Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
341 PM EDT Fri May 23 2025
Day 1
Valid 16Z Fri May 23 2025 - 12Z Sat May 24 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM SOUTHEASTERN
KANSAS AND NORTHEASTERN OKLAHOMA INTO THE OZARKS...

...Kansas, Oklahoma, Missouri, Arkansas...

There remains a good model signal for the development of heavy
rainfall later today, in what is expected to be the first of
several rounds impacting the region over the next few days.

The synoptic pattern will evolve today into one that will support
heavy rain within periods of organized convection/MCS beginning
tonight. This will be in response to the central part of the CONUS
being sandwiched between an elongated trough to the northeast and a
building ridge over the Four Corners, which leaves broad W/NW flow
across the area. Within this flow, ascent will occur through
periodic impulses moving through the flow combined with the RRQ of
a jet streak aloft. The resultant frontogenesis from the RRQ of the
jet streak will efficiently overlap a warm front wavering in the
area, providing additional lift, which will then be enhanced by
isentropic ascent of the LLJ atop this front.

Forcing will be pronounced, and will act upon favorable thermodynamics,
which will become increasingly robust the latter half of today and
tonight. This will occur as the impressively broad 850mb LLJ
emerges from the Gulf and surges to 40+ kts, angling orthogonally
into the warm front. This will produce strong moisture convergence
as reflected by the moisture transport vectors, and resupply
elevated CAPE of 3000 J/kg northward into the Plains. As convection
blossoms this evening, generally after around 04Z, it will expand
rapidly and organize thanks to 40-50 kts of bulk shear. This will
result in clusters of storms, potentially growing upscale into an
MCS, and training of echoes is likely due to aligned flow to the
warm front. There continues to be some uncertainty into how quickly
the convection will grow into an MCS allowing it to push faster to
the southeast towards daybreak Saturday, but until that happens,
some upstream growth to the NW is expected which will prolong
training of echoes. With a high probability for rain rates
exceeding 2"/hr, any training could lead to rainfall of 2-5", with
locally higher amounts possible (10-20% chance), especially near
the MO/KS/AR/OK intersection.

Another fly-in-the-ointment for the risk area is how elevated
convection north of the warm front, especially early tonight, will
impact the progression SW. There are a few high-res CAMs, and
several global models, that suggest an outflow boundary will be
pushed south more into Oklahoma tonight, which will be aided by
residual boundaries from any dry line convection moving east, and
the front itself. This could result in a bi-modal distribution to
rainfall, and a secondary maxima of flash flood risk across eastern
OK. While confidence is higher in the isentropic/elevated heavy
rain risk farther north, the SLGT risk was tailored a bit SW from
inherited to account for this potential. Otherwise, the inherited
risk areas were modified only cosmetically for recent guidance,
primarily being pulled NW (the MRGL risk area) more into KS for
backbuilding convection during the development stage.


...South Florida...
A weakening front across South Florida will serve as a focus for
slow moving thunderstorms once again today, drifting east to the
Gold Coast through the aftn/eve. This front and a lingering tail of
an upper jet streak will provide synoptic lift into favorable
thermodynamics characterized by PWs of 1.75 to 2.00 inches, and
SBCAPE above 2000 J/kg. This overlap will support rainfall rates
within thunderstorms for which the HREF indicates has a 40% chance
of exceeding 2"/hr, and the 15-min HRRR suggests brief rainfall
intensity of 4"/hr possible. Where these storms slow/stall,
primarily along the southeast coast where they interact with the
sea breeze, this could result in spots of 3-5" of rainfall. Should
this rain occur atop urban areas or over locations that have
received heavy rainfall the past 48-hrs, isolated flash flood
instances could result, so the MRGL risk was maintained.

Weiss

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