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

Valid 00Z Mon Dec 31 2018 - 00Z Thu Jan 03 2019


...Southern Rockies into the Southwest...
Days 1-3...

Upper trough will dig southward through the Great Basin and then
close off near the Four Corners before very slowly drifting east
through the end of the forecast period. This occurs at the same
time a cold front drops south through the Plains and towards the
mountains of CO/NM, with winds behind this feature turning E/NE, a
favorable upslope direction. The combination of upslope flow and
slowly moving robust synoptic ascent through the favorable
diffluent region of an upper jet and height falls will produce a
long duration of snow from Colorado and Utah, southward into
Arizona and New Mexico. The heaviest accumulations are likely in
the terrain of Southern Colorado, much of New Mexico, and into
southeast Arizona where WPC probabilities are high for 12 inches.
In Arizona, up to 2 feet of snow is possible in the Gila and White
mountains where heavier snow rates are possible as low-level
frontogenesis sinks southward, and the duration of synoptic ascent
to produce snow persists. Elsewhere above 4000 feet WPC
probabilities are high for 4 inches. By day 3 the upper low will
begin to eject eastward so only light additional snows are likely
in southern NM/AZ.


...Central Rockies through the Northern Plains...
Day 1...

A broad swath of heavy snow is likely from MT/WY eastward towards
the western Great Lakes tonight through Monday. In the terrain of
MT/WY, easterly winds behind a cold front and around a sinking
high pressure will drive upslope flow, which will work in tandem
with increasing synoptic ascent beneath a jet streak to produce
snowfall. High column moisture will generally sink south quickly
during Day 1 as noted by decreasing 1000-500mb RH fields, but
strong enough lift should produce a period of moderate to heavy
snow into early Monday and WPC probabilities are high for 4 inches
of accumulation, with 8 inches possible especially in the Big
Horns and Black Hills.

Further east, a jet streak ejecting to the northeast will produce
a stripe of vertical ascent through diffluence from SD
northeastward towards the U.P. of Michigan. Guidance has come into
good agreement that QPF will be significant, especially from MN
eastward, and snowfall forecasts reflect this upward trend. The
strong lift through diffluence within a coupled jet structure will
combine with low level confluence and subtle 700mb deformation to
produce heavy snowfall. The highest accumulations are likely from
central MN into the western U.P. where WPC probabilities are above
70% for 4 inches of snow, and more than 6 inches is possible.
Further west, 4 inches or less is most likely.


...Great Lakes and Northern New England...
Day 2...

Northern and southern stream energy expected to phase allowing a
surface low pressure to strengthen and lift northeast through the
Great Lakes and along the Canada/US border into Maine. Two areas
of heavy snow are likely with this setup.

In the Great Lakes, as precipitation spreads northward, it will
encounter colder air with lift being driven by WAA and enhanced by
a weak TROWAL as the WCB rotates cyclonically into the cold air
across WI/MI. At the same time, a band of mid-level deformation
collocated with 925-850mb frontogenesis will enhance snowfall
rates where forcing aligns with a saturated DGZ and weakly
negative EPV. These parameters combined with a noted increase in
forecast QPF/snowfall from the models suggests that intense
snowfall rates will overcome the slowly northward advecting warm
nose, and a brief period of heavy snow is likely NW of the surface
low. WPC probabilities have increased for 4 inches and show a high
risk from eastern WI through the L.P. of MI, with even a slight
risk for 8 inches now existing in the far northern U.P. where the
best forcing and cold profile combine.

Further east into New England, robust WAA on 850mb winds rising to
50 kts or more will drive ascent, into a column which is initially
cold enough for all snow. Although the cold surface high is in an
unfavorable position to lock in cold air and will retreat, strong
UVVs suggest intense snowfall rates, especially in the terrain
from the Adirondacks eastward into Maine. The cold air will likely
erode more slowly than progged due to precipitation reinforcing
the high, but with the 700mb low tracking near the Canadian
border, all locations with the exception of northern Maine are
expected to turn over to a mix and eventually rain. Still, heavy
snow accumulations are likely and WPC probabilities are moderate
for 8 inches in northern Maine. Elsewhere a slight risk exists for
4 inches from the northern Adirondacks eastward through the
terrain of VT and NH.

Before precipitation transitions to rain, a period of freezing
rain is likely in Upstate NY, VT, and NH. Accretion is expected to
be mostly less than 0.1 inches, but some low probabilities exist
for 0.25 inches as well.


...Arklatex...
Day 3...

Low confidence forecast by day 3, but a closed upper low moving
out of the Four Corners will combine with upper jet energy to
produce ascent through mid and upper level diffluence. Downstream
ridging from this feature will drive increasing warm southerly
flow from the Gulf of Mexico, and precipitation will overspread
east Texas and into the Arklatex. Guidance varies considerably in
precipitation placement and timing, but cold surface temperatures
in the vicinity of large high pressure to the north supports a
threat for freezing rain on day 3, and low WPC probabilities
one-quarter inch of accretion have been introduced.


Weiss