Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
356 AM EST Mon Nov 05 2018

Valid 12Z Mon Nov 05 2018 - 12Z Thu Nov 08 2018

...Northern and Central Rockies...

Days 1-3...

Areas of heavy snow will fall across much of the mountainous
terrain from the northern Cascades of Washington through the
ranges of ID and western MT, due to a  prolonged period of
mid/upper level confluent and moist flow from the Pacific, with
lift provided from orographics plus periods of upper divergence
and enhanced layer relative humidity.
3 day Snowfall totals in the highest peaks of Idaho, western
Montana, and northwest WY may reach 2 feet by Wednesday night.

On day 1 (Monday)the heaviest snow is expected in the ranges of
ID, where the ECMWF shows stronger  700 mb ascent.  Another area
of locally heavy snow is forecast in the ranges of western MT,
where increasing frontogenesis along a stalled cold front banking
against the mountains will further enhance lift.  WPC
probabilities are moderate for 8 inches or more of snow.

On Tuesday into Wednesday, a persistent trough over the northern
Rockies favors continued enhanced layer relative humidity, with
sporadic 700 mb ascent coinciding with 700 mb convergence maxima
in the ranges of ID and western MT combines with low-mid level
frontogenesis in western MT to support several additional inches
of snow.
The highest accumulations are favored in the high terrain of the
Bitterroots, and ranges of western MT, where WPC probabilities
feature a slight to moderate risk for 8 inches or more. 

...Days 1-3 Upper MS Valley/Western Great Lakes...

A 700 mb circulation and associated low level cold front will move
southeast across the upper MS Valley and adjacent upper Great
Lakes on Tuesday and Wednesday, bringing an increase in 1000-500mb
RH, and weak mid level cyclonic vorticity advection to provide
modest ascent. There exists a likelihood for light snow
accumulations from northern MN into WI and the UP of MI, and even
a slight risk for 4 inches.

On Wed, the snow accumulations are focused to the lee of Lake
Superior in the UP of MI, as west northwest boundary layer winds
cause lee shore convergence. The convergence produces ascent,
which is further aided by steepening lapse rates in the low level
cold advection as the GFS indicates 850 mb temperatures dropping
below -10C. The 00z NAM shows a quarter inch liquid equivalent,
indicating potential for 4 inches of snow in the western UP of MI.

...Day 3 eastern CO into western KS/southwest NE...

The models indicate a 700 mb wave develops as it moves across
eastern CO into KS Wed night to Thu morning.  An area of enhanced
moisture and 700 mb theta-e advection and convergence develops as
the wave approaches and associated mid level ascent produces
precipitation.  Where it is cold enough on the northern edge of
the precip shield light snow is expected to develop in eastern CO
to western KS or southwest NE.  Precip type uncertainty grows with
precip likely mixed in southern KS, cutting down on potential
amounts. Additionally, the GFS is on the light edge of the QPF
spectrum, and the 00z UKMET again the heaviest, so there remains
QPF uncertainty as well with a quarter to half inch spread amongst
the forecast liquid equivalent solutions.

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

Petersen