Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
318 AM EST Fri Jan 24 2020

Valid 12Z Fri Jan 24 2020 - 12Z Mon Jan 27 2020

...Pacific Northwest...
Days 1-3...

Satellite images indicate a plume of enhanced moisture is
streaming onshore tonight from the eastern Pacific into the
Pacific northwest.  Valley rain and mountain snow have developed
and should continue as the upper wave progresses inland out of the
Pacific northwest and across the northern Rockies.  The approach
of an upper ridge signals a lull in the action tonight. The day 1
snow indicates potential for higher amounts in the northern WA
cascades.  otherwise, the progressive wave leads to relatively
short duration snows and modest amounts across the northern
Rockies.

As the next wave amplifies off the coast, the building upper ridge
Sat leads to rising snow levels in northern CA into southwest OR. 
The primary areas of concerns would be further north in the Blue
Mountains and also northern WA Cascades, where an additional
several inches of snow is possible.

The upper wave is forecast to move onshore Day 3, with cooling
aloft lowering snow elevations and expending coverage of snow.
Several inches are expected with the wave passage across
the ranges of ID to adjacent northeast WA and northwest MT.

The 00z ECMWF indicates a secondary jet maxima moving onshore in
OR late Sunday and continuing north into WA by Mon morning.  The
lift from this jet max produces several inches of snow in the
WA/OR/CA Cascades.

...Midwest/Great Lakes/Mid-Atlantic/Northeast...
Days 1-2...

The closed low-mid level circulation and associated sfc low over
southwest MO will drift northeast to Lake MI or nearby lower MI
through Sat morning. Snow in the mid level deformation zone over
northern MO and western IL should continue and drift north across
northern IL into WI.
Several inches are expected in northern IL and adjacent southern
WI. The high res windows and NAM/NAM Conus Nest show this
potential.  The warmer 00z GFS was given less weight, given the
continuity of snow already occurring, plus more snow in the 00z
GEFS Mean solution than the operational run.  A secondary max is
expected in the central UP of MI due to lake enhanced snow is
expected over Lake Superior on an easterly wind.

A secondary low at the end of the occlusion will develop over the
southern Mid-Atlantic in the lee of the Appalachians late Friday
and take an inland track up the Eastern Seaboard through Sunday.
The inland track leads to strong low-mid level warm advection
across eastern NY and New England.  This is turn leads to a rapid
change over from snow to sleet and freezing rain.

With the warm air aloft overrunning the low-level cold air,
freezing rain is likely, WPC probabilities are moderate for a
tenth inch of ice from the Allegheny Plateau in WV/western MD into
PA, across portions of northern PA/the Catskills and Adirondacks
of eastern NY and the Berkshires/Green Mtns/White Mountains/ranges
of western Maine.
The event comes to an end as the low departs interior Maine on day
3.

Day 3...
In the lee of the Great Lakes, cyclonic flow brings an expected
period of westerly boundary layer flow across both Lake Erie and
Lake Ontario.  This results in lee shore convergence and lift that
is aided by topography east of Lake Ontario in the Tug Hill.
Where moisture and loft coincide, several inches of snow is
expected in the lee of both Lakes Erie in southwest New York and
Lake Ontario in upstate New York.

The probability of 0.25 inches of freezing rain is less than 10
percent on Day 1 and Day 3.

Petersen