Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1255 AM CDT Tue May 20 2025
Valid 201200Z - 211200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS IN THE MID/DEEP
SOUTH TO TN VALLEY AND CUMBERLAND PLATEAU...
...SUMMARY...
Multiple rounds of severe thunderstorms are expected today through
this evening, centered on the Mid/Deep South, Tennessee/Lower Ohio
Valleys, and Cumberland Plateau. Tornadoes, some of which could be
strong, along with scattered to widespread damaging winds, and large
to isolated significant severe hail will be possible.
...Synopsis...
Multiple shortwave impulses and attendant mid-level jetlets will
rotate through the southern to eastern portion of the broad
mid/upper trough over the central states. A belt of 60-75 kt 500-mb
southwesterlies should be centered on the Ark-La-Tex to Lower OH
Valley by late afternoon. This should weaken thereafter, as the next
shortwave impulse strengthens flow across the central Great Plains
by evening. Primary surface cyclone should initially drift east from
the IA/MO border into IL through the day, before accelerating
eastward across IN/OH tonight.
...Mid/Deep South to the TN/Lower OH Valleys and Cumberland
Plateau...
An extensive swath of convection, currently from the Mid-MS Valley
to the Red River, will likely be ongoing across parts of KY to the
Ark-La-Miss at 12Z. The northern portion of this convective swath is
consistently progged to persist into the diurnal heating cycle,
aided by low-level warm theta-e advection along with mid-level
height falls, and downstream destabilization. The latter is a
primary source of uncertainty given the outpacing of convection thus
far relative to 00Z CAM guidance. It does appear that there should
be at least modest surface-based buoyancy being maintained
downstream to warrant potential for a severe-producing QLCS into the
afternoon. All severe threats are possible, but damaging winds may
be the predominant coverage hazard. Tornado potential should peak
where the QLCS approaches the surface warm front, before an abrupt
drop-off in surface-based instability to its northeast.
Greater boundary-layer destabilization is expected to the
west-southwest of the early-day activity with relatively rapid
airmass recovery amid low 70s surface dew points. Mid-level lapse
rates will initially be steep over the Mid-MS Valley, but be more
muted with southern extent into the Lower MS Valley. Still, moderate
to large buoyancy with MLCAPE of 2000-3000 J/kg and minimal MLCIN
should be common by mid-late afternoon. During that time frame,
scattered supercells will likely develop along and just ahead of the
eastward-moving cold front from the Lower OH Valley to the
Ark-La-Miss. The modest mid-level lapse rates will be a limiting
factor, but isolated significant severe hail (up to around tennis
ball-size) will be possible in the initial supercell corridor. A
broad swath of 30-40 kt low-level southwesterlies should be
sufficient for at least a few strong tornadoes as supercells mature,
most likely from western/southern KY across TN into northern
portions of MS/AL. Guidance is insistent on upscale growth by early
evening though, which would yield small-scale bows and potentially
multiple damaging wind swaths. Embedded supercells should still
persist well into the evening as storms spread east-southeast in the
Southeast and southern Appalachians. But the aforementioned relative
decrease in mid-level winds and muted lapse rates lower confidence
on significant severe wind and greater tornado probability
highlights.
...IL/IN...
Within the left-exit region of the aforementioned mid-level jetlet,
a dry slot and attendant corridor of steep lapse rates will likely
evolve across portions of MO to IL/IN from midday into the
afternoon. To the north of this, a cluster of sustained storm
development is initially anticipated towards mid-afternoon, centered
on central IL ahead of the surface low. Additional storms should
develop farther south in time towards the Lower OH Valley. A mix of
supercells and multicell clustering is anticipated, with isolated to
scattered coverage of severe and all hazards possible.
..Grams/Weinman.. 05/20/2025
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