Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
347 PM EST Sun Jan 20 2019

Valid 00Z Mon Jan 21 2019 - 00Z Thu Jan 24 2019


...Western U.S....

Heavy snows are expected to focus on the CA Sierra Nevada as the
upper jet max streams onshore tonight with the ECMWF indicating
110-130 kt, supporting upper divergence maxima  that combines with
low level convergence and upslope flow to produce sustained snow
tonight.  The upper trough moves inland Monday, with favored left
exit jet dynamics providing lift to the ranges of northern Nevada,
and then Utah and the ranges of western WY. Persistent upper
divergence maxim in the mountains of northern UT should lead to
locally heavy amounts with potential for a foot of snow in the
northern Wasatch and Uintas. The activity in UT winds down Mon
night as the upper trough progresses in CO, with several inches
likely in the mountains starting in the western CO ranges and then
progressing to the front range.  The activity winds down in the CO
ranges Tue as the upper trough moves east on to the Plains.

On Day 3/Tue night-Wed, the models show confluent low-mid level
flow producing a 700 mb jet max streaming onshore from the
northeast Pacific across WA state into ID and then northwest WY. 
The resulting warm/moisture advection and low level
convergence/upper divergence combine with upslope flow in windward
terrain to produce locally heavy snow in the northern WA Cascades
and ranges of ID. 

...Northern and Central Plains, Upper Mississippi valley and Upper
Great Lakes...

On Monday afternoon and evening, a mid-upper level trough crosses
the Northern Plains and departs the CO Rockies across the central
Plains. The forming sfc low on the high Plains of CO helps focus
low level convergence with accumulating snow on the CO high Plains
to the adjacent high plains of Nebraska and southeast WY.
The northern stream portion of the trough produces an area of
warm/moisture advection and 700 mb convergence, leading to
accumulating snows across the upper MS Valley and Upper Lakes
Tuesday. A secondary band of 700 mb pre-frontal convergence
supports snows developing in northeast Nebraska across northern
Iowa, with precip uncertainty in southeast NE to southern IA.

The low in the mid MS Valley is forecast to cross the Great Lakes
Tue nigh to Wed morning, with timing differences developing.  The
low passage across MI focuses precip and snow across the state,
with several inches possible if the slow cyclone timing of the
ECMWF and/or NAM are more correct than the faster GFS.

...Lower Great Lakes and Northeast...

Snows are expected to continue into early tonight with a 700 mb
low slowly approaching and arriving in northern Maine, preceded by
700 mb convergence and deformation, which supports several inches
of additional snow. Synoptically driven snows are expected to
diminish as the low lifts into Atlantic Canada tonight.

Tonight into Monday morning, strong northerly to northwesterly
flow on the backside of the system across the Great Lakes will
support lake effect snows.  These may produce several inches of
snow on the south shore of Lake Ontario.

After a respite Tuesday, 850-700 mb warm/moist advection commences
across the Great Lakes into the Tug Hill and western Adirondacks,
so show showers should develop Tue night, with light accumulations
expected.

The probability of a quarter inch of freezing rain is less than 10
percent Days 1-3.

Petersen