Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
408 AM EDT Thu Oct 11 2018

Valid 12Z Thu Oct 11 2018 - 12Z Sun Oct 14 2018


Day 1...

...Minnesota...
Surface low pressure lifting northeast out of the Great Lakes will
result in residual comma-head snows across northern Minnesota.
Moisture and forcing weaken through the morning so total
additional snowfall will likely be light, and WPC probabilities
for 2 inches of accumulation are low.

...Northern and Central Rockies...
Shortwave digging southward into the Mountain West will be
accompanied by the diffluent right entrance region to an upper jet
moving into Wyoming to produce lift across the region. Low-level
moist advection ahead of this trough will combine with modest
frontogenesis to produce snow across the mountainous regions of
WY/CO/UT. There is potential for significant accumulations across
the high terrain of CO where upslope enhancement is likely, and
WPC probabilities show a high chance for greater than 4 inches of
snow, with moderate probabilities for 8 inches or more. Elsewhere
high elevation snow accumulating up to a few inches is possible
across the Wasatch, Uintas, Big Horns, and mountain ranges of
northwest Wyoming. 

The probability of significant icing (quarter inch or more) is
less than 10 percent.

Day 2...

...Northern Rockies...
A shortwave diving southeast from western Canada in conjunction
with the left exit region of an upper jet maximum will support
snowfall across the northern Rockies Friday night. WPC QPF is
light during this time range, but SLR of greater than 10:1 is
expected, especially in the high terrain near Glacier National
Park, where WPC probabilities are high for more than 2 inches of
snow.

The probability of significant icing (quarter inch or more) is
less than 10 percent.

Day 3...

...Northern Plains...
Surface low pressure will move from southern Manitoba towards Lake
Superior in response to a shortwave digging into the Northern
Plains. This will produce an increasing area of snow to the
northwest of the 700mb low, within the favorable deformation axis
across southern Canada and into North Dakota/Minnesota. The
strongest frontogenesis appears to stay north of the CONUS, but
light to moderate snow will drop into far northern North Dakota
and Minnesota where WPC probabilities are moderate for more than 2
inches of snow.

...Northern and Central Rockies...
Combination of a shortwave, diffluence within the right entrance
region to an upper jet, and increasing moisture will produce
widespread mountain snows on Saturday. Lift will be strongest
across the Rockies of Colorado and Laramies in Wyoming where
frontogenesis maximizes and upslope flow enhances the synoptic
ascent. It is across these ranges where WPC probabilities are high
for 4 inches of snow or more. A secondary maximum of snow
accumulation of more than 4 inches is possible across the Big Horn
range as well. Otherwise, much lighter snow accumulations are
expected across the remainder of Wyoming, northeast Colorado, and
southern Montana.

The probability of significant icing (quarter inch or more) is
less than 10 percent.

Weiss