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Mesoscale Discussion 220
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   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
   this evening.

   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

   ..Picca.. 02/28/2017


   LAT...LON   40478675 40308850 40429011 40679170 41079226 41679204
               42219103 42358855 42308698 42058612 40738612 40478675 

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