Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
339 PM EST Tue Dec 25 2018

Valid 00Z Wed Dec 26 2018 - 00Z Sat Dec 29 2018


...Western U.S....
Days 1-3...

Two potent shortwaves will dig into the West Coast before closing
off near the Four Corners. The first of these will dig into
California tonight and then tilt negatively and close off near the
TX panhandle before lifting off into the Central Plains. This will
quickly be replaced by a second shortwave following a very similar
track, and then closing off near the Four Corners area late on day
3, Friday.

The leading system will produce heavy snow to the mountains of
Utah, Colorado, Arizona, and New Mexico, with the heaviest snows
likely in the San Juans above 8000 feet where WPC probabilities
show a high risk for 8 inches of snow. Elsewhere, WPC
probabilities are moderate for 4 inches, with accumulating snow
likely above 5000 feet across most of the area. Well south near
the Mexico border, small accumulations of snow are likely as well,
but confined to the highest terrain above 7000 feet.

The second impulse will move into the Pacific Northwest late on
Day 1 and then drop southward through day 2 into day 3, closing
off late in the forecast period. This impulse initially moves
quickly, but is accompanied by ample moisture due to an
accompanying Pacific Jet maximum. This will provide ascent, which
combined with upslope enhancement will produce heavy snow in the
Cascades of Washington and Oregon, with moderate accumulations
elsewhere across the terrain as far south as Arizona. On day 3,
more significant snowfall is probable into New Mexico, and WPC
probabilities are high for 4 inches above 7000 feet, with the
maximum accumulations likely in the Sangre De Cristo Range.


...Central U.S....
Days 1-3...

A significant winter storm remains likely from eastern Colorado,
northward through the Dakotas, and eastward into the Western Great
Lakes.

The deep upper trough traversing the Southwest is expected to lift
out into the Plains, with a closed low developing over the central
Plains Wednesday night into Thursday morning.  Precipitation will
spread well out ahead of the low.  Driven by mid level energy and
low level warm air advection, precipitation is expected to spread
northeast from the central Plains into the upper Midwest on
Wednesday, beginning as a period of snow across portions of the
upper Midwest.  Meanwhile, northern stream energy emerging from
the northern Rockies will support snows further to the west,
producing generally light accumulations across the Dakotas, and
WPC probabilities for 4 inches through day 1 are only above 50
percent in ND and SD.

By Wednesday night, as the upper low begins to close off and
deepen, rain changing to snow can be expected in the associated
deformation zone and intensifying mid-level TROWAL setting up from
the High Plains of eastern Colorado northeastward across central
Nebraska into eastern South Dakota.  Widespread moderate to heavy
snow accumulations can be expected across this region, with WPC
probabilities indicating a widespread slight risk for 8 inches of
snowfall. In a stripe from central Nebraska through eastern South
Dakota and into Minnesota, the heaviest accumulations will likely
exceed 12 inches and may approach 18 inches in a few locations
where the best frontogenesis pivots overhead producing intense
snow rates. A secondary maximum is possible along the NW shore of
Lake Superior where 12 inches of snow is possible due to upslope
enhancement into the Iron Ranges of the Arrowhead of MN.

As the upper low begins to shear out to the northeast and the
surface low follows, the 700mb trough will open and lag back to
the SW. This will maintain weak to moderate forcing in a slowly
drying environment, and a few inches of additional snowfall are
possible Friday from eastern SD into the U.P. of Michigan.


...New England...
Day 3...

Warm advection well ahead of the surface low moving into the Great
Lakes will spawn precipitation across New England on Friday.
Initially, the column will be cold enough for all snow from the
Adirondacks of NY through Northern New England. However, warming
at 850mb on W/SW flow will produce a warm nose sufficient to
change precip over to sleet, then freezing rain, and eventually
rain. Guidance is likely too fast to warm the column as the LLJ
weakens upwind of the retreating high pressure, but still expect
p-type to transition to freezing rain, and eventually rain, across
most of the area by the end of day 3. Enough forcing within the
cold layer will produce a few inches of snow before p-type
transition, and WPC probabilities show a slight risk for 4 inches
of snow in the high terrain of NH and Maine. Freezing rain
accretions of up to one-tenth of an inch are also likely before
the changeover to rain, but WPC probabilities for 0.25 inches
remain below 30 percent.


Weiss