Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
307 PM EST Tue Nov 23 2021

Valid 00Z Wed Nov 24 2021 - 00Z Sat Nov 27 2021


...Pacific Northwest to the Northern Rockies...
Days 1-3...
A lead cold front and trailing upper shortwave will continue to
push through the northern Rockies this evening/overnight. Snow
levels will drop below 2000-3000 ft in northern areas as
precipitation diminishes, with heavier amounts confined to the
Blue Mountains and portions of the northern Rockies through
Wednesday morning. WPC probabilities continue to indicate that
widespread accumulations of 4 inches or more are likely for these
areas. Farther south, generally light snow is expected over the
Colorado Rockies above 7000 ft with some heavier amounts over six
inches possible above 10,000 ft.

Deep ridging will support dry conditions across much of the region
on Wednesday into early Thursday. Then on Thursday and continuing
into Friday, a series of shortwaves will begin to flatten the
ridge, bringing another frontal system and onshore flow back into
the Northwest. However, snow levels will be high across the
northern Cascades, with significant snow accumulations remaining
above pass level through early Friday before falling a bit Friday
afternoon as the front comes in.

...Great Lakes/Northeast...
Days 2-3...
The system currently impacting the Northwest will move across the
Upper Midwest tomorrow and through Lake Superior into Thursday
morning. General consensus of the models show the system
amplifying aloft, with a closed upper low developing by Friday
morning over southeastern Ontario and into the Hudson Valley by
Friday evening. This will bring a return of lake effect snow
showers developing off of Lake Superior first, then downwind of
the other lakes through Friday as surface low pressure deepens
across southern Canada. With 850mb temperatures forecast to drop
to -10C to -14C by Friday morning, snow-to-liquid ratios should
increase which will compensate for decreasing QPF. Into northern
New York and New England, low pressure will move through the St.
Lawrence valley Friday afternoon/evening wrapping in colder air
behind the front over the Adirondacks and northern Green and White
Mountains, where snow will pick up Friday afternoon into the
evening. WPC probabilities indicate a 10-20 percent chance of at
least 4 inches of snow through 00Z/Sat across northern areas of
the Northeast, with a higher chance (30-60 percent chance)
downwind of Lake Erie.


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

Fracasso/Pereira