Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
337 AM EST Mon Dec 31 2018

Valid 12Z Mon Dec 31 2018 - 12Z Thu Jan 03 2019


...Southern Rockies across the Southwest...
Days 1-2..
...Heavy snow likely across the ranges of southern Colorado,
eastern Arizona, and New Mexico...

An Upper trough will dig southward across Utah and AZ with the
upper jet maxima crossing from AZ across NM and southeast CO.
Upper divergence maxima, especially in the left exit jet region,
combine with upslope flow in windward terrain to produce a long
duration of snow from southern Colorado and Utah, southward across
Arizona and New Mexico.

The heaviest accumulations are likely in the San Juans and Sangre
de Cristo Mountains of southern CO and northern NM, mountains in
the Santa Fe National forest of NM to the Zuni Mountains, and Gila
and White Mountains of southeast Arizona.

700 mb vertical motions subside in the afternoon Tue in the ranges
of AZ and then Tue night in the ranges of NM. By day 3 the upper
low will begin to eject eastward so the combination of higher
layer relative humidity and ascent tracks east northeast from NM
across the Tx panhandle into parts of western to central OK.


...Southern Plains to Arklatex...
Day 3...

A closed upper low moving out of the southwest and across the
southern high Plains
induces an upper jet maxima moving across Texas and OK into AR and
MO.
The southerly component of the mid and upper level flow will drive
increasing warm southerly flow from the Gulf of Mexico, while
surface high pressure across the central Plains reinforces low
level cold air, resulting in a mixed precipitation event.

Upper level divergence maxima along and adjacent to the upper jet
maxima induces ascent across central to east Texas and Oklahoma
into the Arklatex, continuing up towards southern MO.  Guidance
varies considerably in precipitation placement and timing, but
cold surface temperatures supports a threat for freezing rain on
day 3 in portions of central to northeastern Texas, southeast OK,
western AR, and southwest MO.  Low WPC probabilities of a quarter
inch of ice accretion remain in place, following the NAM and Short
Range Ensemble guidance.


...Northern Plains across the Upper MS Valley and the western UP
of MI...
Day 1...

Areas of snow have developed in the vicinity of a wavy front
stretching across SD to central and northern MN. The upper level
jet streak ejects to the northeast across Lake Superior will
produce a stripe of ascent in northern MN across the western U.P.
of Michigan. Guidance has come into good agreement that the lift
through diffluence in the vicinity of the jet max will combine
with the low level front to produce bands of  snowfall near the
Lake Superior shoreline today.  The highest accumulations are
likely from central MN into the western U.P. of Michigan.

...Great Lakes and Northern New England...
Day 1 to early 2...

Northern and southern stream energy expected to phase with ascent
provided by 700 mb theta-e advection and convergence that moves
northeast through the Great Lakes and along the Canada/US border
into Maine. In the Great Lakes, as precipitation spreads
northward, it will encounter colder air with isentropic lift
resulting in snow in parts of WI across central to northern lower
MI and the adjacent UP of MI.  A band of mid-level deformation
collocated with 925-850mb frontogenesis ads lift today.  These
parameters combined
suggests that intense snowfall rates are possible in northern
lower MI. 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 warm advection and low level
frontogenesis will drive ascent, into a column which is initially
cold enough for all snow.
Strong vertical motions suggest high snowfall rates, especially in
the terrain from the Green Mountains of VT across the White
Mountains of NH eastward into Maine.

Areas of northern NY, VT, NH, and southern ME are expected to turn
over to a mix and in most cases eventually rain.  Before
precipitation transitions to rain, a period of freezing rain is
likely in higher elevations of 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 in the White Mountains of NH.

Northern Me has potential to remain as all snow.  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.

The wave of low pressure and corresponding low level frontogenesis
persists in Maine early Tue before subsiding and snow ending as
low pressure moves off the ME coast.

Petersen