Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
345 PM EST Tue Nov 6 2018

Valid 00Z Wed Nov 7 2018 - 00Z Sat Nov 10 2018

...Northern Rockies...

For the Day 1 period (Tuesday night into Wednesday), a shortwave
impulse tracks towards the southeast and enhances lift and
increases low level RH, along with a strong upper jet and low-mid
level frontogenesis in place to aid in ascent and produce several
inches of snow.  The highest accumulations are favored for the
high terrain of the Bitterroots, and ranges of western Montana and
northwest Wyoming, where WPC probabilities feature a moderate to
high risk for 4+ inches.  The heaviest snow is expected for the
first 12 hours of day 1 through early Wednesday.  A drier weather
pattern returns by Thursday for this region with just a few
lingering snow showers remaining for the Absarokas and Big Horn
mountains.


...Minnesota to northern Michigan...

An intensifying surface low lifting towards the northeast across
eastern Ontario and western Quebec, with a surface trough
extending southwestward across the northern Great Lakes, is
expected to produce snow showers to the northwest of the surface
low through Wednesday.  This will mainly affect parts of northern
Wisconsin and the western Upper Peninsula of Michigan, where 1 to
3 inches of snow appears likely.  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.


...Central plains to the Great Lakes...

A mid level impulse tracking southeast around the base of the
large scale trough over the north-central U.S. is expected to
produce a band of accumulating snow from eastern Colorado to
northern Kansas and southern Nebraska.  Although no well-defined
surface low is expected with this, there should be enough
mid-upper level forcing to support a 6 to 12 hour period of light
to moderate snow.  Easterly low level flow will result in an
upslope component and further aid with lift.  Snowfall rates may
be locally enhanced owing to some elevated instability and steeper
lapse rates, promoting more vigorous lift in the dendritic growth
zone.  There are multiple EC and GEFS ensemble members on board
with the potential for 4 inches or more of snowfall, with the
greatest consensus for this across the northern third of Kansas. 
WPC probabilities are low to moderate for 4+ inches of snow during
the Day 2 period Thursday. 

By Thursday night, the disturbance tracks out of the central
Plains across the Midwest states and then northeast towards the
Great Lakes by Friday afternoon.  A region of mid-level ascent is
expected in the moisture plume from Kansas, across northern
Missouri, and then over northern Illinois and southern Wisconsin. 
Temperatures will be cold enough for light snow on the northern
edge of the precip shield, on the order of 1 to 2 inches in most
cases.  A few ensemble members are indicating the potential for 4+
inches of snow across northern Wisconsin and the northern half of
Michigan on Day 3, and the WPC probabilities remain low for 4+
inches of snow during the Day 3 period Thursday night into Friday
morning.  

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

D. Hamrick