FOUS30 KWBC 021533
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1133 AM EDT Wed Apr 2 2025

Day 2
Valid 12Z Thu Apr 03 2025 - 12Z Fri Apr 04 2025

...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR EXTREME SOUTHEAST
MISSOURI TO WESTERN KENTUCKY WITH MAJOR FLOODING EXPECTED...

..A PROLONGED LIFE-THREATENING FLASH FLOOD EVENT WILL BEGIN TODAY
WITH SEVERAL DAYS OF HEAVY RAINFALL IMPACTING A LARGE PORTION OF
THE LOWER MISSISSIPPI AND OHIO VALLEY'S..

The D2 period will be an ongoing atmospheric evolution with the
first half of the time frame being a continuation of the
significant rainfall from the D1 as the succession of cold pool
mergers and convective clusters propagate across the Ohio and
Tennessee Valleys before finally fading as they lose steam moving
into the wall of subsidence to the east. Scattered showers and
storms will still be ongoing within the confines of the quasi-
stationary front just due to continued low-level convergence and
smaller mid-level perturbations ripping through the "atmospheric
speedway" setup from southwest to northeast across the Southern
Plains up through the Ohio Valley. Despite the expectation for
lower rates during the early to mid-afternoon period on Thursday,
any additional rainfall will act as a priming mechanism for what
will transpire later in the evening as the second of 3 major
surface waves takes shape across the Lower Mississippi to Ohio Valleys.

The introduction of yet another stout LLJ pattern and attendant
low-level moisture flux will yield another round of widespread
convection within proxy to the surface front that will modestly
undulate north to south within the confines of the aforementioned
areas. The positioning of the front will make all the difference in
the inevitable convective cell motions and subsequent QPF
distribution, something that will be important when assessing where
the worst flooding will occur as the grounds around much of the Low
to Mid-Mississippi Valley and western Ohio Valley will be flood
prone from the previous convective impacts. As of this time, there
is ample agreement from global deterministic, as well as the
relevant ensemble packages for an axis of heavy precipitation to
impact a zone spanning central and northeastern Arkansas up through
western Tennessee and Kentucky with the northern edge of the
heavier precip aligned with the KY/OH border near the Ohio River
basin. Additional amounts of 2-4" with locally up to 7" will
impact those above zones, overlapping several areas that will have
seen significant rainfall the previous 24 hrs. 01z NBM
probabilities for rainfall amounts >6" over the D1-2 period are now
upwards of 40-60% located from Pine Bluff, AR up through western
KY between Paducah and Bowling Green. The area with the highest
probabilities is across western TN, just north of the Memphis
metro, but very close by the major metro center. Considering the
elevated theta_e alignment and +2 to +3 deviation PWATs present,
rainfall rates within matured convective cores will likely breach
1"/hr with 2-3"/hr within reason, especially in the early evening
when instability is highest and the LLJ is at peak intensity. The
deep moist convergence regime will be well-established and cell
motions will remain generally parallel to the frontal boundary
thanks to the stagnant synoptic scale pattern in place. This will
lead to significant, life-threatening flash flooding to impact a
large area where FFG's will likely lean critically low.

Considering all the above variables, there was no reason to remove
the High Risk. Instead, the risk area was expanded to account for
the latest probabilities and relevant run to run continuity from
the pertinent NWP. A Moderate Risk spans from southwestern Arkansas
to the northeast, just outside Cincinnati on the Kentucky side of
the Ohio River. Memphis, Little Rock, Pine Bluff, Bowling Green,
Louisville, and Evansville are all major urban corridors within the
Moderate Risk with areas like Paducah and Union City the larger
towns/metros embedded within the High Risk. The travel routes most
susceptible to flash flooding will be along the I-69 and I-55
interstates within western KY, western TN, and northeastern AR.

Kleebauer


Day 3
Valid 12Z Fri Apr 04 2025 - 12Z Sat Apr 05 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR A LARGE
PORTION OF THE LOWER AND MID-MISSISSIPPI VALLEYS...

The final surface wave within the atmospheric parade across the
Southern Plains to Mississippi Valley will show itself by late-
Friday 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 24 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. When
assessing the theta_e environment, there is a noticeable push north
of more modestly unstable air with the most pertinent area of
available instability situated across southern MO down through AR,
southeastern OK, and even northeastern TX. A sharp uptick in
regional PWATs with deviations between +2.5-3.5 anomaly-wise are
forecast within that zone correlating with a substantial rise in
area convection blossoming late Friday afternoon, onward. This
setup is fortunately further west compared to the previous periods,
so the overlap of the heaviest rainfall is not anticipated to be
as problematic as the previous period.

Having said that, the pattern will yield high precip totals due to
widespread convective schema, falling over some areas that will
have seen appreciable rain, and over other areas that are more
prone to flooding due to complex terrain situated over the Ozarks.
The pattern is such that several areas from northeast of DFW up
through IL will see rainfall amounts of 3-6" with locally higher
beginning Friday afternoon with the heaviest rainfall likely to
occur by the evening with the implementation of yet another LLJ
structure thanks to the surface wave mentioned previously. Further
to the west, another closed upper reflection across the Southwest
U.S. will only entice greater upper forcing within a broad axis of
difluence situated over the Plains. This setup is more privy to
convection forming as far west as the Western Rolling Plains of TX
to points east and northeast. Even there, modest 1-2" with locally
higher totals are forecast in-of those zones. As a result, there
was a general expansion of the Slight Risk further southwest into
Central TX, an expansion of the Moderate Risk to include the
eastern Red River Basin up through the southeast half of Missouri
into southern Illinois, and a high-end Moderate impacting
southeastern OK up through western AR and much of southern and
southeast MO. The threat of widespread flash flooding is
increasing for areas within the Moderate risk and aforementioned
higher-end Moderate designation. Be sure to stay up to date for the
latest changes as we move through the remainder of the week as this
pattern remains very active and prone to significant life-
threatening flash flood concerns.

Kleebauer

$$
--- SBBSecho 3.20-Linux
* Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (618:250/1)