Mesoscale Discussion 1854
   NWS Storm Prediction Center Norman OK
   1257 PM CST Wed Dec 16 2020

   Areas affected...Northeast West Virginia...northern
   Virginia...western Maryland...and central to eastern Pennsylvania

   Concerning...Heavy snow 

   Valid 161857Z - 170000Z

   SUMMARY...Heavy snowfall rates around 2 inches per hour are expected
   to increase in coverage through the mid to late afternoon hours
   across portions of the Mid-Atlantic. Heavy snowfall will likely
   continue into the overnight hours for portions of
   eastern/northeastern Pennsylvania.

   DISCUSSION...Precipitation continues to gradually increase in
   coverage across much of the Mid-Atlantic region within a broad swath
   of strong warm air advection between 850-700 mb (per recent RAP
   soundings and mesoanalysis). Mixed precipitation types continue to
   be reported within the precipitation transition zone along the
   Roanoke to Fredericksburg, VA line. While a wintry mix, including
   pockets of freezing rain, will continue in the near term, this zone
   should become more limited spatially through the mid/late afternoon
   hours. Further north, moderate, to at times heavy, snowfall rates
   have been observed (as evidenced by visibility reductions to
   one-quarter mile and observed snow rates around 2 inches per hour
   for some locations in northeast WV). 

   Heading through the afternoon, the low that is currently advancing
   northward along the Carolina coast will continue to deepen. As this
   occurs, mid-level winds will strengthen and begin to tighten the
   850-700 mb baroclinic zone and augment warm advection across the
   region. As isentropic and frontogenetical lift increase, snowfall
   rates will see a corresponding uptick in intensity. Lift within the
   dentritic growth zone and weak instability closer to the coast will
   help further augment snowfall rates. Widespread snowfall rates above
   1 inch per hour over the region are likely, and the potential for
   widespread snowfall rates exceeding two inches per hour will
   increase through 00 UTC as the potential for organized
   frontogenetical/deformation-driven snow bands increases.

   ..Moore.. 12/16/2020

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

   ATTN...WFO...PHI...BGM...CTP...LWX...

   LAT...LON   39387921 40367890 40907814 41177723 41227627 41087554
               40717526 40257540 39857630 39497679 39097723 38737763
               38477811 38707904 39387921