Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
509 PM EDT Thu May 07 2020

Valid 00Z Fri May 08 2020 - 00Z Mon May 11 2020


...Interior New York and New England...
Days 2-3 Friday - Saturday

A significant late-season winter storm is expected to produce
accumulating snows from in the Northeast, with heavy amounts
possible across portions of interior New York and northern New
England.

Models continue to show a late-season winter storm developing
Friday as a 700 mb wave rotates beneath the Great lakes trough,
with a low developing in advance of the wave over the mid Atlantic
states and moving northeast to near Cape Cod Sat morning and then
near or off the Maine coast before exiting through southeast
Canada.  While the column is expected to be marginal initially for
frozen precipitation, strong forcing may dynamically cool the
column supporting accumulating snow across higher elevations of
northeast PA and central to eastern New York and western new
England.

Expect accumulating snows across the Poconos and Catskills Friday
evening and overnight. Accumulating snows will continue to shift
east overnight into inland portions of New England, with impacted
areas expected to include the Berkshires, southern Greens and
Worcester Hills early Saturday morning.  Saturday morning, as the
shortwave lifts north ahead of the upper low, surface low pressure
is expected to further strengthen and track north along the
northern New England coast.  This will set the stage for
potentially heavy snowfall accumulations across portions of
interior northern New England.  Daytime guidance as a whole still
showed a fair amount of spread, with continuing model to model
differences, as the NAM and UKMET have lighter snow than the 12z
ECMWF and Canadian global.  There is general agreement the greater
potential for heavier snow is across northern New Hampshire and
interior Maine - where probabilities indicates chances persist for
8 inches or more snow.  The precip type change over zone in
downeast Maine and southern NH adds uncertainty to the forecast,
with more than one precip type possible.


Days 2-3 Northern Plains...
The models indicate a 700 mb wave moves slowly southeast from
south central Canada into the northern Plains.  Low pressure
develops.  In turn, a band of low level frontogenesis develops
northeast of the low center.  Low level moisture convergence and
frontal-induced lift then should produce a band of snow.  Cooling
as the low develops increases the chances precip will change to
snow in northern ND.  There are individual solutions that support
a mesoscale band of several inches of snow, including the NAM, Nam
Conus Nest, and Canadian global models.  There are uncertainties
also related to QPF amounts, as the mesoscale guidance amounts are
higher than the global runs.

On Day 3, more guidance shows a mixture of precip types as the
system moves into MN and WI, lowering confidence on accumulating
snow.

The probability of significant icing is than 10 percent Days 1-3.

Petersen