Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
408 AM EST Mon Feb 15 2021

Valid 12Z Mon Feb 15 2021 - 12Z Thu Feb 18 2021


...Central Gulf Coast to the Northeast...
Days 1-2...
Heavy snow and significant icing are expected to extend from the
central Gulf Coast to New England as a fast-moving, major winter
storm moves from the central through the eastern U.S.

As a well-defined shortwave moving through the base of a deep
longwave trough centered over the central U.S. begins to pivot
from the southern Plains into the lower Mississippi valley,
moderate to heavy precipitation will continue to organize along an
axis of strong ascent supported by favorable upper jet forcing and
strong low-to-mid level frontogenesis that extends north from the
lower Mississippi valley.  Heavy snow will continue to develop and
spread northeastward from the ArkLaTex region into the lower Ohio
valley during the day today, with widespread accumulations of
4-inches likely, with locally heavier amounts expected. 
Meanwhile, freezing rain with significant ice accumulations is
expected to develop farther to the southeast of the area of
heavier snow -- with ice accumulations of 0.10-inch or more likely
from as far south as southeastern Louisiana, extending
northeastward into eastern Kentucky and West Virginia on Tuesday. 
Locally heavier ice accumulations are expected, with some portions
of Mississippi, northern Alabama and Middle Tennessee likely to
see accumulations exceeding 0.25-inch during the day on Monday.

As the previously noted shortwave continues to lift steadily to
the northeast, the axis of heavy snow is forecast to shift farther
to the north, extending across the lower Great Lakes region into
the Northeast by early Tuesday.  Strong low-mid-level
frontogenesis will continue to contribute to the generation of
heavy snows as it lifts from the Ohio valley into the Lower Great
Lakes and Northeast Monday night into early Tuesday.  During the
overnight and morning hours, snow accumulations of 4-inches or
more likely to the extend from Indiana to northern New England,
with heavier totals likely near the Lower Great Lakes into
northern New York.  Northeasterly flow is expected to support lake
effect snows and locally heavy totals along the southwestern
shores of Michigan as the surface-low level wave passes to the
southeast of the region late Monday into Tuesday.  Meanwhile,
accumulating ice, with locally heavy amounts will likely extend
from Tennessee valley northeastward across the central
Appalachians into the the northern Mid Atlantic, southern New York
and southern New England.  This will likely raise total ice
accumulations up over 0.25-inch across portions of eastern
Kentucky.

During the day on Tuesday, moderate to heavy snows will continue
to shift east across northern New England, with an icy mix farther
to the south across central New England and coastal Maine. 
Precipitation is expected to diminish as the system moves into
Atlantic Canada Tuesday evening.

...Pacific Northwest to the Lower Mississippi Valley...
Days 1-3...
A second far-reaching major winter storm will continue to develop
over the Northwest as another upper level shortwave drops
southeast across the region today.  Contrasting with the previous
system, snow levels are on the rise, with areas across the
Washington lowlands already transitioned over to rain.  However,
icing will remain a concern early in the period farther to the
south along the Columbia Gorge into the Portland metro before
temperatures increase later today.  Heavy snows are likely across
the Olympics and along the Cascades early on, spreading south and
east across the Intermountain West into the northern and central
Rockies on Monday.  As the leading shortwave continues to dig
southeast, re-amplifying the upper trough over the Southwest, the
threat for heavy snow is expected to reach once again into the
higher elevations of northern and central Arizona as well as
portions of the southern Rockies Monday night into early Tuesday. 
Meanwhile, deep northwesterly flow with embedded energy aloft will
continue to support unsettled weather, including periods of heavy
mountain snow from the Cascades to the northern Rockies through
Tuesday.  By late Tuesday, snow is expected to develop once again
across portions of the southern Plains, with some potential for
heavy accumulations developing from the eastern Texas Panhandle
across Oklahoma.  A more significant threat is expected to develop
farther to the east on Wednesday.  As the upper trough moves east
and amplifies over the southern Plains on Wednesday, models once
again show a strong signal for heavy precipitation developing
within an area of strong ascent supported by favorable upper jet
dynamics, along with low-to-mid level frontogenesis.  Overnight
guidance suggests heavy snows likely to develop from southeastern
Oklahoma and northeastern Texas northeastward across Arkansas into
southern Missouri and the lower Ohio valley on Wednesday. 
Meanwhile, freezing rain with heavy ice accumulations is expected
from eastern Texas through the ArkLaTex region into southeastern
Arkansas and northern Mississippi.

Pereira