Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
416 AM EST Wed Feb 10 2021

Valid 12Z Wed Feb 10 2021 - 12Z Sat Feb 13 2021

...Southern Plains, Mississippi Valley, Tennessee Valley...
Days 1-3...

...Damaging freezing rain event through Thursday...

A southern stream trough digging across northern Mexico today
through Friday will continue to open the Gulf of Mexico and spread
moisture up over a stationary front draped across central and
eastern TX and across the Southeast to the Carolinas this morning
that will slowly drift south as particularly cold air from the
north gradually advances. Increasing jet dynamics from a backing
jet streak over the Midwest/Mid-Atlantic brings improved lift
through Thursday night. Therefore the light precip currently over
the Mid-South increases in intensity through tonight, expanding
southwest to southwest TX and northeast to the central
Appalachians. North of the surface boundary where shallow cold air
resides at the surface and a warm nose from the Gulf moisture
lifts through, expect a swath of freezing rain. 00Z guidance was
pretty steady with the swath of ice compared to the previous run.
Locally moderate to perhaps heavy precipitation will allow some
runoff before freezing, there is a high probability for 0.25" of
ice accretion in the Day 1.5 ice probabilities from just southwest
of Little Rock, AR northeastward just north of Memphis across
northwest TN and the length of KY to the KY/WV border. Some areas
along this swath may receive more than 0.5" (a 40 percent chance
also in Day 1.5), resulting a damaging icing event for much of
this swath. In a much longer this axis of freezing rain, light
accretions are likely from north TX, southern OK and much of AR
where locally backed flow will lead to better isentropic ascent
around a secondary wave of low pressure and prolonged light
freezing rain and/or freezing drizzle is likely.

North of this freezing rain axis, some sleet and snow is expected,
though probabilities of an inch or more are limited to an area of
low probabilities in northeast KY (and increasing east from there
as discussed in the next section). By Thursday night the best
forcing expands eastward and moisture gets shunted to the south,
allowing rates to decrease and shift east. The next wave looks to
produce of light ice farther southeast Friday night.


...Central Appalachians and Mid-Atlantic...
Days 1/2...

Prolonged periods of accumulating wintry precipitation are
expected today through Thursday night.

A weak impulse moving along a stationary boundary is allowing
precip to break out across KY/southern IN this morning which will
shift east across the central Appalachians this afternoon and the
central Mid-Atlantic this evening. An exiting jet streak to the
north will provide right entrance lift over the area with warm air
advection over the existing stationary front and developing cold
air damming wedge from high pressure over Ontario will keep
precipitation going through Thursday night. Within the cold sector
of this system, a period of moderate snow is expected in the
upslope northern WV and western MD into the Laurel Highlands of PA
as well as the Potomac Highlands east of the Appalachian crest.
Here Day 1 WPC probabilities for 4 or more inches remain as high
as 50%. The best moisture and lift occur Thursday with the CAD
wedge fully in place making for a tight boundary between snow to
the north and freezing rain to the south. Day 2 snow probabilities
focus a bit farther south over east-central WV and across
north-central VA to southern MD with 30 to 50% probability of 4 or
more inches. South of the snow, light freezing rain is likely as
the WAA drives a modest warm nose atop the developing cold surface
wedge, with some light sleet in between. WPC probabilities for
0.1" of freezing rain on Day 1 are generally 10-30% in the
Allegheny Highlands of southwest VA to southeast WV. The true CAD
wedge appearance to the freezing rain is on Day 2 with a swath of
50% probabilities for 0.1" or more ice from southern WV then along
the VA/NC border to southeast VA (west of Hampton Roads) with 20
to 30% probabilities for 0.25" or more ice in a swath over
southern VA.

This activity shifts east Friday morning with the next wave
approaching from the southwest late Friday night with another
round of wintry precipitation then at least through Saturday.


...The West onto the Central Plains...
Days 2-3...

The next wave approaches the northern CA/southern OR coast
Thursday with ample Pacific moisture ahead leading to moderate to
locally heavy precip up Thursday afternoon/night north from San
Fran to near Seattle. However, the key feature for lower level
population centers in the Pacific Northwest is the persistent
presence of an Arctic front that shifts south from Seattle this
morning and lingers around the Portland metro area tonight before
drifting a bit further south Thursday into Thursday night until
the low arrives. This cold air will make for most any
precipitation Thursday/Thursday night accumulating snow for the
Portland and Seattle metro areas. Day 2 and Day 3 snow
probabilities for 4 or more inches are generally 20 to 30 percent
for both metro areas a little higher in the slightly higher
terrain between them and up the Columbia Gorge.

This wave pushes inland Thursday night, crossing the central
Rockies (and getting onto the central High Plains) Friday night
which is when the next wave arrives at the WA/OR Coast. Ample lift
and Pacific moisture cause heavy mountain snows across much of The
West for Days 2 and 3. Day 2 snow probabilities are moderate for 8
or more inches for the WA/OR and highest CA Cascades as well as
the northern and central Sierra Nevada, the Sawtooth and Salmon
River Mtns of ID and the Teton and Wind River Ranges of WY. These
probabilities for 8 or more inches persist on Day 3 for these
ranges except the Sierra Nevada and expand to southern WY/northern
CO. Lee cyclogenesis and an exiting jet streak push snow onto the
central Plains with broad 20 to 40% Day 3 probabilities for 4 or
more inches over much of Neb and northern KS.


...Great Lakes...
Days 1 and 3...

Continued west-northwesterly flow across the Great Lakes will
allow LES bands of snow to continue today off Lake Superior on the
Keweenaw Peninsula and eastern U.P. as well as generally north of
Syracuse to the southern Tug Hill. The snow crystal growth zone is
generally above DGZ over Lake Superior, leading to inefficient ice
crystal growth there, but in an ideal position off Lake Ontario.
Day 1 WPC snow probabilities are moderately high for 4 or more
inches in these narrow corridors.

A reinforcing trough rounding the persistent and deep low centered
over Ontario will swing through the Great Lakes Thursday night
with lake enhanced snow


Jackson