Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
349 AM EST Tue Nov 06 2018

Valid 12Z Tue Nov 06 2018 - 12Z Fri Nov 09 2018

...Northern Rockies Days 1-2...

Multiple mid level perturbations and maxima of 700 mb convergence
combine with lapse rates of 7-8 degrees c/km within northwest flow
aloft through Wednesday to produce widespread mountain snow
showers from the Bitterroots of central-northern Idaho, to the
ranges of western MT and northwest WY, with lesser amounts for the
WA Cascades.  On day 1 (Tuesday) the WPC probabilities are
moderate for 8 inches or more of snow for the Bitterroot
mountains.

For the Day 2 period (Wednesday), another shortwave impulse tracks
towards the southeast and enhances lift and increases low level
RH, along with a low-mid level frontogenesis to aid in ascent and
produce another round of light to moderate snow.  The
accumulations are favored for the windward terrain of the
Bitterroots, and ranges of western Montana and northwest Wyoming,
where WPC probabilities feature a moderate to high risk for 4+
inches.  A drier weather pattern returns by Thursday with most of
ID seeing 700 mb relative humidity drop below 70 percent, with
drying spreading across western MT as Thu progresses.  Light snows
may linger into Thu for the Absarokas and Big Horn mountains.


...Minnesota to the western Great Lakes Days 1-2...

A mid-level impulse and cyclonic vorticity advection maxima
produce light snow across eastern ND into northern MN ,followed by
northern Wisconsin and the western Upper Peninsula of Michigan,
where 1 to 3 inches of snow appears likely.  On Wed.,
northwesterly flow across Lake Superior will provide some lake
enhancement as steeper lapse rates develop owing to cold air
advection.  Lee shore convergence will also provide lift over this
region, and WPC probabilities are slight for 4+ inches.  On day 3
the light snow pivots to the arrowhead of MN and Lake Superior
downstream from the next amplifying mid level wave.

...Central Plains on Day 2 and mid MS Valley to Great Lakes Day
3...

On day 2 (Wed), A mid level impulse is expected to produce 700 mb
moisture/warm advection, resulting in a band of accumulating snow
from eastern Colorado to northwest Kansas and southwest NE. 
The models still differ on the amplitude of the wave and resultant
QPF and snow amounts.There are multiple EC and GEFS ensemble
members that are indicating the potential for 4 inches or more of
snowfall spread across northeast CO to northwest Kansas and
southwest NE.

On day 3 (Thu), the wave tracks out of the central Plains across
the mid MS Valley and then northeast towards the Great Lakes.  A
region of lift occurs in the moisture band between 700-600 mb out
of KS across northern MO and then into northern IL. Temperatures
will be cold enough for light snow on the northern edge of the
precip shield.
A minority of ensemble members indicate potential for 4 inches,
but without much overlap.
WPC probabilities remain low for 4+ inches of snow during the Day
3 period Thursday.  

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

Petersen