Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
343 AM EST Sun Dec 30 2018

Valid 12Z Sun Dec 30 2018 - 12Z Wed Jan 02 2019


...Northern to Central Rockies Day 1...

The models show moist confluent mid level flow with embedded mid
level vertical velocity maxima crossing the ranges of western and
central MT, central ID, and northeast OR early today, then
continuing downstream across northern WY. These vertical velocity
maxima occur in conjunction with favorable upper level divergence
maxima as the upper trough amplifies as it moves from the northern
Rockies, with the upper level circulation near northwest WY by 12z
Mon. Several inches of snow are likely in the ranges of these
areas, WPC probabilities are high for 4 inches of snow, and low to
moderate for 8 inches of snow.

...Southern Rockies and Southwest Days 2/3...
...Heavy Snow possible in the ranges of southern CO, eastern AZ,
and NM...

An amplifying upper trough building into southwest from the Great
Basin drives a jet out of AZ across NM. The heaviest snow is
likely once again across the ranges of southern CO and across New
Mexico and eastern Arizona. Here, the best combination of jet
level diffluence, low-level convergence and moist advection, and
upslope enhancement will combine to produce heavy snowfall from
the ranges of southern Colorado through much of New Mexico and
eastern Arizona. The heaviest snowfall is likely in the southern
Sangre De Cristos, as well as the mountains in western New
Mexico/SE Arizona, where the most persistent 300 mb jet core is
likely, with associated upper divergence maxima
combining with upslope flow to produce snow. The higher WPC
probabilities shift south from southern CO (San Juans and Sangre
de Cristo mountains) and northern NM/eastern AZ day 2 to southern
Arizona/central and southern NM on day 3 (Tue). Timing of the
upper trough leads to QPF differences and resultant snow amounts
differences, with the UKMET standing out as slower than other
models and keeping the precip/snow axis further west in AZ.

...Central and Northern Plains/Upper Mississippi Valley Day 1...

 A northern stream shortwave will eject eastward across the
Northern Plains, preceded by
a swath of light to moderate snow where the best upper divergence,
height falls, and low level convergence along a low level
frontogenetic zone combine. Due to limited moisture, total snow
accumulations are likely to be modest across the Dakotas and
Minnesota. A low risk for 4 inches is indicated in the
probabilities.

....Great Lakes and Interior New York/England...Day 2 into early
Day 3...

A southern stream shortwave will move northeast, with a surface
wave progressing across the eastern Great Lakes and into New York
and New England Monday night-Tuesday morning. Robust ascent due to
warm advection and lift beneath a jet maximum will produce
widespread precipitation.  Low-mid level deformation and
frontogenesis aid on producing lift where it is cold enough for
snow across parts of WI and MI and then downstream into northern
New York and northern New England.  A maximum is expected in
southern ME, where a low level frontogenesis and convergence
maxima occurs along the path of the wave. Further south, precip
type uncertainty in northern IL, southern lower MI and southern
New York/southern New England leads to rapidly dropping
probabilities for snow.

There remains considerable spread in the guidance into how much
phasing of the northern and southern streams will occur, with the
ECMWF/NAM producing more snow in WI/MI than much of the other
guidance.The 00z NAM Conus Nest shows less snow in WI than the
parent NAM. The northern stream wave produces day 2 snow from
eastern MN across northern WI and the UP of MI as the synoptic
lift is followed by lake enhancements in the proximity of Lake
Superior.

On Tue morning, the triple point low develops near the ME coast
and moves offshore, leading to the snow and mixed precip in Maine
ending as the peak ascent moves off the coast with the low
pressure center.

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

Petersen