Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
339 AM EST Sat Dec 29 2018

Valid 12Z Sat Dec 29 2018 - 12Z Tue Jan 01 2019

...Day 1...
Pacific Northwest/Northern Rockies...

Shortwave dropping through the Pacific Northwest from Day 1 will
feature a well defined mid-upper jet maximum moving across WA 
this evening and extending inland and then moving south into
northern Or and southern ID by 12z Sun. Upper divergence maxima
near the jet core combine with mid level moisture advection and
orographic lift in windward locations
to produce locally heavy snowfall from the Washington Cascades
through the ranges of northern ID and northwest MT. WPC
probabilities are high for 8 inches in these areas.

...Day 2...
...Northern to central Rockies across the central high plains and
northern Plains...

A northern stream shortwave will dig across WY and CO  and then
the Northern Plains, producing a band of low-mid level convergence
along the path of the wave.  The lack of  along duration ascent
limits heavy snow potential, but a wide swath of the northern to
central Rockies and portions of the adjacent high plains has a
risk of receiving several inches of snow. The upper jet maxima
crossing the Dakotas induces coupled upper level divergence and
low level convergence, so areas along the wave path could also
receive several inches of snow, including the Black Hills of SD.
The phasing among different models varies, with differences from
run to run, producing variance in snow amounts in ND/SD. The
NAM/ECMWF produce snow further north than the GFS/UKMET.

...Day 3...
...Northern Plains/Upper MS Valley/upper Great Lakes/northern New
York and northern New England...

An area of snow is expected to depart the northern Plains and move
northeast across the upper MS Valley and upper Lakes. Typical
differences in snow fall amounts and location exist due to the
uncertainty of phasing northern and southern stream systems.
The increasing 700 mb jet produces well defined theta-e advection
and convergence across the upper MS Valley, upper lakes,
downstream into northern New York and New England, supporting
widespread precipitation.  A band of deformation and mid-level
frontogenesis will likely produce a swath of snowfall where it
remains cold enough for snow.
A precip type transition zone of multiple types is possible south
of the band of snow
from southern WI and southern lower MI to interior New York and
New England.

...Southern CO/NM/AZ...
The models indicate an amplifying mid-upper level trough crossing
UT into AZ, bringing an upper level jet maximum south across AZ
and NM. Lowering heights/temperatures will bring lowering snow
levels with snow expected across most mountain ranges of the
Central and Southern Rockies, extending into other ranges in
Arizona and New Mexico.
Coupled maxima of 300 mb divergence/700 mb convergence combine
with deep layer relative humidity of 90 percent and 700 mb ascent
to produce widespread precip with potential for another round of
heavy mountain snows in favored upslope areas of eastern AZ,
southern CO, and New Mexico.

The probability for significant freezing rain (0.25 inches or
more) is less than 10 percent.

Petersen