Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
354 AM EST Wed Jan 13 2021

Valid 12Z Wed Jan 13 2021 - 12Z Sat Jan 16 2021

...Pacific Northwest and Northern Rockies...
Day 1...

An amplifying ridge quickly trailing the main shortwave trough
currently pushing across the Salish Sea will quickly bring an end
to precip across the Pacific Northwest this morning. Progressive
precip will pushing across the Continental Divide today in advance
of a cold front from the surface low that will track across
southwestern Canada through Wednesday. A weakening cold front
reaches the Pac NW coast Thursday night, but with little push to
get it inland there's not much of a snow threat from it. A welcome
respite from the rest barrage of waves into the Northwest CONUS.
Day 1 snow probabilities are moderately high for 6 or more inches
for the northern WA Cascades, Bitterroots and Lewis Range as well
as the Tetons and portions of the Absarokas.


...Upper Midwest...
Days 2/3...

The shortwave trough pushing into the Pacific Northwest coast this
morning will quickly push along the Canadian border today before
the trailing amplifying ridge allows a reinforcing shortwave
trough to dig a trough down the northern High Plains tonight. This
closes into a low over the upper MS Valley Thursday where it slows
to a southeast drift across the Midwest through Friday night. This
will be an anomalously deep closed low (with 500mb heights falling
to the bottom 1-percentile according to NAEFS ensemble tables.) At
the surface, low pressure developing over the upper MS River is
likely to be nearly stationary Thursday night before it
occludes/fills as it shifts into northern IL Friday.

Ahead of the wave, a brief surge of southerly flow with
frontogenesis could produce a slushy rain/snow mix, with some
light freezing rain Thursday as precip pushes from MN/IA into
WI/IL. The ice accretion risk with this is for a few hundredths of
an inch. As the low closes/develops late Thursday a TROWAL is
expected to develop across northern WI arcing across
central/southern MN and extending down into IA. The slow motion of
the system after developing is expected to allow this TROWAL and
its related precip to remain in essentially the same area for 18
to 24hrs. While precipitation rates are not expected to be
particularly heavy, the steady precip and increasingly cold
conditions should allow for several inches of snow to fall. WPC
QPF is based on the well agreeing 00Z ECMWF/GFS(v15)/UKMET/CMC.
Day 2.5 snow probabilities contain the entire TROWAL related
precip and have moderately high probabilities for 6 or more inches
in an arc from northern WI through south-central MN and into
northern IA. Lake Superior enhancement looks to be mainly across
southwestern portions of the lake based on the consensus low
position and northeasterly low level flow there Thursday night,
backing to northerly on Friday. That northerly flow then persists
into Friday night raising Day 3 snow probabilities to 30-40
percent for far northern WI/western MI UP.


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

Jackson