Mesoscale Discussion 1339
   NWS Storm Prediction Center Norman OK
   1044 AM CDT Sat Jul 24 2021

   Areas affected...Parts of central/northern lower Michigan

   Concerning...Severe potential...Watch possible 

   Valid 241544Z - 241815Z

   Probability of Watch Issuance...40 percent

   SUMMARY...Strong thunderstorms may begin developing by 2-4 PM EDT,
   particularly near and southwest through south of the Houghton Lake
   vicinity, where a supercell or two posing a risk for a tornado is
   possible initially, before strong wind gusts become the more
   prominent potential hazard.

   DISCUSSION...Near the base of large-scale mid-level troughing
   primarily shifting east of the Canadian Prairies into the Hudson
   Bay/northwestern Ontario vicinity, a southwesterly to westerly
   lower/mid tropospheric jet streak is in the process of propagating
   across the Upper Midwest and Great Lakes vicinity.  This includes
   speeds on the order of 30-50+ kt in the 850-500 mb layer, with a
   couple of associated mid-level cyclonic vorticity maxima.  One is
   migrating across and east-northeast of the Houghton Lake area, with
   another trailing to the west-southwest (now southeast of Oshkosh,
   WI).  It appears that forcing for ascent with the trailing
   perturbation may begin impacting areas near/southwest through south
   of Houghton Lake, and northwest of Saginaw, as early as the 17-19Z
   time frame.

   Along a diffuse warm frontal zone across the region, a seasonably
   moist boundary layer, with surface dew points near 70F, appears
   characterized by CAPE on the order of 1000 J/kg.  Mid-levels are
   relatively warm, but lapse rates still appear modestly steep, and at
   least some gradual boundary-layer warming may contribute to a
   further increase in CAPE through mid afternoon.

   Given the moderately strong deep-layer shear (including sizable
   clockwise-curved low-level hodographs), the increasing instability
   and forcing for ascent may become conducive to the development of
   organized convective development by mid afternoon.  Isolated
   supercells posing at least some risk for a tornado or two may be
   possible initially, before activity tends to grow upscale with
   potentially damaging wind gusts becoming a more prominent hazard as
   activity spreads eastward.

   ..Kerr/Hart.. 07/24/2021

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

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

   LAT...LON   45108473 44968330 44038299 43178515 44078592 45108473