Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
439 PM EDT Thu Apr 16 2020

Valid 00Z Fri Apr 17 2020 - 00Z Mon Apr 20 2020

...Central Plains, mid MS Valley, upper Ohio Valley, Southern
Great Lakes, New York/New England...

Days 1-2...
A rather positively tilted upper trough over the northern Plains
and north-central Rockies this afternoon will eject east tonight,
sharpening into a tighter, still positively tilted trough over the
southern Great Lakes Friday evening, then swing to a neutral tilt
as it zips off the Northeast Coast Saturday. The associated
surface trough is currently stretching from a surface low center
on the OK Panhandle to eastern KS. Another surface low will break
off this eastern end and track along the existing stationary front
as it crosses MO tonight, the Midwest on Friday, and translates to
a coastal low south of Long Island Friday night.

A strengthening low level jet from the S/SW with will transport
anomalously high moisture atop this boundary. The combination of
isentropic ascent and intensifying frontogenesis along the
temperature gradient will continue to drive heavy snow in a narrow
band from southern IA across the MO border and into northern IL
where the probability for 6 or more inches of snow tonight is
moderately high.

The strengthening westerly jet lifting from the Mid-south to the
southern Midwest late tonight through Friday will speed the system
and allow it to become neutrally tilted by the time the mid-level
trough reaches the northern Mid-Atlantic coast Saturday afternoon.
This makes for slightly lower snow threats along the swath (best
chances for 6 inches are moderately low over northern
IN/northwestern OH) until the higher terrain of the Alleghenies in
northern PA/into NY state is reached Friday afternoon. Here, the
combination of strong southerly flow, topographic lift, and
frontogenesis ahead of the 850mb low center will allow banding of
heavier snow at these higher elevations. Day 1.5 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.

The accelerating low develops as it reaches the northern
Mid-Atlantic coast Friday night and track south of Long Island
into Saturday with a continued threat for frontogenetical heavy
precip across southern New England. However, thermal profiles are
rather marginal as of now, so the main threat areas for Day 2 are
low probabilities for 4 or more inches in the Berkshires and
southern Green Mtns as well as and the Worcester Hills of
north-central MA into the Monadnocks of southwest NH.


...Northern Rockies and Immediate High Plains...

Days 2-3...
The next trough rounding a deep vortex anchored north of Hudson
Bay 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, descending
to 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 for
Day 2.5.


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

Jackson