Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
338 AM EDT Fri Nov 02 2018

Valid 12Z Fri Nov 02 2018 - 12Z Mon Nov 05 2018

...Northern and Central Rockies...

Days 1-3...

High elevation snow is expected in the Northwest and northern
Rockies.  A shortwave trough crosses the Pacific NW Friday.
Mid-level vorticity advection and upper level divergence maxima
combine with upslope enhancement to produce lift into the ranges
of northwest WY and adjacent MT/ID, with several inches of snow
likely. The mid-level wave moves out of WY into CO and resulting
in snow developing in the ranges of northern CO Fri night.

The arrival of the next upper level jet max Sat night to Sun
morning brings another surge of moisture and lift, so snow is 
expected to redevelop in the ranges of northwest WA across western
MT and ID, supported by 300 mb divergence maxima in the right
entrance region of the upper level jet.  The snow eventually
reaches northwest WY by Sun morning.

Strongly confluent flow over the Pacific northwest and Rockies
leads to both a powerful upper level jet forecast to be 130-150 kt
and a large area of 700 mb relative humidity over 90 percent and
vertical velocity maxima extending in the left exit region of the
elongated upper jet across the northwest to Great Basin, producing
snow from the ranges of ID and western WY to northwest CO on
Sunday/Sunday night. Several inches to a foot of snow is possible
in the windward terrain from ID to western WY and western Co. The
00z ECMWF shows a multi-day total of 18-24 inches in the ranges of
northwest WY.


...Northern Plains and Upper MS Valley...

Days 1-3...

An Alberta Clipper which will move steadily southeast into the
northern Plains on Friday night and Saturday. 700mb warm advection
ahead of this feature will provide moisture and lift into the
Dakotas, which will wrap into the developing comma head north of
the mid-level low center.
There are is more confidence on longer duration temperatures cold
enough for snow in far northeast MT to the ND border.  There are
moderate to high probabilities for a stripe of 4 inches or more of
snow across northeast MT into northwest ND late Friday through
Friday night.

Further south in central ND, Antecedent temperatures are likely
too warm for snow, with cooling allowing a change over from rain
to snow. The disturbance moving southeast out of the northern
Plains toward the upper MS Valley should end the threat for the
northern Plains Sat night.

The initial disturbance is forecast to gradually decay, with a new
low forming somewhere near the IA/MO border 12z Sun that moves
north to WI and possibly western Lake Superior by Mon morning. 
Mid level ascent allow precip to develop in advance of the low
with snow accumulations possible in the arrowhead of MN and
northern WI late Sun-Sun night.

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

Petersen