Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
330 PM EDT Wed Oct 31 2018

Valid 00Z Thu Nov 01 2018 - 00Z Sun Nov 04 2018

...Northern and Central Rockies...
Days 1-3...

Several shortwaves embedded in a weak but persistent atmospheric
river into the weekend will produce widespread mountain snows from
the northern Cascades through the northern and central Rockies.
Two distinct shortwaves, the first on day 1, and the second, more
potent, on day 3, will lift onto the British Columbia coast before
dropping southeast through the inter mountain west. Anomalously
high column moisture will be present due to the atmospheric river,
which will be tapped by increasing lift due to jet level
diffluence, height falls, vorticity advection, and upslope
enhancement. Many of the mountains will experience periods of
moderate snow, accumulating to heavy amounts due to the long
duration event. Temperatures are marginal, so snow levels will
generally remain above 6000 feet all 3 days. Although many
mountainous regions have high WPC probabilities for 4 inches of
snow each day, the highest total snowfall is likely in the
vicinity of Yellowstone National Park across northwest Wyoming. In
this area, there is a good chance for more than 12 inches during
the entire 3 day time period.

The probability for significant icing (0.25 inches or more) is
less than 10 percent all 3 days.


...Northern Plains...
Day 3...

An impulse rotating atop the Pacific ridge will spawn cyclogenesis
in the form of an Alberta Clipper which will race southeast into
the northern Plains on Saturday. 700mb warm advection ahead of
this feature will provide moisture and lift into the Dakotas,
which will wrap into the deformation zone/developing comma head
north of the mid-level low center. Antecedent temperatures are
marginal, and best forcing appears to occur during the early
morning into the afternoon, so this will need to be overcome for
significant accumulations. However, an upward trend has been noted
in model QPF output on better moist advection, and moderate omega
through the dendritic growth zone should allow for periods of
heavier snow rates. WPC probabilities have increased, and now show
a slight risk for a stripe of 4 inches or more of snow across
central North Dakota.

The probability for significant icing (0.25 inches or more) is
less than 10 percent.

Weiss