Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
309 PM EDT Thu Apr 3 2025
Day 2
Valid 12Z 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...

...21Z Outlook Update...
Both global models and CAMs have come into agreement with
development of at least one north-northeast to south-southwest
oriented band of convection located generally from the ArkLaTex
into southeastern Missouri. Convection should be 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.

Expansion of Moderate Risk (and attendant probabilities) was
considered across more of Illinois and central Indiana, but pre-
existing dry conditions and uncertainty about duration of
precipitation were mitigating factors for a higher flash flood
threat. This region will be evaluated for a possible upgrade in
later outlooks.

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

See the previous forecast discussion below for more details.

Cook

...Previous Discussion...

The previous synoptic scale forecast was relatively unchanged
within the run to run assessment over the course of Friday into
Saturday morning. 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 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. 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 +3 to +4 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 activity, 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 east 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. 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 probabilistic output of the NBM, the consistency
within guidance for a significant corridor of very heavy rainfall,
and a well-defined axis of training precip potential, a broad
Moderate Risk was maintained and even expanded to account for the
expanse of likely flash flooding. A high-end Moderate is a likely
tag for places within southeast OK up through western and northern
AR through the Ozarks of Missouri as these areas will have the
greatest threat of enhanced rates over top of either complex
terrain or primed soils. An upgrade is plausible in future updates
pending additional convective probability details.

Kleebauer


Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt