Mesoscale Discussion 1732
   NWS Storm Prediction Center Norman OK
   0552 PM CDT Sun Sep 12 2021

   Areas affected...Parts of southern lower Michigan

   Concerning...Severe Thunderstorm Watch 493...494...

   Valid 122252Z - 130045Z

   The severe weather threat for Severe Thunderstorm Watch 493, 494
   continues.

   SUMMARY...An evolving cluster of thunderstorms may become better
   organized with increasing potential for damaging wind gusts, in
   addition to large hail, while progressing along a corridor near or
   just north of Grand Rapids and Detroit through 9-10 PM EDT.

   DISCUSSION...A pair of mid-level cyclonic vorticity centers have
   contributed to ongoing vigorous thunderstorms development while
   migrating eastward along a 50-70 kt westerly 500 mb jet.  The
   convection appears to be generally rooted within moist air above/to
   the north of a quasi-stationary frontal zone, near the northern
   periphery of plume of warm and capping elevated mixed-layer air
   emanating from the Great Plains.

   One cluster developed fairly far removed from the lead cyclonic
   vorticity center, now progressing across Lake Huron, and the warm
   elevated mixed-layer appears to be suppressing it as it approaches
   the international border area north of Detroit.  The upstream
   cluster appears just ahead of the trailing cyclonic vorticity
   center, migrating inland off of Lake Michigan, to the
   northwest/north of Grand Rapids.  

   The stronger mid-level forcing for ascent with the trailing
   thunderstorm cluster seems likely to maintain it across lower
   Michigan, near/north of Grand Rapids, Lansing and Ann Arbor, into
   the Flint vicinity and areas near/north of the Greater Detroit area
   by 01-02Z.  Thermodynamic profiles characterized by steep mid-level
   lapse rates and sizable CAPE will remain conducive to large hail, in
   the presence of strong cloud-bearing layer shear.  Although
   low-level lapse rates are generally weak, profiles are still
   relatively warm and saturated, and latent cooling due to melting
   hail in downdrafts may gradually contribute to downward transfer of
   the strong momentum in mid-levels, perhaps eventually accompanied by
   increasing risk for potentially damaging wind gusts.

   ..Kerr.. 09/12/2021

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

   ATTN...WFO...DTX...GRR...

   LAT...LON   43568540 43428412 43128258 42448281 42558393 42758533
               43028594 43228539 43568540