Mesoscale Discussion 1855
   NWS Storm Prediction Center Norman OK
   0548 PM CST Wed Dec 16 2020

   Areas affected...Northeastern PA...northern NJ...southern
   NY...western CT...and southwestern MA

   Concerning...Heavy snow 

   Valid 162348Z - 170515Z

   SUMMARY...Snowfall rates will increase to 2-3 inches per hour and
   continue through 10 pm to 1 am EST from northeast Pennsylvania
   across northwestern New Jersey into southern New York/the lower
   Hudson Valley.

   DISCUSSION...A surface cyclone located near the mouth of Chesapeake
   Bay as of 23z is expected to deepen some while moving northeastward
   to just off the southern New England coast overnight.  A broad area
   of moderate to heavy precipitation is ongoing to the north of the
   cyclone, in a zone of intense low-midlevel warm advection and
   frontogenesis, within a coupled left-exit region of the midlevel jet
   and right-entrance region of the upper jet.  A surface warm front
   arcs north-northwest of the cyclone into southern MD, and then
   northeastward across southern/central NJ, denoting a transition to
   mixed precipitation and rain (also apparent in correlation
   coefficient data from KDIX from southeast PA across central NJ, and
   about 25 n mi south of Long Island from KOKX).

   Precipitation rates are expected to maximize over the next 3-6 hours
   as the cyclone matures and with the approach of the ejecting
   midlevel trough from the southern/central Appalachians.  Snowfall
   rates will likely reach 2-3 inches per hour in the zone immediately
   northwest of the warm front and where the warm nose aloft is not
   expected to warm above 0 C, and on the edge of the midlevel dry slot
   where relatively steep midlevel lapse rates could support embedded
   convection/lightning.  Closer to the coast, initially heavy snowfall
   up to 2 inches per hour will transition to mixed precipitation from
   central NJ into Long Island and the NYC metro area by about 04-06z.

   ..Thompson.. 12/16/2020

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

   ATTN...WFO...BOX...OKX...ALY...PHI...BGM...CTP...

   LAT...LON   41647645 42047575 42407454 42427335 42227270 41827225
               41397221 40927274 40467424 40427460 40567518 40627562
               40857627 41177660 41647645