Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
307 PM EST Wed Jan 13 2021

Valid 00Z Thu Jan 14 2021 - 00Z Sun Jan 17 2021

...Upper Midwest...
Days 1-2...

A potent trough digging out of western Canada will evolve into an
anomalously deep upper low across the Midwest Thursday night and
then stall briefly, before ejecting to the east/northeast into
Saturday. As this upper low deepens, a potent jet streak will
rotate through the base of the amplifying longwave trough, and the
combination of the diffluent LFQ of this jet streak with height
falls and PVA will produce a surface low over WI Thursday night,
which will then pivot nearly in place, possibly retrograding SW
briefly into Friday before lifting away through D3. Two rounds of
precipitation will accompany this feature, with moderate to heavy
snow likely, as well as some light freezing rain.

Round 1 will involve a period of WAA precipitation spreading
eastward ahead of a cold front as mid-level flow backs to the SW
and divergence supplies ascent. As the precipitation expands
northeastward, the WAA aloft will drive a warm nose atop cold
surface temperatures. This suggests, and there exists good model
consensus on this, that a narrow band of freezing rain is likely
to move eastward which may accrete a few hundredths of an inch
from central MN through eastern IA and much of WI. As the cold
front pushes eastward and forcing begins to intensify, precip may
mix with or transition to snow, but accumulations on D1 are likely
to be primarily light, with WPC probabilities for 4 inches less
than 30%.

Round 2, which will lag significantly from Round 1 in some places,
but may just evolve in place across eastern MN and WI, is expected
to be more significant. As the upper low closes off and stalls,
spokes of vorticity will rotate cyclonically around it. The
interaction of these features will lead to some enhanced mid-level
deformation, which is evident on most of the guidance which has
come into better positional agreement this aftn. At the same time,
a ridge of higher theta-e air (TROWAL) will rap all the way around
the upper low and drop southward across MN, coincident with some
WAA from the north (an indicator of how wrapped up this system
becomes). While the overall column cools, the combination of the
TROWAL, deformation axis, and mid-level WAA will produce briefly
intense ascent, but also deepen the DGZ while driving an
isothermal layer within the DGZ. This could enhance SLRs across
parts of MN and into northern IA, and snowfall forecasts have
increased this aftn. While most of the snow will likely be light
to moderate, a long duration of snow combined with at least
periods of heavier snow has led to WPC probabilities increasing
for the area. There is now a high risk for 6" from just north of
the Twin Cities southwest into the Coteau des Prairies and the
Buffalo Ridge. It is this area that has the best potential for
some heavier snowfall, and a longer duration of pivoting snowfall,
and local amounts to 10" are possible. Elsewhere, WPC
probabilities are moderate for 4 inches surrounding the higher 6"
probs, and extending into the U.P. of Michigan.


...Central Appalachians and Northeast...
Day 3...

The closed low from the Midwest will broaden and begin to lift
east/northeast Friday into Saturday, spreading waves of
precipitation eastward both along the front and associated with
vorticity impulses rotating around the gyre. The primary surface
low is expected to occlude over Michigan, with a secondary wave
developing along the triple point to the east near New England.
Height falls to steepen lapse rates beneath the mid-level vortex
will combine with waves of energy to produce periods of snow
showers over the Central Appalachians, with increasing NW flow
driving upslope flow into the favored windward slopes of WV and
PA. WPC probabilities are low for 4" on D3. Further east,
precipitation spreading northward on WAA ahead of the secondary
low will move across upstate NY and New England. Much of this
precipitation is expected to be rain, or briefly snow changing to
rain, except in the terrain of the Adirondacks, Greens, and
Whites, where snow levels may rise above 3000 ft. Above these
levels, especially in the White Mountains, heavy snow is likely.
WPC probabilities are high for 4 inches in the Adirondacks and
White Mountains of NH.


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

Weiss