Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
245 PM EST Sat Dec 01 2018

Valid 00Z Sun Dec 02 2018 - 00Z Wed Dec 05 2018

...Northern to Central Plains into the Western Great Lakes...
Day 1...

Deep surface cyclone across the Plains will occlude while lifting
northeastward into the Great Lakes during Sunday. As this occurs,
frontogenetical forcing will begin to weaken, but potent mid-level
deformation will persist, especially northwest and west of the
700mb low as a trough extends well back into Montana. Despite
subtle weakening in overall forcing, height falls, mid-level
stretching within the deformation axis, and moist advection will
continue moderate snow in a stripe from eastern WY, through NE,
and northeast into the U.P. of MI. Frontogenesis may begin to
increase once again, especially across WI into MI as the mid-level
low pulls away and colder air advects southward, and this will
combine with the axis of deformation to potentially enhance
snowfall rates in an otherwise modestly forced environment. WPC
probabilities for 4 inches are highest in WI into the U.P. of
Michigan, where some Lake Effect enhancement off Lake Superior is
also possible.

Further west into IA/NE, enhanced ascent within spokes of PV
anomalies rotating around the closed mid-level low will produce
periods of more intense snowfall. The duration of any of these
should be limited, and with low-level temperatures near freezing,
SLR's are expected to be modest. There is a signal for additional
moderate accumulations in this area, and WPC probabilities show a
high risk for 4 inches, much of which is likely overnight and
early on Sunday.


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

A potent mid-level trough and associated upper Pacific jet maximum
will drop into the California coast this evening and then continue
southeast towards the Four Corners region by Monday. This feature
will advect enhanced moistured noted by an increase in PWAT as
well as 1000-500mb RH across much of the west, with height falls
and jet level diffluence combining with 700mb W/NW flow to produce
ascent. Snow levels will fall to as low as 2000 feet, so
accumulating snow is likely across much of the states of OR, ID,
NV, and UT, with accumulating snow also likely in SE CO, and
northern AZ/NM. The heaviest snow on day 1 is expected from the
ranges of Idaho, southeast through Utah and into northern New
Mexico where the longest duration of synoptic ascent is expected.
In these ranges, WPC probabilities show a high risk for 4 inches
of snow, with a moderate risk for 8 inches, especially above 7000
feet. Lesser amounts are likely between 2000-7000 feet. As the
moisture spills further south and east on day 2, lighter snows are
possible in the high terrain from WY southward into NM.

Late on day 3, Monday night into Tuesday, a deep low pressure is
progged to approach the California coast spreading moisture
onshore. Initially, temperatures through the column are too warm
to support snow due to shortwave ridging out ahead of this
feature. However, enough forcing and moisture is likely to reach
the high and cold terrain of the California mountains late in the
forecast period, and WPC probabilities show a slight risk for 4
inches of snow in the northern Sierra, the Trinities and Siskiyous
of California, as well as the southern Cascades of Oregon.


...Upstate New York and Northern New England...
Day 1...

Surface low pressure beneath a closed mid-level circulation will
lift slowly northeast from the Central Plains into the Great Lakes
while occluding to a secondary surface low north of New England by
Sunday night. Deep SW flow ahead of this feature will flood into
New England, and this will overrun a slowly retreating surface
high pressure. The combination of warm advection and enhanced
isentropic lift will cause precipitation to overspread the region
Sunday morning.

Initially, the entire column is below freezing across much of
central and northern New England. However, the warm advection will
drive a robust warm nose northward changing the snow over to
sleet/freezing rain quickly except in the Adirondacks of NY, as
well as the Whites of NH and into ME. Although 850mb temps will
climb above 0C in these areas as well, it will take longer,
especially in Maine, so a few inches of snow is likely before the
p-type transition occurs. The highest amounts are expected in
Maine, where WPC probabilities are moderate for 4 inches of snow.

As the warm air floods overhead, surface temperatures will be slow
to warm as the wedge of high pressure retreats. This suggests a
period of freezing rain is likely, especially in the terrain of
upstate NY and northern New England. The setup does not support a
prolonged duration of freezing rain, but there remains a
multi-model signal for around 0.1 inches of accretion, and WPC
probabilities have increased to 30-40 percent for up to 0.25
inches in the high terrain.

Days 2-3...
The approach of the low-mid level front and arrival of colder air
provides a window of opportunity of light snow. The cold front
will be progressive and moisture is somewhat limited, but a few
inches of snow are possible where W/NW winds enhance snow
potential due to upslope in the mountains of northern New England,
as well as downwind of the Great Lakes, especially east of Ontario
and Erie.

Weiss