Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
310 PM EDT Mon May 19 2025
Day 1
Valid 16Z Mon May 19 2025 - 12Z Tue May 20 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHERN ARKANSAS, SOUTHERN MISSOURI, AND EASTERN OKLAHOMA...

16Z Update: No major changes were necessary for the MDT risk
inherited with some slight expansion to the southwest to include
the far eastern sections of OK. Latest trends are for repeated
cells to occur within a alignment southwest to northeast between
Southeastern OK up through the Ozarks in proxy to a ribbon of
elevated theta_E's coupling with intense large scale ascent between
a coupled jet maxima with southern stream jet influence. 12z HREF
probs are indicative of elevated flash flooding concerns within the
MDT forecast, especially when assessing the EAS prob fields for >2"
showing a broad expanse of 30-60% encompassing the area of
interest. A secondary maxima is also "bullseyed" over Northern MO
with the second area of focus later this evening as multiple rounds
intersect that area northeast of Kansas City. The key difference
between the two regional maxima are the FFG's within the southern
tier max being much more conducive for localized flash flood
concerns due to impacts yesterday priming grounds for today's
event. Assessment of NASA SPoRT soil moisture anomalies shows a
relative min across Northern MO meaning some of the initial
rainfall will be more beneficial than anything else before it
eventually becomes more of a problem with the repeated nature of
the convection.

General theme of the setup is broad convective coverage will induce
region-wide flash flood concerns beginning later this afternoon
across parts of the Central and Southern Plains with an advancement
into the Missouri and Mississippi Valley areas later this evening
as the large scale pattern pivots east. Areal average of 1-3" is
forecast with a dual maxima over Eastern KS up through Northern MO
(2-4" w/ locally up to 5") and across Eastern OK up through the
Ozarks of AR/MO (2-4" w/ locally up to 6") forecast. The setup will
lead to heavy rains early tomorrow morning over the Mid-Mississippi
River Basin and adjacent Plains to the east.

Kleebauer

..Previous Discussion..

A digging longwave trough will rapidly become negatively tilted as
a vigorous upper level shortwave rounds the base of the trough over
the Texas and Oklahoma Panhandles. This will greatly increase the
forcing downstream of this feature across Arkansas and Missouri.
The forcing will run into a very unstable and moisture-rich air
mass, due to a vigorous low level jet extending from deep South
Texas north into Missouri. A well-defined dry line will demarcate
the westernmost extent of the upper level jet, and as the dry line
pushes east, it will further enhance low level forcing for ascent.
The result of all of these ingredients coming together will be the
explosive development of several rounds of storms, initiating
around peak heating this afternoon, then tracking east along the
AR/MO state line. The storms will subsequently weaken as they
outrace the forcing. This should limit the eastward extent of the
heaviest convection to no further than the Mississippi River.

CAMs guidance as come into better agreement, albeit far from
unanimous, about how the convection will evolve this afternoon and
tonight. Due to the extra forcing from the upper level shortwave,
expect multiple rounds of storms to track across northern Arkansas
this afternoon and tonight, becoming the southern end of an MCS
that is largely over Missouri. The southern end of the MCS will
have the best feed of moisture from the LLJ, and will therefore
have the best opportunity to convert that moisture into heavy rain
via the various thunderstorms. As is very typical with convective
scenarios, the small details become very important, as its likely
the initial storms' cold pools feed the development of the next
round of storms. This is especially true in this environment with
the LLJ very effectively replacing the moisture lost to rainfall.
Nonetheless, with the 2 upper level shortwaves, both the one at the
base of the trough and a second rounding an upper level low further
north, likely driving and enhancing the convection in a way that
favors training and subsequent flash flooding, where they track
will be critical. It is in this critical detail that the high-
resolution CAMs differ some, which greatly impacts where the
heaviest rainfall ultimately occurs. Much of the guidance suggests
this will be across the Moderate Risk area in northern Arkansas and
southern Missouri, and given the sensitivity of the soils there,
this was enough to push the confidence level into the low-end
Moderate category.

Elsewhere, as mentioned above there will be a secondary maximum of
heavy rainfall, likely near the MO/IA border, which will be forced
by a shortwave rounding the upper level low embedded within the
broader trough over the central Plains. While various guidance
suggest this area may see even more rain than areas further south,
that solution was discounted as contrary to the conceptual model
that if there are two simultaneous rounds of storms occurring, the
southern-more of the two will be the dominant due to unobstructed
inflow of Gulf moisture from the LLJ, whereas the northern most
set of convection gets the "leftover" convection, and is therefore
very frequently weaker. Should the heavier rainfall totals verify
in this area, then Moderate Risk level impacts will also be
possible here as well. For now however, the area is in a higher-end
Slight, along with the rest of the state of Missouri. No
significant changes were made to the risk areas elsewhere as the
overall pattern remains well-resolved.

Wegman


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt