Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
305 AM EDT Thu Nov 01 2018

Valid 12Z Thu Nov 01 2018 - 12Z Sun Nov 04 2018

...Northern and Central Rockies...

Days 1-3...

On Day 1 (Thu), a low level warm front crosses MT and WY, with
periods of warm/moist advection and 700 mb convergence supporting
several inches of snow where these conditions overlap in the
ranges of northwest WY, with help from lift from orographics in
windward terrain. A secondary max is forecast in the Bighorn
Mountains of northeast WY.

The next in the series of 700 mb waves moves across MT/ID to the
ranges of northwest WY on day 2 (Fri).  Several more inches of
snow is expected across the ranges of northwest WY to the MT
border as mid level vorticity advection, and upper level
divergence maxima combine with upslope enhancement to produce
lift. The combined day 1 and day 2 events provides 2 day potential
for a foot of snow in favored terrain of northwest WY.
The mid level wave is progressive, moving out of WY into CO and
resulting in snow developing in the ranges of northern CO Fri
night.

The departure of the upper trough onto the Plains Sat provides a
respite for the northern Rockies, but the arrival of the next
upper level jet max Sat night to Sun morning brings another surge
of moisture and lift, so light snow area expected to redevelop in
the ranges of western MT and ID, supported by 300 mb divergence
maxima in the right entrance region of the upper level jet.
Additional light snows are expected on the nose of the upper level
jet in the northern WA Cascades.

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


...Northern Plains...

Day 2/3...

An Alberta Clipper which will move steadily southeast into the
northern Plains on Saturday. 700mb warm advection ahead of this
feature will provide moisture and lift into the Dakotas, which
will wrap into the deformation zone/developing comma head north of
the mid-level low center. 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
towards the upper MS Valley should end the threat for the northern
Plains Sat night.
WPC probabilities show a slight risk for a stripe of 4 inches or
more of snow across northwest ND to adjacent northeast MT.


Petersen