Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
321 PM EDT Fri Nov 01 2019

Valid 00Z Sat Nov 02 2019 - 00Z Tue Nov 05 2019


...Great Lakes...

Days 1-2...
A cold front draped from a weak low pressure moving eastward
through Ontario Province will move across the Great Lakes, turning
winds W/NW Saturday into Sunday. The CAA behind this front moving
in favorable trajectory across the Lakes will produce the first
widespread LES event of the season. The favorable wind shear, deep
boundary layer mixing, and instability of 500-800 J/kg atop the
warm lake will support Lake Effect precipitation downwind of all
the Lakes, but marginal temperatures are expected to limit total
snowfall accumulations. The heaviest snowfall is forecast across
the upper L.P. of Michigan, as well as the hilly terrain east of
Lake Erie and Ontario. In these locations, WPC probabilities for 4
inches low, but spot totals to 6 inches are possible where higher
rates can overcome the marginal temps, and where focused
precipitation can persist longest.  The event winds down Sun as a
ridge approaches from the Ohio Valley, with drying aloft and
warming temperatures bringing an end to the LES. 


...Northern Rockies...

Day 3...
Increasing RH through the column in part due to confluent mid/upr
level flow will produce an environment favorable for light to
moderate snow. Within this regime, pieces of shortwave energy will
rotate down atop the Northern Rockies, embedded within the broad
trough across the east. One of these will drive a cold front
southward which will then bank against the terrain from Montana
through Wyoming. Upslope low-level flow behind this feature will
aid in the synoptic ascent to produce periods of light snow,
mostly in the higher terrain of the Big Horns and Absarokas where
WPC probabilities are high for more than 4 inches.


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

Weiss