Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
326 PM EST Sun Nov 04 2018

Valid 00Z Mon Nov 05 2018 - 00Z Thu Nov 08 2018

...Northern and Central Rockies...

Days 1-3...

Periods of heavy snow will fall across much of the mountainous
terrain from the northern Cascades of Washington through the
northern and central Rockies. This snow will be due to a 
prolonged period of mid/upper level confluent and moist flow from
the Pacific, shown by nearly continuous 700mb RH of over 70
percent. Shortwave troughs and jet maximums racing through this
flow will provide ample forcing for ascent, while increasing
frontogenesis along a stalled cold front banking against the
mountains will further enhance lift, especially by Tuesday.

Monday, shortwave and intense left exit region diffluence will
drive strong omega through a saturated 1000-500mb layer to produce
heavy snow across the Rockies. The heaviest snow on day 1 is
likely in the Rockies of Colorado, as well as the Bitterroots of
Idaho where WPC probabilities are high for 8 inches or more of
snow. Much of the rest of the high terrain above 4000 feet has a
high probability for 4 inches of snow.

Tuesday into Wednesday, another shortwave and jet maximum will
drop southeast around the Pacific ridge to provide the impetus for
further lift from the Washington Cascades southeast into the
Colorado Rockies. At the same time, a cold front will drop slowly
southward to enhance low-level frontogenesis while prolonged NW
flow will upslope in portions of the northern Rockies of MT/ID and
through northwest WY. Although column moisture will begin to wane
Wednesday, the combination of high SLR's and lowering snow levels
will allow for snow across much of the area, with accumulations
likely even on the valley floors. The highest accumulations are
favored in the high terrain of the Bitterroots, northern Rockies
near Glacier National Park, and the Absarokas/Tetons of Wyoming
where WPC probabilities feature a slight risk for 8 inches or
more. Elsewhere, lighter amounts are likely with a few inches
possible even in the valleys of Montana and Idaho Tuesday night
and Wednesday.

Snowfall totals in the highest peaks of Idaho and Montana may
exceed 2 feet by Wednesday.

...Western Great Lakes...

Days 1-3...

A deepening low pressure will move from Iowa through western Lake
Superior on Monday. Precipitation ongoing this evening will
persist into the first part of Monday across Wisconsin, Minnesota,
and the U.P. of Michigan as warm advection continues east of the
negatively tilted low. Temperatures will remain marginal so
snowfall accumulation will be confined to the Arrowhead of MN
where temperatures are sub-freezing, moisture will persist longer,
and frontogenesis will be maximized as the low slows on Monday.
Eventually 850-700mb temps will climb above 0C even into the
Arrowhead, so snow will changeover and end during Monday. Before
that time however, moderate accumulations are possible especially
just inland from the lake shore where upslope enhancement into the
Iron Ranges will occur. WPC probabilities are moderate for 4
inches of snow in extreme NE Minnesota, with lesser amounts
possible in other areas surrounding the lake.

A second system will drop southeast towards the Great Lakes on
Tuesday and Wednesday bringing an increase in 1000-500mb RH.
Height falls and weak vorticity advection will combine with at
least subtle jet level diffluence within a jet streak moving south
of the region to provide modest ascent. The system is progressive
and forcing is marginal, but there exists a slight risk for light
snow accumulations from northern MN into WI and MI, with WPC
probabilities below 30 percent for 4 inches.


The probability of significant icing is less than 10 percent days
1-3.

Weiss