SPC AC 211245
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0645 AM CST Wed Feb 21 2024
Valid 211300Z - 221200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FOR PORTIONS OF
THE WESTERN OZARKS REGION AND OVER NORTH-CENTRAL PARTS OF ILLINOIS
Areas of strong thunderstorm development are possible across parts
of the Ozark Plateau and adjacent Great Plains and across central
Illinois to far western Indiana late Wednesday evening into early
Thursday, with some risk for severe hail.
The mid/upper-level pattern will trend more toward zonal through the
period, as a well-developed southern-stream shortwave trough now
near coastal southern CA moves quickly east-northeastward and
penetrates a low-amplitude mean ridge over the central CONUS. This
perturbation should cross the central/southern High Plains around
00Z, then deamplify somewhat as its northern part enters confluent
flow and height falls ahead of a northern-stream trough crossing the
Upper Midwest. By 12Z, the southern perturbation should reach from
northern parts of IL/IN to southwestern MO and northwestern AR.
At the surface, 11Z analysis depicted a wavy, slow-moving frontal
zone from northernmost Lower MI across southern WI, through a weak
low near DBQ, to near MKC, EMP and through a broad area of low
pressure over western KS. Cyclogenesis is forecast today over the
western KS region, shifting eastward across KS to near MKC by 06Z,
trailing a cold front southwestward over northern OK. The Great
Lakes/Mississippi Valley frontal segment should move southeastward
then stall. A secondary low is possible along the cold front over
northern OK by 06Z, when a warm to stationary front should extend
from the MKC-area low across central parts of IL/IN.
...Ozarks to southern Great Lakes region...
Isolated to scattered, elevated thunderstorms are expected to
develop across portions of IL/IN and perhaps the western Ozarks
region overnight, shifting/expanding eastward especially across the
IL/IN region. Activity will attain greatest coverage after 06Z, and
may pose a risk of marginally severe hail.
Low-level warm/moist advection above the boundary layer is underway
in a relatively dry, early-stage return-flow event across a broad
corridor from the southern Plains to the Ohio Valley and southern
Great Lakes States. The warm-advection plume should become more
confluent and moist through today, and especially this evening into
tonight, as it intensifies with the approach of the southern
shortwave trough. Though the return-flow regime still will be far
from mature or optimal, moist advection/transport will persist such
that increasingly high-theta-e parcels are involved, and ultimately
lifted to an LFC.
The two areas with the most such progged lift are in and near the
"marginal risk" outlooks. The northern area should have greater
lift and thunderstorm coverage thanks to contribution from elevated,
low-level frontal forcing, while theta-e and midlevel lapse rates
will be greater above a relatively stable boundary layer in the
southern area. Albeit with much-different vertical distributions,
both areas should attain around 500-1200 J/kg MUCAPE, with greater
effective shear in the northern area (35-45 kt), and a stronger
proximate LLJ (45-55 kt) for the southern area across the Ozarks.
However, despite a favorable environment for severe hail, given
thunderstorms, the southern region appears more conditional because
of stronger CINH, imparted by the same EML that is providing the
high lapse rate. That implies less convective coverage, and some
(not all) convection-allowing progs with weaker indicators of
isentropic ascent indicate none -- despite greater moisture content
in the potential inflow region.
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