Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
313 AM EST Sun Nov 04 2018

Valid 12Z Sun Nov 04 2018 - 12Z Wed Nov 07 2018

...Northern and Central Rockies...

Days 1-3...

A strong upper level jet and moist confluent flow leads to periods
of snow across the mountains of the northern WA inland across the
ranges of ID MT, and WY. Jet-level diffluence and upslope flow
will allow for widespread 1-2 feet of snow in many of the mountain
ranges, with the highest amounts likely in the Absarokas, Grand
Tetons, and Bitterroots.

The first shortwave and associated jet streak will drop across the
northern Rockies on day 1 before shifting eastward into the Plains
on day 2. The heaviest snow associated with this feature will be
across Idaho, northwest Wyoming, and western CO, where WPC
probabilities are high for 8 inches of snow, with 12 inches likely
in the Grand Tetons.

On day 2 (Mon),  the persistent west-northwest jet crossing from
the Pacific northwest to the northern Great Basin favors enhanced
moisture with forecast high 700 mb relative humidity extending
across the ranges of ID to western MT, where the UKMET has
persistent 90 percent plus RH.  Consequently, the ranges of
central ID and adjacent western MT are targeted for several inches
of snow.
  
On day 3 (Tue),  this round of snow continues where the best
combination of difluence and upslope enhancement come together
across the Bitterroots of Idaho southeast towards the ranges in NW
Wyoming. An additional 4-8" of snow is possible in the highest
terrain.

...Western Great Lakes/Upper MS Valley...

Days 1-3...

An area of low pressure will strengthen as it moves northeast from
near the Iowa/MO border this morning towards Lake Superior Monday
morning.  Precipitation will expand and overspread the area from
the south on increasing warm and moist advection and
frontogenesis, and temperatures initially will be cold enough for
snow. 850-700mb temps will climb above 0C and the precipitation
will gradually change over to rain across Wisconsin and the U.P.
of Michigan, with snow persisting only across the Arrowhead of MN.

The most likely location for 4 inches of snow or more will be
across the Arrowhead of MN which will experience a pivot of
mid-level frontogenesis as the negatively tilted low slows down,
and it is in this region where WPC probabilities are moderate for
4 inches of snow and low for 8 inches of snow. Elsewhere, 2-4" is
possible across far northern Wisconsin and into the U.P. of
Michigan before precipitation type transitions to rain.

A lull in between systems is expected on day 2.  On day 3 (Tue),
the enhanced moisture in
the mid level trough and possible embedded closed low with
periodic modest ascent leads to light snow across northern MN and
WI into the UP of MI. A few inches of snow is expected.
A few ensemble members of the GEFS show potential for 4 inches of
snow, so a low risk is indicated.

The probability of significant icing is less than 10 percent days
1-3.

Petersen