Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
428 PM EDT Thu Apr 23 2020

Valid 00Z Fri Apr 24 2020 - 00Z Mon Apr 27 2020


...North-Central Rockies...
Day 1...

A shortwave riding a 120kt northwesterly jet is pushing into WY
from ID this afternoon and will cross the north-central Rockies
tonight before shifting onto the southern Plains and cutting off
mountain snow Friday. The north-central Rockies will be in the
left exit region of the jet, maximizing ascent along with the
mid-level trough axis to allow increased snow production in the
higher ranges of WY/CO tonight. Snow levels are currently 7000 to
8000ft in these states, and will drop to 6000 to 7000ft tonight.
Moisture is modest in the continental air mass, so snowfall rates
will also be somewhat modest though upslope post-frontal flow will
aide snow for eastern/front ranges. Day 1 WPC snow probabilities
are moderate to high for 6 inches or more in the Big Horns,
Laramie, and Medicine Bow ranges in WY, and the Front and Park
ranges in northern CO.


...Northeast...
Day 3...

The shortwave trough crossing the north-central Rockies tonight
will swing across the southern Plains and lift into the Midwest
Saturday as it interacts with additional northern stream energy
from the northern Plains. This should promote cyclogenesis as the
systems shifts east across the northern Mid-Atlantic states
Sunday. There is uncertainty with the track and intensity of this
low, but the favored 12Z ECMWF does have low enough heights in the
closed low and is dynamic enough to warrant a threat for a TROWAL
to develop across a stripe of PA/NY with snow levels around 3000ft
by Sunday afternoon. Day 3 snow probabilities are limited to the
highest Adirondacks for a 4 inch or higher threat, but there would
be an accumulating snow threat where the TROWAL sets up,
particularly at higher elevations in the PA/NY area.


For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Jackson