Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
428 PM EST Sun Nov 25 2018

Valid 00Z Mon Nov 26 2018 - 00Z Thu Nov 29 2018


...Great Lakes/Mid MS Valley...
Day 1...

...Blizzard conditions possible tonight into early Monday in the
Chicago metro area...

A winter storm will continue to develop over the central U.S.,
producing a swath of heavy snowfall and high winds from northeast
Missouri across northern Illinois to Lower Michigan tonight into
early Monday.  Blizzard conditions are possible at the height of
the storm tonight for some locations, including in the Chicago
area, and other portions of northern IL.  Snowfall rates of 1-2
inches per hour have been observed within mesoscale bands this
afternoon, and this should continue over northern IL and possibly
parts of southern lower MI. The snow occurs within a well defined
low-mid level frontogenesis maxima that tracks east northeast with
the cyclone.

The general trend in the models was toward a slightly more
southerly axis of snow in IL/MI/northeast MO.
Guidance has now come into good agreement on the cyclone
track/intensity.
By late Monday with the low moving into eastern Ontario, models
show only some light snows on the backside of the system, along
with some lake effect snow showers remaining across Michigan in
the lee of Lake Michigan and Lake Superior.

...Northeast/Lower Great Lakes...
Days 2-3...

The models continue to show cyclogenesis either on the mid
Atlantic coast or over northern NJ Monday evening, with the
developing cyclone lifts to the north across Long Island and New
England. The west track favors mostly rain along the coast and
transition to snow inland, and at higher elevations.
Ample moisture with strong upper forcing, along with low to mid
level frontogenesis will raise the potential for moderate to heavy
precipitation rates.  Heavy snow is possible in the higher
elevations of the NY Adirondacks, VT Green Mountains, White
Mountains of NH, and ranges of western ME.
Amounts are significantly lower in valley areas of VT and NH and
downeast ME.
The NAM and ECMWF show potential for a foot of snow from the White
Mountains to mountains of western Maine.
The cyclone weakens on day 3, with the remaining snow threat
mostly in northwest ME and winding down as Wed progresses.

On day 3 Tue night- Wed, lake effect snow showers are expected in
the lee of Lakes Erie and Ontario, with locally heavy snow in the
Tug Hill Plateau of NY and in southwest NY to northwest PA.


...Pacific Northwest to the Northern Rockies...
Days 2-3...

Models continue to show accumulating ice within the mountain
passes and valleys of the northern Washington Cascades on Monday
into Mon evening.  While southwesterly winds ahead of a
approaching shortwave warm the low levels, lingering cold air
supported by easterly boundary layer winds is expected to produce
some freezing rain, with WPC probabilities continuing to show a
low risk of a quarter inch accumulation on both day 1 and day 2
within the region. 

On Tuesday into early Wednesday, low level convergence out ahead
of the 700 mb trough combines with upslope flow to produce several
inches of snow in the Blue Mountains of OR.  As the wave continues
further inland Tue evening into early Wed., snow continues in the
ranges of southern ID, with heavy amounts possible at higher
elevations of northeast Oregon into the central Idaho ranges, as
shown by the WPC probabilities. A respite is expected to develop
Wed with the departure of the 700 mb wave and dry air advection
aloft.

The probability of 0.25 inches of ice accumulation on day 3 is
less than 10 percent.

Petersen