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Mesoscale Discussion 169
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   Mesoscale Discussion 0169
   NWS Storm Prediction Center Norman OK
   0859 AM CST Sun Feb 12 2017

   Areas affected...Portions of northern and eastern

   Concerning...Heavy snow 

   Valid 121459Z - 122000Z

   SUMMARY...Areas of heavy snow will spread east-northeastward through
   mid-day, and continue through the afternoon. Snowfall rates of 1.0
   to locally 1.5 inches per hour will be possible.

   DISCUSSION...Forcing for ascent preceding a low-amplitude, sheared
   midlevel wave crossing the southern Great Lakes area has supported
   the development of a broad shield of precipitation from northern
   parts of the Mid Atlantic toward southern New England. The continued
   progression of the wave will correspond to this precipitation
   overspreading increasingly colder/sub-freezing profiles (e.g., as
   indicated by the 12Z Albany NY sounding). Sufficient ascent
   interacting with saturated dendritic growth zones will support areas
   of heavy snow with rates of 1.0 to 1.5 inches per hour possible.

   The heavy snow will likely develop east-northeastward through around
   18Z. Secondary surface cyclogenesis over the Delmarva vicinity --
   where more pronounced surface pressure falls (around 1.5 mb per 2
   hours) are ongoing -- will likely correspond to sustained deep
   ascent/frontogenesis through the day. As a result, the heavy snow
   should continue into the afternoon hours. Upslope-flow-enhanced
   precipitation will contribute to the highest snowfall rates along
   east/southeast slopes of mountains.

   Later in the day, notable midlevel drying may spread across southern
   New England, perhaps related to ongoing clearing in precipitation
   across western PA per mosaic radar loops. As such, precipitation
   growth may tend to become increasingly shallow in the column,
   resulting in a lack of snow growth aloft. As such, precipitation
   could change to light freezing rain across southern parts of the
   area after 20Z, before diminishing.

   ..Cohen.. 02/12/2017


   LAT...LON   41737108 41547206 41947347 43387550 44137605 44737537
               44907418 43927191 42647080 41737108 

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