Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
458 PM EST Sat Feb 06 2021

Valid 00Z Sun Feb 07 2021 - 00Z Wed Feb 10 2021


...Southern and Central Appalachians to the Northeast...
Day 1...
A southern stream shortwave moving across the Tennessee valley
overnight, before lifting northeast along the Mid Atlantic to the
Northeast coasts on Sunday, is expected to produce a fast-moving
storm, with winter weather impacts from the southern Appalachians
to New England.  Initial winter weather impacts will begin this
evening and continue overnight across the Cumberland Plateau into
the southern and central Appalachians.  Areas impacted by the
heaviest snow accumulations are expected to include the Blue Ridge
and southern Allegheny mountains, where localized amounts of
4-inches or more are likely.  Overnight, as a surface low begins
to develop along the Carolina coast, snow will likely begin to
spread northeastward across the Piedmont into the Virginia and
Maryland I-95 corridor.  There is still a fair amount of
uncertainty as to how far southeast accumulating snows will reach
into southeastern Virginia, however an inch or more does look
likely as far south as the Richmond metro.  Heavier accumulations
are more certain farther to the north, especially from
northeastern Maryland to southern New York and southern New
England.  While the NAM is along the western edge of the guidance
and is probably showing its typical bias of being too far north
and west with its heavier amounts, the overall consensus of the
guidance does show the potential for banded, heavier precipitation
developing, that is supported by low-to-mid level frontogenesis
and strong upper forcing on the northwest side of the low.  This
is most evident as the low moves southeast of Long Island and
southeastern New England during the day on Sunday and is reflected
in the higher probabilities for accumulations of 6-inches or more
centered across Connecticut, Rhode Island, eastern Massachusetts
and far southern New Hampshire.  Unlike the previous system, this
storm is expected to move quickly to the northeast, tracking east
of Nova Scotia Sunday night.  This will brush the northern New
England coast with at least a few inches of snow, with heavier
amounts more likely across Down East Maine.

...Northern Cascades to the Central Rockies...
Days 1-3...
Shortwave energy embedded with strong northwest flow aloft is
expected to bring another surge of deeper moisture while nudging
an arctic air mass farther south.  This will support additional
periods of heavy snow, with heavy accumulations likely Saturday
night into Sunday across portions of the region.  Impacted areas
are expected to include the northern Cascades, the Bitterroot
Range and the western Montana ranges west of the Divide -- where
additional accumulations of a foot or more are likely.  Unsettled
weather is forecast to continue, however the potential for heavy
accumulations is expected to wane Monday into Tuesday.

...Central Plains to the Mid Mississippi Valley...
Days 1-2...
Shortwave energy emanating from the West, along with a strong
low-to-mid level baroclinic zone, will support another round of
dry, fluffy snow spreading southeast from the northern High Plains
into south-central South Dakota and central to eastern Nebraska
overnight.  Snow will continue to move east into the lower
Missouri and mid Mississippi valleys during the day on Sunday. 
While widespread heavy amounts are not expected, strong forcing
along with high snow-to-liquid ratios may support some totals of
4-inches or more, especially across portions of central to eastern
Nebraska into southwestern Iowa on Sunday.  Additional rounds of
snow are expected late Sunday into Monday as upstream energy moves
out into the central Plains, with some amplification of the flow
and right-entrance region upper jet forcing supporting an axis of
heavier snows centered across the Mid Mississippi valley on Monday.

...Great Lakes to the Northeast...
Day 3...
While a deep vortex remains centered over central Canada, models
show a well-defined shortwave moving south of the center across
the Great Lakes into the Northeast on Tuesday.  This is expected
to support widespread light to moderate synoptically driven snows,
followed by some lake effect activity late in the period.  A
coastal low developing by late Tuesday may begin to support
organized heavier snows along the northern New England coast
before the end of the period.

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

Pereira