Mesoscale Discussion 0220
NWS Storm Prediction Center Norman OK
0524 PM CST Tue Feb 28 2017
Areas affected...Portions of eastern Iowa...northern
Illinois...northwestern Indiana...and southwestern Michigan
Concerning...Tornado Watch 42...
Valid 282324Z - 010100Z
The severe weather threat for Tornado Watch 42 continues.
SUMMARY...Severe thunderstorms capable of tornadoes, large hail, and
damaging winds will continue across portions of Tornado Watch 42
DISCUSSION...A cluster of supercells has organized across northern
Illinois late this afternoon on the nose of a stout 850-700-mb jet.
A 21Z sounding from Lincoln, IL indicated a strongly veering with
height low-level wind profile, topped by steep lapse rates in the
approximate 850-650-mb layer, which is fostering vigorous updraft
accelerations in these storms. Additionally, low-level
storm-relative helicity is maximized in the vicinity of a warm front
across northern Illinois currently. As a result, several supercells
continue to evolve eastward across the area, with recent reports of
a large tornado in eastern La Salle / western Grundy counties.
As these cells move east towards northwestern Indiana and Lake
Michigan, cooler surface temperatures and lower dew points may favor
some slight weakening of these cells and/or less organized storm
modes. However, any organized, rotating updraft may be able to
sustain itself even in a weakly stable low-level environment, due to
a strong upward pressure perturbation gradient force. Moreover, some
upscale growth of cells into small bowing segments may yield an
attendant damaging-wind threat with these storms as they progress
LAT...LON 40478675 40308850 40429011 40679170 41079226 41679204
42219103 42358855 42308698 42058612 40738612 40478675