913
ACUS01 KWNS 210555
SWODY1
SPC AC 210554

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1254 AM CDT Tue May 21 2024

Valid 211200Z - 221200Z

...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS IN PARTS OF
IOWA...NORTHERN MISSOURI...NORTHWEST ILLINOIS...SOUTHWEST
WISCONSIN...AND FAR SOUTHEAST MINNESOTA...

...SUMMARY...
An outbreak of severe thunderstorms, including the potential for
strong tornadoes, is probable today, peaking in the afternoon to
early evening. The area most likely to be impacted will include the
Mid to Lower Missouri Valley across the Mid to Upper Mississippi
Valley to the Lake Michigan vicinity.

...Mid-Lower MO Valley into the western Great Lakes...
An MCS and other remnant storm clusters are expected to be ongoing
across parts of the Mid-MO Valley to Upper MS Valley at 12Z. How
exactly this morning activity evolves eastward is somewhat
uncertain. An initial bout of severe wind and hail may persist with
this activity through midday. This will likely be over a focused
mesoscale corridor, given a stout elevated mixed layer to its south
and progressively limited instability to the north. Quick air mass
recovery is expected in the wake of this early-day convection within
a progressive/highly dynamic synoptic pattern.

Pronounced surface cyclogenesis is expected with amplification of a
vigorous shortwave trough from the central High Plains to the Upper
MS Valley. This will yield substantial amplification of low to
mid-level flow, with a belt of 70+ kt 500-mb winds extending from
eastern KS and western MO through at least IA this afternoon. As the
surface cyclone deepens, an attendant cold front will surge eastward
into central IA through northwest MO by 00Z.

Very steep mid-level lapse rates will overspread the increasingly
moist warm sector, supporting aggressive diurnal destabilization
especially on the southern fringe of lingering convection/cloud
cover. Initial supercell development should occur near the deepening
surface cyclone by midday to early afternoon over the Mid-MO Valley.
Increasingly widespread thunderstorm development is expected through
the afternoon from the Upper MS Valley southward to the Lower MO
Valley. Deep-layer shear will support an initial mixed mode of
semi-discrete supercells and embedded supercells in clusters. But by
late afternoon, upscale growth into a fast-moving arc of bowing line
segments and embedded supercells is expected. Additional supercells
should also form downstream along the effective warm front/remnant
large-scale outflow that advances north in southern to central WI.

Very large hail potential will be most likely with the initial
supercell development, and with southern extent where semi-discrete
storms should persist longest. Tornadoes, some of which should be
strong, appear most probable from northern MO northward into
southeast MN and southwest to central WI from both supercell and
mesovortex processes. How far north strong-tornado potential can be
realized will be modulated by the degree of destabilization in the
wake of the early-day convection, but low-level SRH will be quite
large. Damaging winds will become increasingly likely later in the
afternoon into the evening with gusts of 75-90 mph possible as a
well-organized QLCS forms. This should tend to be maximized across
parts of central/eastern IA into western IL and northern MO. QLCS
intensity should subside after dusk, as convection outpaces larger
buoyancy, but a damaging wind threat could persist into Lower MI
through the Mid-MO Valley.

...Ozarks to central TX...
Convective coverage will decrease with southward extent as
large-scale ascent focuses northward toward the Upper MS Valley and
western Great Lakes. Nevertheless, scattered severe storms are
possible during the late afternoon/evening near where the cold front
overtakes the dryline in the Ozarks. More isolated convective
coverage is expected southwestward along the dryline in central TX.
Large buoyancy and ample deep-layer shear will support a primary
threat of very large hail in TX. While a mix of large hail, damaging
winds, and a few tornadoes will be possible over the Ozarks.

...New England and northeast NY...
A low-amplitude shortwave impulse/remnant MCV will move across
southern QC and northern New England. This will result in a band of
enhanced westerly deep-layer flow from northern NY into ME. A
seasonally moist airmass will be in place with surface dewpoints
into the low 60s F, with strong heating and steepening low-level
lapse rates. Modest MLCAPE and effective shear from 30-35 kt will
support a few organized cells. Moderately elongated hodographs
suggest marginally severe hail will be possible along with locally
damaging wind gusts.

..Grams/Barnes.. 05/21/2024

$$