Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
438 PM EDT Thu Oct 04 2018

Valid 00Z Fri Oct 05 2018 - 00Z Mon Oct 08 2018

Day 1...

...Mountains of south central MT, western WY and northern UT...
Upper level trough forecast to be over NV 12z Thu moves steadily
northeast into the northern Plains. Low-mid level moist advection
ahead of this feature will combine with upper divergence maxima
produce mountain snows across the ranges of northern UT and
western WY early Thu before the wave departs on to the Plains. 
WPC probabilities show the potential for several inches of snow in
the highest peaks of the Uinta and the Wind River ranges. A
northern stream trough crossing MT does likewise, with upper
divergence driving a period few snow at higher elevations briefly
in southwest MT and then across south central MT to the WY border.
 
...Northern Plains... 
As the filling shortwave lifts northeast into the Plains, mid
level warm/moist advection develops an leads to snow developing
across the Dakotas into MN.  The 18z NAM had heavier snow in MN
thanks to strong 300 mb divergence in the right entrance region of
an upper level jet streak across northern MN to the upper Great
Lakes.
The probability for snow is highest across south-central North
Dakota where the column will initially be entirely below freezing
before slowly warming.
 
The probability of significant icing is less than 10 percent.

Day 2...

...WY/CO Rockies... 
The next system in the series of upper troughs crossing the
intermountain west moves east from the Great Basin across WY and
CO. As an upper jet maxima crosses CO, upper divergence in the
left exit region favors lift across much of southern WY and Co. As
Fri progresses the wave continues east and deamplifies as it moves
on to the Plains, limiting both the duration and intensity of
snow. 

...Ranges of Wa/Or...
The next upper trough is forecast to move slowly southeast from
the Pacific Ocean towards the Pacific northwest.  Moisture return
ahead of the upper trough combined with upper divergence in the
left exit region jet leads to snow showers in the Wa Olympics and
Cascades, eventually reaching the Blue Mountains.

...Northern Plains...
The area of low-mid level warm/moist advection moves quickly out
of the Dakotas and MN into the Canadian provinces, so the snow
should taper with only minor additional accumulations due to the
steady forward progress of the upper trough.

The probability of significant icing is less than 10 percent.

Day 3...

...Ranges of the Great Basin to the northern and central Rockies...
An amplifying upper trough will depart the Pacific northwest and
cross the northern Rockies and Great basin.
Areas downstream from the trough are expected to be in favored
couplets of 70 mb convergence and 300 mb divergence, leading to
periods of snow in the ranges of ID and then moving into southern
MT, western WY and northern UT.
Several inches of snow are possible where longer duration high
layer relative humidity and lift occur in the ranges of northern
UT across western WY and then moving into western CO Sat night.

The probability of significant icing is less than 10 percent.

Petersen