SPC MD 2202

SPC MD 2202


Mesoscale Discussion 2202
NWS Storm Prediction Center Norman OK
0643 PM CDT Wed Sep 27 2023

Areas affected…Southern Illinois and southern Indiana into

Concerning…Severe potential…Watch possible

Valid 272343Z – 280145Z

Probability of Watch Issuance…40 percent

SUMMARY…Thunderstorms developing across southern Illinois are
expected to intensify as they migrate east into the Ohio River
Valley. Large hail and severe wind gusts will be possible with the
more intense cells, but thunderstorm coverage and mode remains
uncertain. Trends will be monitored for watch issuance.

DISCUSSION…Over the past hour, a few transient attempts at
convective initiation have been noted across southern IL ahead of a
subtle mid-level impulse and in the vicinity of a decaying surface
low. Additional attempts at CI are expected over the coming hours as
isentropic ascent increases over a warm front draped from southern
IL into the OH River Valley. Temperatures across the warm sector –
which is bound to the east by a residual outflow boundary – have
begun falling with the onset of nocturnal cooling. MLCAPE values
have been falling due to this cooling, but MUCAPE values should
remain near 1000 J/kg as saturation of the surface-850 mb layer
increases. Additionally, weak south/southeasterly flow across the
region with 30-35 knot flow aloft is supporting somewhat elongated
hodographs with effective bulk shear values near 30-35 knots. Some
veering in the lowest 1 km is noted in regional VWPs, which is
supporting effective SRH values around 100-150 J/kg. This
environment is favorable for organized convection, including the
potential for a few supercells.

However, several modulating factors are noted. Deep-layer shear and
storm motion vectors are forecast to be largely oriented along the
warm front, which should favor a tendency for upscale growth. These
factors may be offset by the isolated nature of storms given the
relatively weak ascent over the region, so storm mode remains
somewhat uncertain. Additionally, the noted nocturnal cooling may
hinder the potential for surface-based convection, limiting the
tornado potential and favor large hail and severe winds as the
primary hazards. Trends will continue to be monitored for the needed
for a watch issuance, which may be conditional on storm coverage and

..Moore/Hart.. 09/27/2023

…Please see www.spc.noaa.gov for graphic product…


LAT…LON 38548523 38288487 37848482 37288516 36898588 36758649
37068764 37328846 37548904 37848943 38288944 38688888
38758804 38768588 38548523

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