Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
402 PM EST Wed Dec 15 2021

Valid 00Z Thu Dec 16 2021 - 00Z Sun Dec 19 2021


...Dakotas...Minnesota and the UP of Michigan...
Day 1...

A very powerful low pressure system continues to develop as it
moves northeast across Neb rest this afternoon and across MN this
evening before pushing into central Ontario overnight. A northern
stream shortwave trough currently over central MT will shift east
across ND tonight and across northern MN into Thursday. 12Z
guidance is still disparate with the low track and more
importantly for winter purposes, the swaths of banded snow on the
NW side of the low tonight. However, a blend of the 12Z GFS/ECMWF
provides a good low track and usage of CAMs such as the HRRR and
NAM Nest provides QPF for the expected bands over the Dakotas
tonight with no CMC guidance included which remains much farther
east with its track and QPF.
The intense deformation axis beneath a pronounced TROWAL wrapping
cyclonically around the low will drive a rapid changeover from
rain to snow this evening near the western MN border then up
through north-central MN toward International Falls with the heavy
snow threat lifting into western Ontario around per recent CAM
consensus. Heavy snow rates are likely soon after transition with
the WPC snowband tool (based on the 12Z HREF) indicating the
potential for 0.2" to 0.3" liquid in the form of snow per hour
which generally correlates to 2 to 3"/hr snowfall with Day 1 WPC
snow probabilities now 40 to 60 percent for 6 or more inches along
this swath from west-central MN to International Falls.
Additionally, the further west solutions over the past day have
lead to more interaction with the northern stream shortwave trough
and enhancing snowband potential over the central and eastern
Dakotas where bands with rates of 1-2"/hr snowfall are possible
and Day 1 WPC snow probabilities for 4 or more inches are to 20-30
percent over central ND and 40-50 percent over northeast SD.

Westerly flow in the wake of this low will allow LES bands to
develop with Day 1.5 snow probabilities for 4 or more inches
moderate over the Keweenaw Peninsula.


...Western U.S....
Days 1-3...

A cold core low off the WA coast will weaken/open as it shifts
southeast across western OR tonight, bringing renewed moisture and
precipitation into the region into OR and northern CA which were
impacted by a major mountain snow storm only two days ago.
Confluent mid-level flow and a modest Pacific jet streak are
advecting moisture onshore, through which ascent will maximize via
height falls and left exit upper diffluence. Heavy snow this
afternoon spreads south from the OR Cascades across the CA
Cascades/Klamath/Trinity Alps and down the Sierra Nevada tonight
with snow levels generally around 3000 ft in OR and 4000 to 5000ft
in the Sierra Nevada. Day 1 WPC probabilities are high for 12
inches (generally expect 1 to 2 feet) in the northern and central
Sierra Nevada with forcing weakening for the southern Sierra
Nevada where one foot probabilities are moderate. While this is a
run-of-the-mill storm for CA mountains, impacts should be greater
since it's on the heels of the greater storm on Mon/Tues.

The wave crosses the northern Great Basin Thursday with Day 1 snow
probabilities moderately high over ranges in northern NV and
western ID with Day 1.5 values moderate for the Wasatch of
northern UT, the Tetons/Wind Rivers of WY, and Bitterroots along
the ID/MT border.

The typical La Nina is alive and well with the next wave/plume of
Pacific moisture reaching WA Friday night with snow levels rising
from about 2000ft at snow onset. Day 3 probabilities are moderate
for 8 or more inches for the higher Olympics, northern WA
Cascades, and low over the northern tip of ID.  


...Northeast...
Days 1 and 3...

A warm front extending from a potent low pressure moving into
Ontario, Canada will lift northeast into New England, and the
accompanying WAA will spread precipitation across the area
tonight. At the same time, surface high pressure currently
centered off New England will drift east, with low-level cold air
persisting as precipitation lifts into the area. This will support
a period of snow and freezing rain this evening into Thursday
morning, before changing over to rain and ending on Thursday.
While snowfall accumulations should be light, just 1-2" outside of
the highest peaks of ME and NH, WPC probabilities for 0.1" of
accretion are 20 to 30% in the White Mtns and interior Maine.

Then a positively tilted trough crosses the Great Lakes Saturday
with surface low pressure over the Mid-Atlantic focusing moisture
advection through NY State and New England where thermal profiles
are cold enough for snow that looks to start Saturday afternoon.
As of now Day 3 snow probabilities are around 10 percent over the
Adirondacks and Green/White Mtns, but will rise on subsequent
shifts as the storm looks to linger through at least Saturday
night.

Jackson