Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
416 AM EDT Fri Apr 17 2020

Valid 12Z Fri Apr 17 2020 - 12Z Mon Apr 20 2020

...Upper Ohio Valley, Southern Great Lakes, New York/New England...

Days 1-2...
A positively tilted trough 700-500 mb crosses over the southern
Great Lakes this evening and across New York and New England. The
700 mb wave amplifies along the way, producing an enhanced jet
across norther n Ohio this morning that continues downstream with
moisture advection and ascent along 700 mb moisture convergence
maxima in norther PA and southern NY to MA.  Snow in the 700 mb
theta-e advection zone currently in northern IL moves east into
northern IN/southern lower MI and northern OH. Several inches of
snow are expected in northern PA and southern NY/MA, mitigated by
melting on streets where prior road temperatures were above
freezing .

The strengthening westerly jet today will speed the system, which
will move off the New England coast Saturday.
Localized maxima area  expected in the higher terrain of the
Alleghenies in northern PA/Catskills of NY/Berkshires of MA
as the combination of warm/moisture advection, topographic lift,
and frontogenesis ahead of the 850mb low center will allow banding
of heavier snow at these higher elevations. Day 1 snow
probabilities are moderately high for 6 or more inches over a
portion of north-central PA with low probabilities extending east
across southern NY to the Catskills despite much of this falling
during daylight.


...Northern Rockies...

Day 2...
The next mid-upper trough shifts down the Canadian Rockies into MT
Friday night before shifting east along the US/Canadian border
Saturday. Anticyclonic post-cold frontal flow will promote upslope
flow for eastern slopes of the MT Rockies including Glacier NP
Saturday, and then northern WY Saturday night into Sunday. Day 2
snow probabilities for 6 or more inches are moderate for the Lewis
Range (Glacier NP), spreading south to the northern Absarokas.
The upper trough departs on day 3, with dry air advection aloft
across MT and then WY in MT bringing the event to a close.

...Utah and CO...

Day 3...
Modest ascent within a pool of moisture across Utah that persists
on day 3 and then moves into the ranges of CO gets lifted within a
region of difluent flow aloft as an upper level trough drifts
south out of ID and WY into northern UT. Most model have QPF that
is light, so only a slight risk of four inches is shown in these
areas due to weak 700 mb lift. because of the weak lift, the
primary modest snow is expected in the Utah Uinta/Wasatch
mountains into the ranges of western CO.   

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

Petersen