Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
353 AM EST Thu Dec 27 2018

Valid 12Z Thu Dec 27 2018 - 12Z Sun Dec 30 2018


...Central Plains through the Western Great Lakes...
...Day 1...

...Band of moderate to heavy snow and local blizzard conditions
today from the central Plains to the upper MS Valley and far
northwest Great Lakes...

Snow has developed in the vicinity of the mid-upper level low in
NM, extending northeast in the mid level deformation in western
KS.  This low is forecast to track northeast across western KS,
central NE to eastern SD, and northern MN before shearing out to
the northeast into Ontario Friday morning. A band of moderate to
heavy snow is expected along the track of the low on favored mid
level deformation/700 mb convergence, with upper level divergence
aiding in the production of ascent.

Cold advection will rapidly transition any precipitation to
snowfall in the central Plains as the low approaches, and WPC
probabilities now show a moderate to high risk for 8 inches of
snow in a stripe from north central Nebraska across eastern South
Dakota, and west central to northeast Minnesota. The maximum of
snow is also likely NW of Lake Superior in the Arrowhead of MN
where SLRs will be slightly higher, and some upslope enhancement
is expected with the onshore component of boundary layer flow.

Probabilities decline rapidly heading south into southeast MN,
Iowa, and northern WI as temperatures are too warm for all snow,
with warm advection leading to a changeover in most of the these
areas.

...Day 2...
On Friday, cold air advection with northerly to northwesterly flow
is expected to support lake effect snow, which may produce some
additional light amounts across parts of the U.P. of Michigan
through Friday night.

...Southwest and Southern Rockies...
Days 1-3...

With the initial burst of snow in northeast NM in the mid level
deformation and band of enhanced moisture, snow should persist
into the morning in the Sangre De Cristo Mountains of northeast NM
before the mid level low departs and drying aloft leads to a
decrease in snow coverage/intensity.

The break looks to be short-lived as the models have converged on
the development of a 700 mb low in the four corners region Thu
night. Strong diffluence within a coupled jet structure around the
base of the trough combines with strong moist advection, low level
convergence and upslope enhancement to produce the potential for
higher snow amounts along the San Francisco,  Black, and San Juan
Mountains.

The highest QPF and snow amounts and probabilities peak across
central to eastern NM, particularly the Sacramento Mountains. The
700 mb low is forecast to drop south in eastern AZ just west of
the NM border through Friday night and early Sat., and as the low
drops south of the border, snow coverage and intensity should
decrease on Sat.

WPC probabilities on day 2 Fri-early Sat feature a moderate to
high risk for 8 inches of accumulation, and 2-day totals may
exceed 12 inches in the mountains of central to eastern NM.

...Northeast New York across Northern New England...
...Day 1...

Warm advection will produce precipitation across northeast New
York and northern New England today. Initially, the presence of
cold high pressure will keep the column cool enough for snow at
the onset across the Adirondacks of NY and points east through
VT/NH/ME. The strong WAA will quickly cause a p-type transition to
sleet/freezing rain and then rain as the column warms above 0C in
New York and adjacent New England. 

...Day 2...
On Friday, the strong warm advection brings snow into northern
Maine.
Strong enough ascent in the warm advection regime will produce
snowfall which has potential to accumulate several inches in
northern Maine Fri, with WPC probabilities of  30 to 50 percent
for 4 inches of snow.

After the snow, a period of sleet and freezing rain is likely in
the White Mountains of northern NH into the ranges of southwest
ME. Areal amounts of freezing rain accumulations will be on the
order of a tenth of an inch with isolated locations possible
getting up to a quarter inch.

With the models in good agreement on the mid level wave moving off
the coast Fri night, drying aloft reduces the precipitation
coverage and intensity, with no significant additional
accumulations expected on day 3/Sat.

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

Falling heights with the mid level ridge drifting south allows
cooling aloft plus development of moisture advection across the Wa
Cascades inland into the ranges of northeast WA on Fri produces
snow as the enhanced moisture is accompanied by ascent in the
700-500 mb layer. Several inches of snow are possible. The next
burst of snow comes with a cold front progressing east across the
ranges of WA/OR on Saturday, continuing into the ranges of ID and
western MT Sat night/early Sun.  Strong lift leads to potential
for several additional inches of snow from the WA Cascades through
the ranges of northern ID, and then northwest MT.  The potential
for higher amounts is staggered over the WA Cascades, closer to
the Pacific moisture source. Up to a foot of snow is possible in
the WA Cascades and ranges of northern ID.

The probability of a quarter inch of freezing rain is less than 10
percent on day 3 (Sat-Sat night).

Petersen