Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
300 PM EDT Tue Apr 22 2025

Day 1
Valid 16Z Tue Apr 22 2025 - 12Z Wed Apr 23 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTH...

...16Z Update...

The inherited Marginal Risk areas across the Panhandles of Texas
and Oklahoma as well as the newly expanded Marginal from the lower
Mississippi Valley to the southern Appalachians were combined to
cover much of Texas with this update. There remain 2 distinct
features forcing the thunderstorms, a stationary boundary draped
from the Lower Mississippi Valley to the southern Appalachians, and
a progressive dry line that will spawn new thunderstorms across
western and central Texas later this afternoon though tonight.
While the area in between...namely east Texas, has a notably lower
chance of flash flooding, enough of the guidance suggests there to
be enough overlap late tonight between earlier convection and those
areas harder hit over the past day or so to justify the combining
of the inherited Marginal Risk areas.

This summer-like pattern of convection is much less predictable
than the colder season larger precipitation shields of the past
several months. It's therefore becoming the season where larger
swaths of the country are broadly under a localized threat for
flash flooding as thunderstorm complexes interact with one another
in largely unpredictable ways, with enhanced impacts should those
interactions occur over urban areas. The combined Marginals thus
highlight these larger features for the potential for training
convection once cell interactions are taken into account. Given the
increasing amounts of atmospheric Gulf moisture advecting into
these features, resultant convection will have the potential to
cause localized rainfall rates of up to 2 inches per hour.
Localized flash flooding is most likely to occur where these rates
coincide with lower FFGs from recent heavy rainfall.

Wegman


Day 2
Valid 12Z Wed Apr 23 2025 - 12Z Thu Apr 24 2025

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

...19Z Update...

The inherited Marginal Risk was greatly expanded towards the south
and east with this update. The area now includes much of northern
and eastern Texas, and the northern portions of the Southeast
through into the Carolinas. This eastward expansion has largely to
do with the increasing Gulf moisture streaming north into the
Southeast with a very persistent souther LLJ, and a similarly
persistent stationary front that will remain draped across the
Southeast. The clash of this warm and humid air with the front will
cause a renewed round of showers and thunderstorms from east Texas
through to the Carolinas. While the large-scale pattern favors more
slow-moving training thunderstorms, there is significant
disagreement in the guidance as to what form those storms will
take, whether groups of cells and MCSs interact, and ultimately
where these interactions may occur. Since the front remains in
place, the storms are likely to move over many of the same areas
being impacted by ongoing showers and storms this afternoon, which
locally increases the flash flooding risk.

There is somewhat better agreement in the guidance that the
wettest areas will likely focusing into southern Nebraska and
northern Kansas on Wednesday. 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.

Wegman

...Previous Discussion...

Several shortwaves will be exiting the Rockies throughout the day
with several pockets of convective development sprawled across the
Central and Southern High Plains by the evening hours. While the
confidence continues to be lower on the maximum amounts there is
however higher confidence on the potential for scattered flash
flood prospects. Although the location of QPF maxima vary, there
is a general consensus of a widespread 1 to 2 inches from the Front
Range of Colorado/Wyoming/Nebraska south to the Red River Basin
near Texas to Kansas/Oklahoma. Some of these areas will have seen
significant rainfall prior to any additional impacts leading to
some localized lower FFG indices once the event occurs. The
inherited Marginal continued to mostly highlight areas across the
Plains that had an elevated threat for heavy rainfall and flash
flooding. The latest guidance and WPC QPF trended higher with QPF
over northeast Kansas, southern Nebraska, southwest Iowa and
northwest Missouri. As such, the Marginal Risk area was modified to
encompass that part of the region.

Campbell/Kleebauer


Day 3
Valid 12Z Thu Apr 24 2025 - 12Z Fri Apr 25 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
KANSAS, OKLAHOMA, ARKANSAS AND MISSOURI...

...19Z Update...

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 associated
therewith, renewed rounds of showers and thunderstorms will
redevelop. Since this is likely to occur over many of the same
areas hit with heavy rain both now and on D2/Wednesday, this will
once again renew the potential for localized flash flooding over
portions of the Southeast.

For the Slight Risk area, these showers and storms will likely
redevelop in areas particularly forecast to be hard hit with heavy
rain from the D2/Wednesday time period, along with the potential
for a new MCS to form Thursday night, resulting in more widespread
heavy rainfall. Thus, the Slight Risk was maintained where it was
already issued, and expanded south and east with this update into
more of Missouri, Arkansas, and Oklahoma. This expansion is largely
a function of much of this area being hit with heavy rain a few
days ago, and the rivers will not have fully drained, and the
typical evolution of MCSs in the guidance to trend south with time.

For all areas, the LLJ out of the Gulf will tap into some of the
deepest moisture yet as compared with the previous two days. This
will support both heavier rainfall with the strongest storms, and
greater coverage of showers and storms in general. Due to inherent
uncertainties with how the MCSs/storms will evolve and interact
with each other, at this point it appears unlikely additional
upgrades to a Moderate will be needed, as errors in thunderstorm
placement and coverage are high by the Day 3 period. However,
should the forecast remain wet and rainfall from the current D1 and
D2 periods outperform, then a targeted Moderate Risk cannot be
completely ruled out by the time more of the CAMs have a chance to
analyze the weather pattern.

Wegman

...Previous Discussion...

The pattern will be favorable for multiple rounds of showers and
thunderstorms to move through the south-central U.S. With a moist,
unstable flow expected and shortwave energy passing through heavy
rainfall will likely develop across the region which may lead to
local flash flooding. Areal averages of 1 to 2 inches are forecast
with locally higher amounts possible. A Slight Risk for excessive
rainfall is in effect for a large portion of Kansas, northern
Oklahoma, far northwest Arkansas and western Missouri. A Marginal
Risk area spans from northern Texas to southern Nebraska and
extreme southwest Iowa.

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