Mesoscale Discussion 1840
   NWS Storm Prediction Center Norman OK
   0901 AM CST Sat Dec 12 2020

   Areas affected...Northern Michigan

   Concerning...Heavy snow 

   Valid 121501Z - 121900Z

   SUMMARY...The onset of heavy snow has begun across northern
   Michigan, and will likely persist into the early afternoon hours.
   Snowfall rates above one inch per hour are likely, and may exceed
   two inches per hour at times.

   DISCUSSION...Morning observations from northern MI indicate that
   heavy snow has already begun for a few locations, notably from
   Gaylord, MI and areas north and east where visibility reductions
   between one-half to one-quarter mile due to snowfall have been noted
   (though gusty winds up to 25-30 mph and blowing snow may be
   contributing somewhat to these visibility reductions for some
   stations). This onset is slightly earlier than suggested by some
   guidance, and recent water-vapor and IR imagery show that the
   strongest region of ascent likely remains further west closer to the
   upper-level low over lake Michigan and parts of southeastern WI
   (where light to moderate snow has been ongoing for the past few
   hours). 

   Moderate to heavy snow will likely become more widespread over
   northern MI through 18 UTC as the upper-level shortwave trough
   continues to translate to the east/northeast. Mesoscale ascent will
   augment synoptic lift via broad warm advection between 850-700 mb.
   Recent RAP Mesoanalysis and hi-res guidance suggest that a
   developing deformation zone within these levels will likely become
   established over northern MI and will introduce the potential for
   organized snow banding. Additionally, 6-7 C/km lapse rates and
   50-100 J/kg MUCAPE rooted aloft were noted in 12 UTC soundings and
   RAP Mesoanalysis to the south across IN, OH, and southern MI. While
   displaced to the south, the strong south/southwesterly mid-level
   flow may help advect some convective elements northward into
   northeastern MI.

   The combined effect of these features will be for an increase in
   snowfall rates through the remainder of the morning. Snowfall rates
   up to 1-2 inches per hour appear likely, and may exceed two inches
   per hour if and where mesoscale snow banding can become more
   organized by late morning. While some guidance suggests that heavy
   snowfall could continue into the afternoon hours for parts of
   northeastern MI, the spatial extent of this potential after 18 UTC
   is unclear due to the progressive nature of the upper-level low and
   the possible displacement of favorable lift to the east/northeast.

   ..Moore.. 12/12/2020

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

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

   LAT...LON   44848621 45408546 45768478 45588379 45248321 44708320
               44408375 44278461 44098550 44098620 44338644 44848621