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
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).
...Please see www.spc.noaa.gov for graphic product...
LAT...LON 40957483 40887529 40947584 41327627 42527639 43097631
43477602 43617579 43677382 43387296 43327280 42967249
42717257 41867312 41147429 40957483