Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
357 AM EST Sun Feb 07 2021

Valid 12Z Sun Feb 07 2021 - 12Z Wed Feb 10 2021


...Mid-Atlantic through eastern New England...
Day 1...
A shortwave trough rounding the sprawling trough around the deep
vortex over Ontario is apparent in GOES-16 mid-level water vapor
imagery as over northern AL with surface low pressure developing
along the Carolina Coast. Snow overnight has been contained to the
higher Piedmont and southern Appalachians with the initial wave of
precip up toward the central Mid-Atlantic and coastal plains
starting as rain until wetbulbing down toward the freezing mark.
This makes for marginal low level conditions that require being
overcome by dynamically forced banding northwest of the of the
surface low. Guidance has wavered considerably about the strength
of this banding across the central Mid-Atlantic this morning with
the 3km NAM waffling on the magnitude of low level frontogenesis
over recent runs. That said the 06Z 3km NAM seems reasonable with
the expected pattern of colder, but less dynamic/less QPF north of
DC seeing accumulating snow and snow struggling to accumulate
through the morning hours along and south of the I-95 corridor.
However, should banding setup well the associated dynamic cooling
should allow some moderate snow rates which could accumulate to a
couple inches quickly. Low probabilities for 2 or more inches are
on the VA Blue Ridge and across the Balt-Wash metro.

Farther northeast, the stronger low and offshore track allows
essentially all snow north from southeast PA. Across northern NJ,
the NYC metro, southeast New England and far eastern Maine, the
combination of colder conditions and low level frontogenesis
allows a stripe of heavy snow. Day 1 probabilities for 4 or more
inches are moderate for northern NJ up through southern CT with
moderate probabilities for 6 or more inches for RI and southeast
Mass and moderate probabilities for 8 or more inches for Downeast
Maine.

...Northern Cascades to the Central Rockies...
Day 1...
Shortwave energy embedded with strong northwest flow aloft is
expected to bring another surge of deeper moisture while nudging
the arctic air mass farther south. This will support additional
periods of heavy snow, with heavy accumulations into tonight
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. In these areas, expect
additional accumulations of a foot or more. Rates decrease tonight
with less moisture available and heavy snow should end by Monday.

...Central Plains to the Mid Mississippi Valley...
Days 1-2...
A shortwave trough shifting into southeastern Nebraska will
continue to move east into the lower Missouri and mid Mississippi
valleys through today. 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 (Day 1 probabilities for
that are 10 to 20 percent), especially across portions of central
to eastern Nebraska into southwestern Iowa and northwest Missouri.
Additional rounds of mainly light 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...
Days 2/3...
While a deep vortex remains centered over central Canada, models
continue to 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 should support
organized heavier snows along the northern New England coast into
Tuesday night.

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

Jackson