Mesoscale Discussion 1103
NWS Storm Prediction Center Norman OK
0834 PM CDT Fri Jun 25 2021
Areas affected...east-central Missouri into central Illinois
Concerning...Severe Thunderstorm Watch 317...
Valid 260134Z - 260300Z
The severe weather threat for Severe Thunderstorm Watch 317
SUMMARY...Thunderstorms will continue across the watch through the
evening. The most likely area for severe will be across portions of
Illinois where a threat for strong-to-severe thunderstorm winds and
a couple brief tornadoes will persist for another few hours.
DISCUSSION...Thunderstorms continue across much of the watch this
evening, with several distinct regimes in play.
Across Missouri, outflow dominant thunderstorms continue to slowly
move south as part of the broader southward progression of an MCS
stretching from Kansas into Missouri. Although these thunderstorms
remain in an very unstable atmosphere, they are moving farther away
from the better deep-layer shear. Thus, the overall severe potential
should remain fairly low, with gusty winds being the main severe
threat. Portions of the severe thunderstorm watch here may
eventually be canceled early.
Across west-central Illinois, a small, linear cluster as developed
associated with the leading edge of the original east/northeastward
expansion of the afternoon Kansas MCS. These thunderstorms remain in
a belt of deep-layer shear around 30-40 knots and most-unstable CAPE
in excess of 2000 J/kg. These thunderstorms will continue along the northwestern edge of the instability axis for the next few hours,
posing a continued risk of strong-to-severe thunderstorm winds.
Farther east, across east-central Illinois, more isolated, discrete
convection has developed along remnant convergent/outflow boundaries
across the area. Although the instability is much less here than
farther southwest, augmented low-level vorticity associated with the aforementioned boundaries will support a continued threat for brief
tornadoes with any strong updraft that can tap into these vorticity
reservoirs. This threat should diminish quickly as the nocturnal
boundary layer develops and deepens with the loss of diurnal
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