Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
333 PM EST Sun Dec 20 2020

Valid 00Z Mon Dec 21 2020 - 00Z Thu Dec 24 2020


Days 1-3...
...Pacific Northwest to the Northern and Central Rockies...

An ongoing atmospheric river event is expected to continue on
Sunday from the Pacific Northwest into the northern Rockies, with
additional locally heavy precipitation likely.
A frontal wave develops over the northeast Pacific and moves
towards northern WA State Monday, northern Idaho Monday night,
followed by Montana on Tuesday.

The approaching low drives the warm front north across OR into WA,
with the heaviest snow on Day 1 north of the front in the higher
peaks of the WA Olympics/northern Cascades and the northern
Rockies, from the northern Idaho and western Montana ranges to
western Wyoming. 

Snow amounts focus where the wave crosses northern ID and
northwest MT.  Several additional inches are expected in the Lewis
Range in northwestern Montana.  Post frontal steepening lapse
rates and low level ascent in windward facing terrain have the
potential to bring several inches of snow to the WA Cascades and
the OR Cascades.  Drying aloft the latter half of Day 2 reduces
snow coverage and intensity.

Models continue to show a well-defined wave with a strong trailing
cold front, driven by an amplifying upper trough, is expected to
plunge southeast across the central Rockies on Tuesday.  Post
frontal moist northwest flow with pooled moisture and low level
ascent crosses the ranges of eastern Wyoming and Colorado Tue
night into Wed., producing a period of snow.  Several inches of
snow are expected in favored upslope areas. The progressive nature
of the system will help limit the potential for widespread heavy
accumulations. The models have a lot of spread as to where post
frontal lift
produces snow in the central high Plains, with confidence low on
where a focused area of accumulated snow may develop.  The GFS is
an outlier, with the UKMET/ECMWF/NAM/Canadian clustering better
with the magnitude of the trough.


...Upper MS Valley to the Central Appalachians...

The models show a progressive clipper system with low pressure
crossing MN tonight leading to low-mid level frontogenesis north
of the low in northern MN. Low level moisture convergence in
northern MN under the frontogenesis max leads to a period of snow.
 An area of 2-4 inches of snow is expected before the system
departs into the Great Lakes on Mon.

As the system crosses the upper Oh Valley, a trailing cold front
crosses the central Appalachians.
Post frontal moist west-northwest flow comes into the ridges of
the central Appalachians from southeast PA across West Virginia. 
A sustained period of lift in windward terrain in the mountains of
West Virginia should lead to several inches of snow.

The event winds down on Tuesday as a low level ridge approaches
from the Ohio Valley with drying aloft reducing available moisture
and lift.  The 12z ECMWF/UKMET/NAM/GFS/Canadian are similar in QPF
amounts and resultant snow totals.

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

Petersen