Mesoscale Discussion 0006
   NWS Storm Prediction Center Norman OK
   1129 AM CST Tue Jan 07 2020

   Areas affected...Parts of northern Virginia...central Maryland and
   south central Pennsylvania

   Concerning...Heavy snow 

   Valid 071729Z - 071930Z

   SUMMARY...A 1 to 3 hour period of snow, perhaps occasionally
   including rates in excess of 1 inch per hour, will impact areas near
   and west through north of the District of Columbia through 2-4 PM
   EST.

   DISCUSSION...Lower/mid tropospheric warm advection ahead of a
   vigorous short wave trough is contributing to an area of strong lift
   now beginning to spread east-northeast of the Blue Ridge. 
   Particularly across northern Virginia, central Maryland and adjacent
   portions of south central Pennsylvania, lift is becoming maximized
   in the 700-500 mb layer, where forecast soundings indicate
   temperature and moisture profiles are becoming conducive to large
   dendritic ice crystal growth.

   Although near surface profiles are initially relatively dry and warm
   across and northwest of the Washington D.C. metropolitan area,
   evaporative cooling/melting within an initial burst of heavy snow
   should cool and saturate this environment sufficiently to allow for
   a period of accumulating snow.  Aided by precipitable water on the
   order of .4 to .5 inches, this may include snow rates of 1/2 to 1
   inch+ per hour, before stronger mid-level ascent spreads
   east-northeast of the D.C. area.  

   Although this lift may overspread the Baltimore and Philadelphia
   metropolitan areas later this afternoon, an initially warmer
   boundary layer (surface temperatures now in the lower/mid 40s)
   across these areas seem likely to result in decreasing potential for
   accumulating snow.

   ..Kerr.. 01/07/2020

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

   ATTN...WFO...CTP...LWX...

   LAT...LON   40707713 39817621 39177664 38217785 38367832 40357831
               40707713