Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
408 AM EST Fri Feb 05 2021

Valid 12Z Fri Feb 05 2021 - 12Z Mon Feb 08 2021

...Great Lakes...
Days 1-3...
The deep low associated with the Arctic outbreak over the
north-central CONUS is currently over centered over the U.P. of MI
and lift north into Ontario today as a large upper gyre sets up
over Ontario through this weekend. Westerly to occasionally
northwesterly flow south of the system will continue to support
lake-effect snow spreading east from MI to the eastern snow belts
as the low tracks north of the Great Lakes through today and
continuing through Saturday. The next wave rounding the gyre
crosses the Great Lakes Sunday which should disrupt LES in favor
of light synoptic snow with a return to LES Sunday night. 72hr
snow in excess of an additional 12" is highest over the Keweenaw,
Pictured Rocks area of the eastern U.P. and in the lee of Lake
Erie near Buffalo and east of Lake Ontario on the Tug Hill Plateau
(and north to the Canadian border). Probabilities for the western
shore of the L.P. of MI are moderate for 6 or more inches on Day 1.


...Northern Cascades to the Central Rockies and Central Great
Plains...
Days 1-3...

A shortwave trough shifting down the BC coast early this morning
will move southeast from central WA this morning to the southern
Rockies this evening, reinforcing the base of the broad scale
trough spread across the interior of the continent and centered
over Ontario this weekend. The base of this shortwave trough
shifts east across the southern Plains late tonight through
Saturday while the northern stream crosses the central Plains. A
stronger wave train shifts into the Pac NW late Saturday this time
crossing the northern Rockies before spilling across the central
Plains in a similar as the previous over the central Plains.
Organized heavy snowfall will shift south from the northern
Cascades down the spine of the Rockies today before a swath of
snow crosses the central Plains/Neb to northeast KS/northern MO
through Saturday. Strong northwesterly flow will to carry the wave
train across the region, with additional periods of heavy
mountains snow continuing into Sunday night with a round of
lighter snow Sunday over a similar swath as the previous track. It
is expected that portions of the northern Rockies from northern
Idaho to western and central Montana, will see storm totals in
excess of two feet over the next three days. Day 1 snow
probabilities for 8 or more inches are high over the northern
Rockies south from Glacier NP to northern CO as well as the
Wasatch of UT. The secondary wave train is featured for Days 2 and
3 with probabilities of 12 or more inches moderate to high for the
northern Cascades and Bitterroots of ID/MT and the Big Belt Mtns
of MT on Day 2, decreasing for similar areas on Day 3. Day 1.5
snow probabilities are moderate for 4 or more inches over central
Neb with low probabilities stretching across northern MO on Day 2.
Then there are additional low probabilities for 4 or more inches
over east-central Neb on Day 2.5.


...Southern Appalachians to the Northeast...
Days 2/3...

The aforementioned trough moving east across the southern Plains
Saturday will cross the far southern Appalachians Saturday night
and turn northeast up the front side of the main trough and off
the Northeastern Seaboard Sunday/Sunday night. Perhaps the 00Z
guidance has brought about a reasonable consensus solution with a
track over the southern Mid-Atlantic Sunday morning, with the
surface low pressure tracking along the Carolina Coast with
typical biases of a faster GFS solution and preference to the
similar 00Z ECMWF/NAM/UKMET. Going forward, the parent wave will
be in the CONUS raob network which will hopefully limit further
track shifts. While track is in better agreement, there is still
thermal and intensity uncertainty. Marginal thermal profiles over
the southern/central Mid-Atlantic likely limits moderate snow
rates to where low level frontogenesis and associated mesoscale
bands set up. These are currently progged over southeast VA to the
southern Delmarva Saturday night into Sunday morning. Strong jet
dynamics allow the low to quickly shift northeast off the
Northeastern Seaboard Sunday into Sunday night with some northern
stream trough supporting mainly light snow into interior New
England Sunday afternoon/evening. Day 2.5 snow probabilities for 2
or more inches are low to moderate from the southern Appalachians
all the way through central New England and moderate for New
England on Day 3. The heaviest snow is likely to be in the
southern Appalachians with low Day 2 probabilities for 4 or more
inches over southwest VA and western NC and more low probabilities
across New England on Day 3.

Jackson