Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
331 AM EST Mon Feb 24 2020

Valid 12Z Mon Feb 24 2020 - 12Z Thu Feb 27 2020


...Northern Rockies/Northern Plains...
Day 1...

A mid-upper level trough moves east of out of the northern Rockies
today and crosses the northern Plains.
Confluent flow produces a low level frontal boundary with
deformation/convergence near the boundary as it stretches across
northeast MT to western SD.  Low level northwest flow produces
upslope conditions in windward terrain of the Black Hills, where
locally heavy snow is expected. Another maxima is anticipated in
the Lewis range of northwest MT/Glacier National Park, where
persistent moisture and upslope flow support a period of snow
before the upper trough passes.

Another maxima is expected over south central North Dakota.  The
NAM and GFS show a 700 mb circulation possibly developing in the
frontal zone over northern SD with low level convergence focusing
north of the low in southern North Dakota.
The high res windows/NAM/HRRR show enhanced QPF and snow in south
central ND, where several inches of snow may occur during the day.
 The snow should taper tonight as the low level convergence signal
fades and drying aloft commences.

...Upper Midwest/Great Lakes...
Days 2-3...

A merger of northern stream and southern stream impulses will lead
to a deepening and slow moving circulation crossing from the
Plains thru the mid-upper MS Valley and into the Upper Great Lakes
and midwest on day 2.
A band of moderate to heavy snow is expected to develop in the mid
level frontogenesis and deformation zone, but the models start
have significant spread and have made run to run changes on the
location and axis of heaviest snow.

The guidance has shown a southward trend tonight, so the threat
for heavy snow in central IA to central WI is easing, it is
increasing in northern IL to southern lower MI.  Locally heavy
snow is possible on the SW lake shore of Lake Michigan where
enhancement is possible due to lake enhancement from cross lake
flow and lee shore convergence.

The day 3 snow is focused where the 850-700 mb low crosses MI into
adjacent Canada, with the synoptic snow winding down as the system
departs.  Lake enhanced snows may persist in the eastern UP of MI,
northwest lower MI, and northern Indiana.
Less weight was given to the GFS and  more to the NAM for better
continuity and agreement with the dominant cluster of solutions
for the cyclone track.

 Northeast...
Day 3...
As the upper trough crosses the Great lakes, the upper jet maxima
moves up the spine of the Appalachians, with well defined 300 mb
divergence maxima crossing PA/NY into interior New England.  This
combines with low-mid level warm advection to produce widespread
precip across the northern mid Atlantic to the northeast Wed. 
The combination of synoptic lift and upslope flow into the NY
Adirondacks, the VT Green Mountains, and NH White Mountains to the
ranges of interior Maine leads to potential for several inches of
snow at these sites.
The warm advection limits snowfall potential at lower elevations.

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

Petersen