Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
409 PM EDT Mon Oct 21 2019

Valid 00Z Tue Oct 22 2019 - 00Z Fri Oct 25 2019

Days 1-3...

...Northern/Central Rockies to the Northern Plains...

A very strong area of low pressure lifting north-northeast across
the Upper Midwest this evening will cross Lake Superior by midday
on Tuesday. Thermal profiles are currently too warm to favor snow,
but later tonight and into the early morning hours on Tuesday,
there will likely be sufficient low-level cold air advection and
dynamic cooling within the comma-head/deformation zone of the
storm system to foster some of the rain mixing with or changing
over to wet snow. This will mainly be the case across west-central
to northwest MN, and locally an inch or two cannot be ruled out
before the storm system pulls away into Canada by Tuesday night.

Meanwhile, a clipper-type low center and associated shortwave
trough crosses British Columbia overnight and moves from eastern
MT to SD by Tuesday night. This system will have a fair amount of
mid and high-level Pacific moisture that will yield some locally
heavy accumulating snow for the higher terrain of the northern and
central Rockies Tuesday and into Wednesday. The heaviest amounts
will tend to be across the Tetons, Beartooth, Absaroka and Big
Horn ranges where as much as 6 to 12 inches of snow is expected by
late Wednesday. Farther to the northeast across the eastern plains
of MT and the Dakotas, there may be a stripe of some light warm
air advection snowfall which will be focused to the northeast of a
clipper low track. Only minor accumulations of as much as 1 to 2
inches are expected.

By later Wednesday and into Thursday, a portion of the energy
associated with the clipper low system is expected to amplify
southward down across CO and NM. Meanwhile, at the surface there
will be a cold front driving quickly south down the Front Range
and adjacent High Plains, reaching all the way down through
eastern NM and west TX by late Thursday. Post-frontal upslope flow
across the High Plains and eastern slopes of the Rockies will
couple with fairly strong mid/upper-level jet forcing/dynamics for
some locally heavy snowfall. The focus will be over the high
country of central CO where as much as 6 to 10 inches will be
possible through late Thursday. Some light accumulations may spill
east down along the Front Range and Palmer Divide where a few
inches of snow will be locally possible.

The probability of significant icing is less than 10 percent.

Orrison