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

Valid 00Z Fri Dec 28 2018 - 00Z Mon Dec 31 2018


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

Strong surface low pressure will move from the upper MS valley
into southeast Canada tonight and Friday. Northwest of this
surface feature, moderate to heavy snow will persist within the
comma head/deformation zone which is likely to push slowly
eastward through Day 1. A band of heavier snow is expected within
this area where the best mid-level frontogenesis occurs in the
vicinity of cooling temperatures along the 700mb trough axis.
Although drying will occur from the NW as the surface low fills
and the mid-level trough ejects to the east, ascent in an airmass
characterized by high 1000-500mb RH will produce several inches of
snow from central MN through the U.P. of Michigan. The highest WPC
probabilities for 4 inches or more are along the north shore of
Lake Superior in the Arrowhead of MN where upslope enhancement due
to onshore boundary layer flow will exist. Elsewhere,
accumulations should be primarily less than 4 inches.


...Southwest and Southern Rockies...
Days 1-2...

...Major Winter Storm will produce near blizzard conditions across
much of Central and Eastern New Mexico...

Mid-level trough will sink southward into the Four Corners tonight
and then drift slowly south/southwest into Sunday. As this occurs,
a strong polar jet of up to 130kts will shift around the base of
this trough and place much of NM into the favorable left exit
diffluent region, which combined with strong moist advection on SW
700mb winds, will produce widespread heavy snow from far southern
Colorado through much of New Mexico and potentially into the
western Panhandle of Texas. The heaviest snow is likely across
central and eastern NM which will be within the favorable LFQ of
the jet the longest, and also experience the best low-level
frontogenesis and upslope enhancement on low level easterly winds
as a cold front drops south.

Guidance has shifted the heaviest QPF subtly NW today, and this
has prompted an increase in WPC probabilities across much of the
area. The heaviest snowfall is likely in the high terrain of the
southern Sangre De Cristos, the Black Mountains, the ranges near
Albuquerque, and the nearby highland. Above 6000 feet, WPC
probabilities are high for 8 inches, with 1-2 feet possible. At
lower elevations, WPC probabilities are high for 4 inches across
nearly all of New Mexico, with probabilities lowering and sinking
southwestward into day 2.

Further east, jet level diffluence will increase as the polar jet
exits northeastward Friday night into Saturday. This will occur in
conjunction with the best mid-level deformation as the 700mb
trough stretches to the southwest. High 1000-500mb RH is of short
duration however, so while a band of heavy snowfall is possible
stretching NE into the Panhandles of TX and OK, WPC probabilities
remain low for 4 inches of snow.


...Northern New England...
Day 1...

Warm advection will produce precipitation across the region
tonight into Friday. Initially, the presence of cold high pressure
will keep the column cool enough for snow at the onset in the high
terrain of 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. Before this occurs, strong enough ascent has the
potential to produce a few inches of snowfall from the Northeast
Kingdom of VT through much of Maine. The highest snow
accumulations are likely in the mountains of Maine as the cold air
hangs on longer, and WPC probabilities are 30-50 percent for 4
inches. Elsewhere, accumulations should be just 1-2 inches.

After the snow, a period of sleet and freezing rain is likely from
the Green Mountains of VT, the White Mountains of northern NH, and
 into the ranges of southwest ME. Areal amounts of freezing rain
accumulations will be on the order of a tenth of an inch with very
low probabilities for up to 0.25 inches.


...Pacific Northwest...
Days 2-3...

Moisture pushing onshore atop a mid-level ridge shifting southward
will produce snowfall into the mountain of the northwest. The most
significant snow is likely Saturday night into Sunday /Day 3/ as a
shortwave trough and associated Pacific Jet energy drive a cold
front onshore. The combination of enhanced moisture, synoptic
ascent due to height falls and jet level diffluence, as well as
upslope enhancement on westerly 700mb winds will produce snow into
the mountains from the OR/WA Cascades eastward into the ranges of
ID/western MT/NW WY. As the cold front surges eastward, snow
levels will fall by the end of the forecast period to below 2000
feet, but the heaviest snowfall will remain above 6000 feet where
WPC probabilities are moderate for 8 inches of accumulation.


Weiss