Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
357 PM EST Sat Feb 15 2020

Valid 00Z Sun Feb 16 2020 - 00Z Wed Feb 19 2020


Days 1-3...

...Pacific Northwest to the Northern and Central Rockies...
Heavy snow is expected across the windward aspects of the OR
Cascades into the ranges of southern ID and western WY tonight and
then further south into the northern UT Wasatch and Park Range of
CO later Sun.
The potential for heaviest snow tracks the 700 mb jet and enhanced
pool of moisture and bands of moisture convergence that persist in
the favored ranges. Locally heavy accumulations remain likely for
the higher elevations of the Cascades, Blue Mountains and the
Boise Mountains across to the Tetons.  The ranges of northwest WY
should receive the heaviest amounts due to having the long
residence time of the 700-300 mb jet maxima and resultant higher
QPF of 1-2 inches liquid equivalent.  WPC PWPF continues to show
high probabilities for accumulations of a foot or more across
these areas on Day 1.

The upper jet continues Sun night across southern WY and CO, with
embedded 300 mb divergence maxima driving additional snows across
the ranges of western to central CO Sunday night and Mon. WPC PWPF
shows high probabilities for accumulations of a foot or more
across the ranges of western and southern WY and northern CO on
Day 2 as well.

The continued south drift of the 700-300 mb jet across CO to
eastern NM allows snow to occur further south on the CO front
range and foothills Mon night-early Tue.  The 12z NAM extends the
snow potential further east on to the Plains, with
other models keeping higher amounts in the elevated terrain, where
lift is aided by upslope flow. The GFS/ECMWF 700 mb convergence is
not as strong on the CO/KS Plains, thus it has lighter snow
amounts than the NAM.

...Northern Plains to the Upper MS Valley...Great Lakes/New York
and interior New England...

Low-mid level warm and moisture advection increases heading from
the mid to upper mS Valley late Sunday night into Monday,
resulting in snow increasing across northern Iowa into southern MN
and southern to central WI. A generalarea of 3-5 inches of snow is
forecast in this region through the day Mon. Difluent flow aloft
in the left jet exit region occurs in conjunction with a 700 mb
wave as itt crosses the notrherhern Plains Mon.  A band of 700 mb
convergence along the wave train supports light snows in eastern
North Dakota acorss northern MN.

On Mon night night-Tue the 2 stream merge with low pressure
crossing the Great Lakes.  Snows continue moving acorss lower and
eastern Upper MI in tandem with the low, ending as the low departs
into Canada.
The low-mid level warm/moisture advection shifts east across the
lower lakes into New York and interior New England.
Several inches of snow are possible in northern NY, VT, and NH,
extending into western ME.
Most models/ensembles show higher amounts on the order of 6-10
inches focused on the Adirondack Mountains of NY, where the
heavier snow occurs in response to the 700 mb theta-e advection
and convergence maximum downstream from the jet maxima.

For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.

Petersen