Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
315 AM EST Fri Jan 25 2019

Valid 12Z Fri Jan 25 2019 - 12Z Mon Jan 28 2019

...Great Lakes...

...Days 1-3...

An active period of lake enhanced and lake effect snow owing to
cyclonic flow will continue across the Great Lakes with episodes
of lake effect snow focused on the day 1 period Friday to early
Sat.  Recent radar/satellite trends show bands of lk effect snow
off Lake Superior and also Lake MI into northeast lower MI and
southwest lower, where longer cross-lake trajectories exist.  A
band of forming along the major axis extending down lake Erie and
that portends a band of heavy snow at the east end of the lake
from northwest PA across southwest NY to the Buffalo area, where
several inches of snow are possible today into Sat.

On Sun, amounts and coverage start to taper as the models forecast
drier air aloft to cross from the upper MS Valley into the upper
Lakes, so the snow off Lake Superior may taper after several
additional inches occur in the eastern UP of MI. The drier air
also is forecast to make it down into Lake Erie on Sun with the
primary snow off Lake Ontario into the Tug Hill, where drier air
arrives Sun night.


...Northern Plains to upper MS Valley and adjacent Lake MI...
...End of Day 2 through Day 3...

An upper level shortwave and 700 mb jet maxima cross from the
Canadian Prairies Sunday and into the northern Plains.  700 mb
moisture advection and convergence along a strong baroclinic zone
will combine with upper divergence maxima to cause to snow to
develop and continue as the combination moves downstream from ND
across northeast SD , southern MN to northeast IA, southern WI,
and possibly as far as southern lower lake Michigan and northern
IL.  Strong low-mid level frontogenesis should result in a heavy
band of snow, with 4-8 inches expected on day 3.  Some slight
axis/orientation differences of the QPF/snow exist among the model
solutions to be resolved. 

...Ranges of central to southern MT/WY/northern CO...
...Days 1 and 3...

An upper level trough moves south across MT and WY today and CO
through this evening.  Along the path of the trough is a strong
north to northwesterly jet streak.  Mid-level forcing will combine
with the moistening column in deep northwest flow to promote
enough vertical ascent for snow in windward facing terrain from
central MT across the ranges of WY to north central CO.  Several
inches are expected in favored upslope areas.  Day 2 sees a
respite as drier air aloft advects across from northwest to
southeast. On Day 3, the next upper trough builds south across MT
into WY, accompanied by a 300 mb jet jet streak and associated
divergence maxima/700 mb convergence maxima across MT to northern
WY around 00z Mon, that then drifts south to the WY/CO border by
12z Mon.  An additional several inches of snow is expected in the
windward terrain from southern MT across WY.

...Northeast...
...Day 3...

The models forecast a persistent mid-upper level vortex over
Ontario.  Downstream the the circulation, a 700 mb broad 50 kt jet
crosses northern VT/NH across ME, with warm/moisture advection
occurring and a modest 700 mb convergence maxima.  Light snow is
expected as a result, with 2-5 inches of snow possible in northern
ME. Abundant cold air across the region allows for all snow in
interior ME. 

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

Petersen