Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
337 PM EDT Sun Oct 27 2019

Valid 00Z Mon Oct 28 2019 - 00Z Thu Oct 31 2019


Days 1-3...

...Northern and Central Rockies/High Plains...

Snow has developed in CO, and should continue tonight as upslope
flow behind a strong cold front, along with low to mid level
frontogenesis and upper divergence will support moderate to heavy
snowfall across central Colorado on Sunday into early Monday. 
While the heaviest amounts are expected to fall across the
mountains - where WPC probabilities show that amounts of 8-inches
or more are likely - several inches are likely to extend east of
the foothills into the High Plains along the I-25 corridor,
including the Denver Metro area.  By late Monday, snows will begin
to taper off across the central Rockies/High Plains as the
associated shortwave begins to accelerate northeast away from the
region.

In MT/WY, snows will redevelop and drop south through the northern
Rockies as a second shortwave trough amplifies while moving south
from Canada Monday, driving a low level front across the region. 
While widespread heavy amounts are not expected, locally heavy
accumulations are possible over the western Montana into the
northern Wyoming ranges by early Tuesday. 

Models show this second system continuing to amplify as it drops
farther south, with a closed upper low developing over southern
Idaho-northern Utah on Tuesday into early Wednesday.  Snows are
expected to intensify ahead of the system, with heavy
accumulations likely to return to portions of southeastern Wyoming
to central Colorado in response to low level frontogenesis and
convergence.  Snows extend back along the 700 mb front into Utah,
aided by upper divergence maxima as the 300 mb jet drifts south
across the state.

Model spread with this second system is greater than the initial
system, with timing differences as to how long snow persists
before the system departs onto the Plains. The consensus of the
models shows heavier amounts over the front range, foothills ,and
high Plains.  There is potential for several inches along the I-25
corridor from Casper to Denver, with amounts gradually decreasing
further east across NE/KS.

...Mid-Upper MS Valley to the Upper Great Lakes...
Models continue to show the leading shortwave accelerating
northeast across the Plains to the upper midwest, with precip on
the northwest fringe of the surface wave possibly changing from
rain to snow before ending Tuesday morning. 
Light amounts are shown in most forecasts across eastern Iowa and
eastern Wisconsin/UP of MI Tuesday morning. 

A similar scenario is possible on day 3/Wed as the next mid level
wave approaches the mid MS Valley from the Plains, with initial
precip possibly changing to snow before ending in the mid MS
Valley.  Differences remain on the duration of each type, limiting
accumulation potential.

For Days 1-3, the probability of significant icing is less than 10
percent.

Petersen