heavy rainfall. Rounds of showers and thunderstorms
will fire up along the the boundary from Texas to the Carolinas
with some training and backbuilding expected. Given there will be
very little movement of the front the storms are likely to track over
many of the same areas being impacted by ongoing showers and
storms of late, which locally increases the flash flooding risk. A
convective complex tracking east across the Hill Country during
the very early hours of Wednesday is spreading 0.5 to 1 inch/hour
rain rates along its path which may lead to isolated urban flash
flooding. Refer to WPC mesoscale precipitation discussion #160 for
more details.

The latest guidance has better confidence that the wettest areas
will likely focus over southern Nebraska and northern Kansas. This
too will be the result of an MCS or clusters of thunderstorms
developing and training multiple times over the same areas. The
Marginal remains in place as the consensus of heaviest rains on the
order of 1-2 inches (locally higher, of course) is in a portion of
Kansas and Nebraska that has largely missed out on recent heavy
rainfall, and the antecedent dry conditions should help to offset
and delay the greatest impacts from any heavy rain.

The area of the Marginal of least concern appears to be over
Oklahoma, which while seeing some scattered shower and thunderstorm
activity, should be between the MCSs/areas of more organized and
persistent convection. However, due to heavy rainfall, lower FFGs
over much of the state lower the threshold needed to achieve flash
flooding, so the Marginal is largely unchanged there.

Campbell/Wegman


Day 2
Valid 12Z Thu Apr 24 2025 - 12Z Fri Apr 25 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL AND SOUTHERN PLAINS...

...1930Z Update...

The guidance surrounding the coverage and intensity of showers and
thunderstorms across the country Thursday remains highly variable,
which has resulted in some bigger changes to the Day 2 outlook.
Generally, there have been increases in the forecast around Iowa,
Oklahoma and North Texas, and the southern Appalachians and north
Georgia. Meanwhile the main area of decreases was in Missouri and
Arkansas. As a result of these changes, the Slight Risk area was
trimmed on the eastern side largely out of Missouri and Arkansas,
and expanded south to cover portions of north Texas, including the
DFW Metroplex.

For the Marginal, it too was expanded west to cover the rest of
west Kansas and the eastern half of the Oklahoma Panhandle, the
eastern Texas Panhandle (for initiation of dryline convection
Thursday afternoon), and more significantly, was expanded north
into eastern Kentucky from the southern Appalachians. All of these
areas have had some heavy rain in recent days, resulting in
depressed FFGs. For DFW, that heavy rain was as recently as this
morning. Only an isolated flash flooding risk is expected in the
Marginal Risk area...with the urban centers assuming the brunt of
that risk.

Wegman

...Previous Discussion...

Multiple clusters of thunderstorms will likely be ongoing in
different portions of the ERO risk areas at the start of the Day
3/Thursday time period. These MCS's will likely weaken and
dissipate through the late morning, as MCS's typically do. However,
with peak heating and the increased instability there will be
renewed rounds of showers and thunderstorms. Since this is likely
to occur over many of the same areas hit with recent heavy rain
this will once again renew the potential for localized flash
flooding over portions of the Southeast. Much of the guidance has
areal averages of 1 to 2 inches with higher maximums possible
falling over much of the same footprint in the day(s) prior. The
potential for a new MCS to form Thursday night will keep the
threat for flash flooding elevated. The inherited Slight Risk
remained largely untouched as it continued to reflect the areas
with the greater threat for flash flooding.

In similar fashion as Day 1, the low level jet will keep a steady
influx of Gulf moisture over the stalled boundary which will
maintain support for heavier rainfall with the strongest storms,
and greater coverage of showers and storms in general. A Marginal
Risk covers the northern Gulf states and Southeast.

Campbell


Day 3
Valid 12Z Fri Apr 25 2025 - 12Z Sat Apr 26 2025

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHERN PLAINS, MID-MISSISSIPPI VALLEY, OHIO VALLEY, AND
PORTIONS OF THE NORTHEAST...

...1930Z Update...

A strengthening surface low developing ahead of a strengthening
upper level shortwave trough will drive the storms from a forcing
standpoint through the period. Starting in the Plains it will track
northeast into the Great Lakes Friday night, with showers and
storms forming along the cold front to the south and east of the
low center. This will bring periodically heavy rainfall to portions
of the Northeast, which has been in a drier period the past week.
Gulf moisture continuing to advect in from the south will lead to
shower and thunderstorm activity starting Friday afternoon across
the Tennessee and Ohio Valleys, and continuing into Friday night as
the front lags behind (south and west) of the surface low.
Meanwhile, the trailing (western) end of the cold front may be the
trigger for additional thunderstorms into southern Oklahoma, north
Texas, and the southern Texas Panhandle. For this reason (and
previous days' rains), the Marginal was expanded to cover this area
for an isolated flash flooding threat, though coverage of storms
looks to be significantly decreased on Friday in this area as
compared to previous days.

Wegman

...Previous Discussion...

Recent and ongoing rainfall over some portions of this broad
region will make some areas susceptible to additional rains. During
this period a surface low pressure system and its attendant front
will likely track from the Mississippi Valley toward the Great
Lakes region and interior Northeast. This system will usher in
showers and thunderstorms to areas with increased soil saturation
and lower FFGs. The broad Marginal Risk was maintained from the
Tennessee Valley to the New Hampshire/Maine border given the
increasing signal for heavier and more widespread QPF.

Campbell

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