Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
350 AM EST Mon Jan 27 2020

Valid 12Z Mon Jan 27 2020 - 12Z Thu Jan 30 2020

...Northwestern CONUS...

Days 1-3...

In a series of progressive shortwaves to impact the Pacific
Northwest, snowfall accumulations are expected to primarily affect
higher elevations of western OR and western WA with the highest
probabilities of heavier snow totals across the WA Cascades due to
relatively high snow levels elsewhere.

The highest amounts through tonight are forecast to be in the WA
Cascades due to the longer duration of overlapping lift/moisture
as the upper level jet maxima and low level warm advection is
expected to produce another round of locally heavy snow. A break
looks to develop on Day 2 in the WA/OR Cascades as the upper
trough moves onshore and inland, with drying aloft developing in
its wake.
On Day 3, the mid mid level trough approaches with another band of
enhanced moisture and 700 mb lift moving onshore and across WA/OR
to produce snow at higher elevations. Several more inches are
likely in the WA Cascades.

Across portions of central/northern ID and northwest MT, several
inches are snow are expected as the repeating shortwaves track
through the central/northern Rockies/the Blue Mountains of
northeast OR. While no particular system is likely to produce
widespread heavy snowfall, the repeated doses over the next 3 days
will produce considerable accumulations for the highest elevations
of ID/western MT. 
The 3 day totals show the potential for a foot to 18 inches across
the ranges of northeast OR to ID, northwest WY, and northwest MT.

Lighter snowfall totals will spread southeast across areas with
upslope flow in CO/northern NM. Snow should taper Tuesday morning
as the upper trough departs and moves east onto the Plains.

...Southern/South Central Plains/Southern Appalachians...

Day 2/3...
A compact shortwave trough and associated area of low pressure
drops out of the central Rockies toward the southern/south-central
Plains tonight thru Tuesday. As the wave digs into the OK/TX
Panhandles, colder air surges on the backside of the system and a
changeover from rain to snow is expected across southeast CO and
southwest KS, then the OK/northern TX panhandles.

Recent model runs have trended towards a more amplified and slower
solution on Tue. 
This allows more precip to wrap into the cold sector in the mid
level frontogenesis zone, presenting a threat for several inches
of snow in southwest KS to the adjacent OK/TX panhandles and
northwest OK. 

Confidence is low given the changing forecasts, and different
axis/orientation of QPF and resultant snow that are different
among the NAM/GFS/Canadian/UKMET, and ECMWF.  Uncertainties also
stem from initial conditions being too warm for snow, and the
timing/duration of the change over period of mixed precip types.
The GFS looks to be warmer than the 00z GEFS Mean/NAM/ECMWF so was
given less weight. 

As the system progresses eastward toward the lower MS Valley,
probabilities of frozen or mixed precipitation decrease due to
relatively warmer boundary layer temperatures. 

Day 1 Southern Appalachians...
At the base of the mid level trough crossing the eastern US, light
precip has developing in the TN Valley.  The pocket of enhanced
moisture and lift crosses the Southern Appalachians early today.
Higher elevations in eastern TN to western NC and adjacent
southwest VA should receive snow at higher elevations, with
potential for 3-5 inches at peaks in the region.  The snow tapers
as the 700 mb trough moves towards the coast this evening and
drier air aloft advects across the southern Appalachians.

Day 3 Southern Appalachians...
Light snow is possible as moisture from the wave reaches the
southern Appalachians Day 3. Most models have the 850 mb and sfc
low tracking further south, keeping heavier precip south of the
mountains.

...Lower Great Lakes/New York/northern New England...

Day 1...

Strong cyclonic flow in the wake of the departing system has led
to a period of lake enhanced snows aided by upslope flow
downstream from Lake Erie and Lake Ontario. The 700 mb trough in
place across the lower lakes this morning allows snow showers to
continue early today.  As the day progresses the 700 mb trough
moves east across NY into New England.  In the wake of the trough,
drier air aloft cross the lower Great Lakes. This leads to a
decrease in snow coverage and rates, and therefore amounts.  An
additional 2-4 inches is possible in southwest NY to adjacent
northwest PA.

The probability of 0.25 inches of freezing rain is less than 10
percent Days 1-3.

Petersen