Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
427 PM EDT Mon Mar 29 2021

Valid 00Z Tue Mar 30 2021 - 00Z Fri Apr 02 2021

...Colorado Rockies...
Day 1...

Positively-tilted upper trough will move through the central
Rockies overnight into Tuesday morning as a deep upper low skirts
the U.S./Canadian border toward Hudson Bay. Surface cold front
will push through Colorado 00-12Z dropping temperatures below
freezing from northwest to southeast as 700mb temperatures drop to
-12 to -15C. Upper jet will sink southeast of the region quickly
late Tuesday as any moisture in the column dissipates by early
Wednesday. Limited moisture will be in place but a band of
0.2-0.3" precipitable water values will be able to squeeze out
0.25-0.50" liquid as upslope enhancement maximizes southwest of
Denver. General 4-6" amounts are expected with higher maxima 6-8"
possible in favored areas. Local maxima may also slide along the
Front Range west and northwest of Colorado Springs around 3-4" but
generally 1-2" below 7000 ft.

Fracasso


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

A reinforcing shortwave trough currently over the WA/OR border is
rounding a parent trough which currently has its axis over
northern Rockies. This shortwave trough amplifies as it digs to UT
tonight, making a positively tilted trough axis connected to the
parent trough that will reach northern Ontario Tuesday. The
shortwave ejects east from the CO/NM border and across the
southern Plains Wednesday. Then Wednesday night the trough becomes
negatively tilted over the Midwest as rapid surface cyclogenesis
occurs along a slowing cold front and under the right entrance
region of the southerly jet over the Mid-Atlantic. Surface low
development continues Thursday it lifts over New England around a
now closed upper low over the eastern Great Lakes.

The combination of the surface low development and nocturnal
effects allows a rapid expansion of precip on the cold side of the
low with widespread snow developing over the eastern Midwest and
interior northern Mid-Atlantic (including the Appalachians)
Wednesday evening. This area of snow lifts north with the low
Thursday, mainly affecting northern PA, Upstate NY to VT. Ample
cold air and the deep cold core low do raise the prospect of snow
showers east of the Appalachians and perhaps all the way to the
Mid-Atlantic coast in the peak heating of Thursday afternoon. Day
3 snow probabilities for 4 or more inches are 10 to 30 percent
from the Allegheny Highlands of WV to northern PA, and 30 to 60
percent over interior Upstate NY (including the Finger Lakes
region) to the northern Greens of VT with 80% confidence for the
higher Adirondacks.

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

Jackson