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Mesoscale Discussion 105
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   Mesoscale Discussion 0105
   NWS Storm Prediction Center Norman OK
   0215 AM CST Fri Mar 02 2018

   Areas affected...Portions of the Mid Atlantic and adjacent New

   Concerning...Heavy snow 

   Valid 020815Z - 021415Z

   SUMMARY...Moderate/heavy snow will become more common from
   northeastern Pennsylvania to western New England through daybreak,
   as rain mixes with/changes to snow in many locations.

   DISCUSSION...Recent surface analysis and water-vapor imagery
   continue to depict the impressive evolution of a powerful
   mid-latitude cyclone over the Northeast US this morning. Surface
   pressure falls upwards of 2-3 mb/hr have been noted within the New
   York Bight, evidencing the ongoing transfer from a decaying surface
   low over Pennsylvania to a rapidly deepening low south of Long
   Island. Concurrent with this evolution, regional VWP data (KENX,
   KOKX, KBOX) show expanding clockwise-looping hodographs through the
   approximate 0-4km layer, indicating amplifying warm-air advection in
   low/mid levels (up to about 600 mb). Strong ascent related to this
   warm advection regime, combined with intensifying frontogenetic
   circulations over the region, should focus heavier precipitation
   over eastern New York and adjacent areas through daybreak.

   Dual-polarization data from KDIX, KOKX, and KBOX depict the edge of
   a melting layer (around 4-6K ft AGL) stretching from eastern PA
   northeastward through Litchfield Co, CT and onward to the MA/NH
   border near the coast, as of 08Z. To the northwest of this melting
   layer, near-surface melting (beneath radar beam height) is ongoing
   where temperatures remain above freezing, especially at lower
   elevations. However, diabatic cooling and surface cold advection is
   causing a slow south/southeastward retreat of the rain/snow line
   over parts of PA/NY, and KBGM CC data illustrate this process over
   the last hour or so. Nonetheless, a corridor of wavering snow/rain
   from the lower Hudson Valley to western Massachusetts is likely to
   persist, due to the complex thermal interplay of diabatic effects,
   frontogenetic circulations, and broad warm-air advection.

   Forecast soundings across the region do not show a very deep
   dendritic growth zone and, for many locations, the strongest ascent
   may reside outside the -12 to -18 C layer. Nonetheless, strong
   vertical motion, generating an array of crystal types, and
   aggregation within the warm-advection layer will foster snowfall
   rates upwards of 1-1.5 in/hr, with locally higher amounts possible.
   Indeed, recent KENX data indicate enhancements in KDP and ZDR over
   the Lower Hudson Valley (around 12-14K ft above radar level),
   indicating more robust planar-crystal growth and a potential for
   higher rates (where the surface p-type is snow).

   ..Picca.. 03/02/2018

   ...Please see for graphic product...


   LAT...LON   40957483 40887529 40947584 41327627 42527639 43097631
               43477602 43617579 43677382 43387296 43327280 42967249
               42717257 41867312 41147429 40957483 

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