Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
348 AM EST Fri Jan 15 2021

Valid 12Z Fri Jan 15 2021 - 12Z Mon Jan 18 2021

...Minnesota and Iowa/nearby Midwest...
Day 1...

A slow moving vertically stacked low pressure system will
gradually eject to the east through tonight bringing an end to the
snowfall across the Midwest. As the upper low begins to fill and
pivot eastward, the primary surface low will weaken as it dives
south towards Missouri, while secondary low pressure develops
across MI. Each of these surface waves will bring moderate
snowfall to the region.

With the primary low, periodic vorticity lobes spinning around the
closed feature will provide ascent, aided by a potent but
weakening axis of deformation collocated with a TROWAL pivoting
all the way around the low to the west. As these features dive
southward, they will enhance snowfall in a narrow band, likely
from southern MN through central IA and into central MO. While the
heaviest snowfall is expected before the D1 period begins, periods
of moderate to at times heavy snow where this axis lasts the
longest could produce an additional several inches of snowfall
with rates potentially reaching 1"/hr for brief periods. WPC
probabilities for 4 inches are as high as 30%. Further to the
northeast associated with the secondary wave of low pressure
moving across MI, an additional band of moderate to heavy snow
will develop within the comma head north of the dry slot, aided by
additional vorticity spokes and near moist adiabatic lapse rates
through the column. Where this band pivots the longest, WPC
probabilities indicate a 30-40% chance for 4 inches of snow across
the northern L.P. of MI.


...Central Appalachians and Northeast...
Days 1-3...

The closed low from the Midwest will advect slowly eastward but
maintain most of its intensity through the weekend as it does so.
Streams of vorticity will rotate cyclonically around the primary
gyre, with the primary vort lobe beneath the LFQ of a 130kt
subtropical jet streak interacting with the low-level baroclinic
zone and a triple point extending from the Midwest surface low to
drive secondary cyclogenesis near the Mid-Atlantic coast which
will then lift northeast into Maine, and may get pulled NW by the
upper low across Maine on Sunday. This surface low will be
accompanied by robust WAA ahead of it, which will spread and
expand precipitation northward from the Central Appalachians all
the way into Canada. Intense warm/moist advection will spread rich
theta-e air northward beginning Friday night, but this will also
have the effect of warming the column. While the guidance has
cooled just subtly for this event across the Northeast tonight,
expect the heaviest snow will be confined to the terrain of New
England as snow levels climb to 1500 ft.

On Day 1, broad WAA could lead to a quick burst of snowfall from
northern PA into the Catskills, Adirondacks, and Greens, but
accumulations should be light. During D2, as the low consolidates,
ascent becomes more robust through intense upper diffluence and
the more pronounced warm/moist advection. It is D2 when the
majority of the northeast snow is expected. After an initial burst
of WAA snow, most of the region below 1500 ft should change to
rain. However, in the higher terrain of upstate NY, VT, NH, and
ME, intense ascent with some upslope enhancement will continue,
and heavy snow is likely to persist much of the day with snow
rates of 1-2"/hr possible at times. WPC probabilities have
increased for D2, and now show a high chance for 4" in these
ranges, with locally more than 8" likely in the White Mountains of
NH. By D3 the best forcing shunts eastward as the low begins to
occlude over Maine. Moderate snow will persist in a few areas, but
WPC probabilities for 4" are generally less than 20% and focused
in northern Maine.

Behind this system, CAA and steep lapse rates beneath the upper
low will combine to drive both upslope snow in the Central
Appalachians, and also periods of LES off Lake Erie and Ontario.
There are high probabilities for more than 4" in the favored snow
zones in WV, as well as downwind of these two lakes. Local 3-day
totals both in WV and downwind of Erie/Ontario could exceed 10".

Additionally, the closed low moving across the Ohio Valley and
into the northeast will create an environment supportive of
widespread snow showers across a large portion of the east. While
accumulations are likely to be light, any location across the OH
VLY, Mid-Atlantic, and much of the Appalachians could see briefly
heavy snow as increased instability interacts with periods of
low-level fgen and low-level theta-e lapse rates approaching 0.


...Pacific Northwest to Northern Rockies Day 3...
Day 3...

A mid-level ridge will retreat inland early Sunday as it gets
replaced by a shortwave and accompanying height falls into the
Pacific Northwest. This feature will be driven eastward by a
modest but expanding Pacific jet streak which will spread moisture
inland while also placing the diffluent RRQ atop the region. Moist
advection from the Pacific jet in the presence of deep layer
ascent will drive snowfall in the terrain from the WA Cascades
southeast as far as the Tetons, and possibly reaching the Front
Range by the end of D3. The highest snowfall is likely in the WA
Cascades and into the Northern Rockies which are closer to the
higher moisture axis and will experience a more prolonged period
of forcing. WPC probabilities for 6 inches are high in these
areas, with generally less than 6 inches expected elsewhere.


For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Weiss