Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
234 AM EST Sat Jan 11 2025

Valid 12Z Sat Jan 11 2025 - 12Z Tue Jan 14 2025

...Central Appalachians...
Day 1...

Exiting storm off the Mid-Atlantic will be closely followed by the
mid-level shortwave, helping to wring out a few inches of snow
over the central/southern Appalachians (but mainly central WV
northward to the Laurel Highlands) this morning that will diminish
later this evening.


...Northern/Central Rockies to the Upper Midwest/Great Lakes...
Days 1-3...

Upper trough that entered the PacNW yesterday will continue
through the Rockies today as a positively-tilted trough with
multiple embedded vort maxes along its axis. This will yield a
broad swath of light to moderate snow over much of the Rockies
today from central Idaho into Montana southward to the CO Rockies
along/ahead of the cold front. Favored areas on D1 for snow include
the Little/Big Belts, Big Snowy Mountains, and into the Bighorns
where WPC probabilities for at least 8 inches of snow are >50% with
the higher peaks likely receiving >2ft of snow through D2.

To the east, the northern extent of the upper trough will carry a
clipper system out of Canada with an area of low pressure tracking
across central ND to southern MN by this evening. Light to perhaps
modest snow is forecast around the low, focused via WAA and beneath
some upper divergence on the northern side of the system from the
Red River Valley eastward across northern MN. QPF should be near
and below 0.25" but with a deeper DGZ snow ratios should be >15:1
which should yield an area of 3-4" between Fargo and Duluth where
WPC probabilities for at least 4 inches D1 are >20%. Low pressure
will turn the corner northeastward across northern WI and across
the western U.P. into Quebec, favoring southerly flow into the
eastern U.P. along with compact convergence over the western
portion of Lake Superior which will favor the Keweenaw Peninsula as
well as Isle Royale along a sfc trough axis. Lighter snow is
expected southward and eastward across much of the Great Lakes
though there could also be some enhancement along the western side
of Lower Michigan via land/lake sfc convergence. WPC probabilities
for at least 4 inches of snow D2 are >50% over the aforementioned
favored areas that also includes the North Shore from Duluth up to
Grand Portage where there are >50% probabilities for at least 8
inches of snow.

By D3, a new upper low out of central Canada will drop southward
into MN and expand across the Great Lakes, maintaining cyclonic
flow across the region with cold 850 temperatures (-24C at the core
of the upper low and -10C dropping to -15C into the Tug Hill by
the end of the period. This will support widespread lake-effect
snow on general northwesterly to westerly flow that favor modest
snows over the typical lake belt areas. Amounts may be light/modest
(several inches) with some locally higher amounts along the
northern coast of the U.P. and also east of Lake Ontario.

Lastly, on the west side of the incoming cold upper low D3,
additional vorticity will stream southward across the northern
Plains which will carry light snow over eastern MT
south/southeastward that could accumulate several inches over the
Black Hills thanks to some favorable upslope.


For the Day 1-3 period, the probability of significant icing of at
least 0.10" is less than 10 percent across the CONUS.


Fracasso