Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
453 AM EST Mon Dec 12 2022

Valid 12Z Mon Dec 12 2022 - 12Z Thu Dec 15 2022

...Great Basin & Rockies...
Day 1...

Anomalous mid-level low will continue to shift eastward across the
Great Basin, with the core deepening into a closed low over the
Central Plains by Tuesday morning. Height falls and PVA through
spokes of vorticity rotating around the primary gyre will drive
widespread ascent, although the intensity and location will vary
with time. Moisture will continue to be enhanced across the region
as a Pacific jet streak and confluent mid-level flow downstream of
this low will funnel Pacific moisture eastward, providing an
environment favorable for widespread precipitation from the Great
Basin through the Central and Southern Rockies. Snow levels will
gradually lower through the period as the cold pool aloft advects
eastward and modest waves of low pressure drag a cold front across
the region. WPC probabilities are high for 6 or more inches of
snow from the terrain of northern Nevada southward through the
Mogollon Rim and White Mountains of AZ, and into the San Juans, CO
Rockies, and much of UT/WY. The heaviest snow this period is
likely to be in the White Mountains and San Juans where more than
12 inches is possible. Late D1 into D2, the consolidating low will
shift the primary forcing for ascent eastward shutting off
precipitation across much of the area by Tuesday morning.


...Central Appalachians and Mid-Atlantic...
Day 3...

The expansive closed low over the central part of the country will
shed a lobe of vorticity eastward Wednesday, working in tandem
with the LFQ of an approaching upper jet streak and WAA on
low-level SW flow to expand precipitation into the Central
Appalachians and Mid-Atlantic states late Wednesday. Impressive
moist isentropic upglide at 300K will lift northeastward slowly,
overrunning an exceptionally dry airmass as a cold wedge of high
pressure remains anchored down the coast. As precip expands
northward, moistening of the column will allow temps to wetbulb
below freezing, but with 850mb temps already above freezing, this
suggests a period of freezing rain and/or sleet lifting across the
Central Appalachians and as far north as the Laurel Highlands by
Thursday morning. This could result in light accretions of
freezing rain even along the I-95 corridor Thursday morning.
Current WPC probabilities are as high as 50% for 0.1" of accretion
in the higher terrain of eastern WV, western VA/MD, but
uncertainty into exact p-type and timing remains high at this
point.


...Northern & Central High Plains through the Upper Midwest...
Days 1-3...

...Significant winter storm to bring a blizzard to the High Plains
and a possible ice storm to parts of the Upper Midwest...

The large bowling ball closed low ejecting from the Central
Rockies late D1 will consolidate and deepen over the Central
Plains Tuesday, with 700 mb heights falling to -4 sigma below the
climatological mean according to the NAEFS ensemble tables. This
deepening low will combine with phased jet streaks south of the
primary trough curving cyclonically and arcing poleward to place
impressive LFQ diffluence overlapping the most robust height falls
and PVA at 500mb. This will result in surface cyclogenesis in the
lee of the Rockies, and this system will move slowly northeastward
in response to the highly amplified pattern and the low pressure
becoming vertically stacked. East of this low, impressive WAA
emerging meridionally out of the Gulf of Mexico will funnel
impressive PWs northward, with the attendant theta-e ridging
lifting cyclonically within the WCB to be an impressive TROWAL
over the Northern Plains. This gulf moisture will lift efficiently
on the 285K isentropic surface with mixing ratios as high as
5g/kg, suggesting an extremely moist environment favorable for
heavy precipitation. While the antecedent airmass is quite dry,
resulting in a slow saturation of the DGZ, by Tuesday evening all
areas north of the warm nose will be snowing heavily, with a slow
collapse of the dry slot to the southeast occurring as dynamic
forcing cools the column and re-saturates the region. This low
will then move slowly eastward with time, spreading heavy snow
from the High Plains of CO, WY, and MT eastward through the
Northern Plains, and into the Upper Midwest/Western Great Lakes by
Thursday. An axis of heavy freezing rain is also likely southeast
of the heaviest snow, generally across the Coteau des Prairies and
Buffalo Ridge of SD/MN.

For the snow area, the heaviest accumulations are likely well NW
of the low center where an impressive deformation axis is likely
to pivot near far western SD and eastern MT. Forcing within the
deformation axis will likely be enhanced by mid-level fgen
produced by the ageostrophic response to the upper jet streak, and
will occur within a region of theta-e lapse rates <0C/km,
suggesting a strong likelihood for CSI. While models are not
deterministically suggesting any -EPV* for upright convection,
even the slantwise ascent should result in snowfall rates that may
reach 2"/hr at times, and where this band pivots and drifts,
especially Tuesday aftn through Wednesday aftn, the heaviest snow
is expected. With SLRs likely to be quite high, 15:20-1 despite
some potential dendritic fracturing, this will result in snowfall
that will likely widespread exceed 12", shown by WPC probabilities
for this threshold exceeding 50% from the Pine Ridge of NE through
the Black Hills of SD and points just east. Here, locally more
than 18 inches of snow is likely. Otherwise, WPC probabilities for
more than 6 inches of snow are above 50% from the High Plains of
NE CO through much of eastern MT, and eastward through central MN
D2, with additional heavy snowfall spinning into the Arrowhead of
MN where WAA will tap Lake Superior moisture and upslope into the
Iron Ranges. This system is likely to be heavily banded, so
anywhere in the higher probabilities of more than 6 inches could
see local maxima of more than 12 inches through Thursday.

While heavy snowfall is nearly certain, the freezing rain
potential is a little more clouded. The high-res is suggesting
some leading freezing rain D1 before the DGZ can saturate which,
while light, could efficiently accrete in the cold air due to
light precip rates. However, the more significant risk develops
late D1 and D2 as the impressive WAA pushes a warm nose northward
into the eastern Dakotas, IA, and MN. The guidance continues to
feature two camps, with the GFS/NAM being the most aggressive with
the warm advection, but nearly all the models have a more
pronounced dry slot rotating around the low before occlusion
tonight through Tuesday. The antecedent airmass is marginally cold
and locked in, such that the environment does not look ideal for a
major icing event. However, in some of the higher terrain,
especially in SW MN and eastern SD, surface temps may be just cold
enough to offset the strong WAA and prevent changeover to rain,
keeping freezing rain longer. This could also occur should the dry
slot be more pronounced, preventing snow growth to result in more
widespread icing. While initially ice may accrete efficiently,
heavy rain and better instability surging northward will likely
yield at least some runoff percentage to lower freezing rain
accretions. However, probabilities for significant freezing rain
have increased, and feature a 10-20% chance for 0.25" across the
Coteau, with 0.1" probabilities above 30% across northern IA,
central WI, and into the L.P. of MI by D3.


Key Messages for December 9-15 winter storm from the Pacific Coast
through the Upper Midwest:
--A winter storm over the Southwest will advance through the
southern/central Rockies today producing mountain snowfall rates
of 1-2"/hr across the Desert Southwest and Four Corners. This will
result in difficult travel.

--This system will strengthen and then stall across the Central
Plains into Thursday, producing several days of significant
impacts to travel and infrastructure due to snow, blowing snow,
and freezing rain.

--The combination of snowfall rates of 1-2"/hr and strong winds
gusting over 40mph will produce blizzard conditions for parts of
the Central and Northern High Plains Tuesday and Wednesday. Travel
will likely become difficult to impossible due to snow covered
roads and near zero visibility.

--An icy wintry mix including freezing rain and sleet is forecast
to begin later today across the Plains and expand into the Upper
Midwest through Wednesday. Freezing rain accretions may exceed
0.25" in some areas creating dangerous travel and isolated power
outages.

--The combination of blowing snow and cold temperatures will
produce bitterly cold wind chills and harsh livestock conditions.


Weiss