Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
503 PM EDT Mon Oct 08 2018

Valid 00Z Tue Oct 09 2018 - 00Z Fri Oct 12 2018

Day 1...

...Northern Cascades to the Northern Rockies...
Good upper level divergence ahead of a well-defined
shortwave/upper jet digging south into the Pacific Northwest is
expected to support mountain snows across portions of the northern
Cascades and Rockies, with WPC probabilities indicating the
potential for locally significant amounts -- particularly across
the central Idaho ranges on Tuesday.

...Central Rockies...
The southern stream trough presently centered over the Southwest
is expected to shift east overnight before beginning to lift out
to the northeast into the southern High Plains on Tuesday.  Low
level convergence and upslope flow combined with a brief period of
low-mid level frontogenesis may support some locally heavier
amounts -- particularly along the Sangre de Cristo mountains,
where WPC probabilities for significant amounts are higher.

...Northern High Plains...
As the previously noted wave continues to lift to the north,
amplifying northeasterly winds are expected support snows
extending east from eastern Wyoming into western North Dakota and
southwestern South Dakota, producing a Slight Risk for additional
accumulations of 4-inches or more across portions of the region. 

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

Day 2...
                                                           
...Northern to the Central Rockies...
The potential for significant snowfall accumulations is forecast
to become more widespread across the area as the aforementioned
shortwave digging into the Northwest continues to amplify, carving
out a new trough across the region Tuesday night into Wednesday. 
WPC probabilities indicate a widespread Moderate Risk for amounts
of 4-inches or greater with more localized risk areas for heavier
amounts across the higher terrain from central Idaho and western
Montana to northern Utah and Colorado.

...Northern Plains and Upper Midwest...
The southern stream shortwave emanating from the Southwest will
continue to lift to the north, assuming a negative tilt as it
moves from the Plains into the mid and upper Mississippi valley on
Wednesday.  Surface low pressure organizing over the mid
Mississippi valley Tuesday night is expected to deepen further,
dropping below 1000mb as it moves toward the upper Great Lakes
region on Wednesday.  Rain changing to snow is expected within the
associated comma-head.  WPC probabilities are indicating a
Moderate Risk for accumulations of 4-inches or greater and a
closely aligned Slight Risk for 8-inches or more extending from
north-central South Dakota to northwest Minnesota.

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

Day 3...

...Upper Midwest...
Models show a well-defined surface low tracking northeast across
the upper Great Lakes Wednesday night.  As the system departs,
some additional light accumulations can be expected across
northern Minnesota and the northeast corner of North Dakota, with
WPC probabilities indicating a Slight Risk for additional
accumulations of 4-inches or more.

...Central Rockies...
A shortwave trough digging south across the Pacific Northwest and
northern California before shifting east into the Great Basin will
introduce another chance for mountain snows, with some locally
significant accumulations possible, especially along the northern
Utah into the south-central Wyoming and central Colorado ranges on
Thursday.

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

Pereira