Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
319 AM EST Wed Nov 24 2021

Valid 12Z Wed Nov 24 2021 - 12Z Sat Nov 27 2021


...Pacific Northwest and the Northern Rockies...
Day 2...
Dry conditions are expected to prevail across much of the
Northwest today into early Thursday as an upper ridge shifts east
across the region.  A series of shortwaves will begin to flatten
the ridge -- bringing a return of onshore flow and unsettled
weather back into the Northwest beginning Thursday and continuing
into early Friday.  Snow levels are expected to be high through
the event, with WPC probabilities indicating little to no
potential for significant accumulations for the primary passes in
the northern Cascades.  Probabilities show little potential for
significant accumulations in the northern Rockies as well.  Dry
conditions are forecast to return as another ridge begins to build
across the region on Friday.

...Great Lakes and the Northeast...
Days 2-3...
A shortwave trough currently centered over the northern Rockies is
forecast to move steadily east across the Plains today, amplifying
as it moves through the Upper Midwest and northern Great Lakes on
Thursday.  Strong northwesterly flow and cold air advection behind
an area of low pressure developing over southern Ontario will
support lake effect snow showers, with increasing potential for at
least a few inches of snow for portions of the eastern U.P. and
northwestern Lower Michigan on Thursday.  Models show the system
continuing to deepen, with negatively-tilted upper trough/closed
low developing over the St Lawrence Valley/Northeast on Friday. 
Primary snow impacts will be lake effect in the lee of the eastern
Great Lakes, and higher elevation snows developing in the upslope
regions on the backside of a deep surface cyclone developing over
Atlantic Canada.  WPC probabilities for Day 3 -- ending 12Z
Saturday -- indicate there is significant potential for snow
accumulations of 4 inches or more in the lake effect belts of
northwestern Pennsylvania, western New York, and the Tug Hill. 
Similar probabilities are indicated for portions of the northern
Adirondacks and along the northern New England-Canada border. 

For Days 1-3, the probability of significant icing is less than 10
percent.

Pereira