Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
418 PM EDT Fri Mar 26 2021

Valid 00Z Sat Mar 27 2021 - 00Z Tue Mar 30 2021


...Northeast...
Days 1-3...
An upper level shortwave and its associated surface low will track
east of the Great Lakes through the evening, with rain changing to
snow expected across northern New York and New England as the
system tracks from Maine into the Canadian Maritimes tonight.  WPC
PWPF probabilities for an inch or two of snow remain high across
northern Maine.  Although remaining relatively low, probabilities
for accumulations of 4-inches or more across northern Maine have
increased some with the 12Z guidance.

Shortwave ridging in the wake of the departing system will support
dry weather across the Northeast into early Sunday.  Wet weather
is forecast to return ahead of an intensifying low moving across
the Great Lakes on Sunday.  As precipitation develops midday into
the afternoon, model consensus shows enough cold air remaining
entrenched to support a wintry mix across northern New Hampshire
into western Maine, with mostly snow across central into northern
Maine.  A secondary low developing along the coast late Sunday may
help to keep precipitation mostly snow across northern Maine
through the event.  Even for areas that do change over to rain, a
change back to snow is expected on the backside of the system as
it moves into the Canadian Maritimes on Monday.  WPC PWPF shows
high probabilities for two-day totals (ending 00Z Tue) of 4-inches
or greater across a good portion of northern Maine.  It also
indicates some 50 percent or greater probabilities for amounts of
8-inches or more across northern Aroostook County.

...Northern Minnesota...
Day 1...
An upper level shortwave ejecting northeast out of a broader scale
trough over the Southwest, is forecast to lift northeast from the
central Plains into the Upper Midwest ahead of a northern stream
trough moving through the Dakotas on Saturday.  Models show
precipitation developing as or changing to snow along and behind
the northern stream boundary as it drifts across northern
Minnesota on Saturday.  This will likely produce at least an inch
or two of snow across the Arrowhead, with some threat for local
amounts of 4-inches or more.

...Pacific Northwest to the Northern Rockies...
Days 2-3...
Heavy, wind-driven snow is expected to return to the mountains of
the Pacific Northwest with the approach of a vigorous shortwave
trough and strong cold cold front on Sunday.  Accumulations of
8-12 inches are likely by Sunday evening across the higher
elevations of the Olympics and northern Cascades.  As post-frontal
onshore flow supports additional snow showers across the Olympics
and northern Cascades into Monday, the front and supporting
shortwave are forecast to press east across the Northwest into the
northern Rockies, with powerful winds and a period heavy snow
expected.  Widespread accumulations of 4-inches or more are likely
across the mountains of northern Idaho and northwestern Montana. 
Locally heavier amounts on the order of 8-12 inches are most
likely along the northwestern Montana ranges for areas along and
west of the Divide.  With the additional snows on Monday, WPC
shows high probabilities for accumulations exceeding a foot over
the northern Cascades and Olympics.  Plummeting snow levels are
expected to bring significant accumulations down into the passes.

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


Pereira