Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
335 AM EST Sun Feb 9 2025

Valid 12Z Sun Feb 09 2025 - 12Z Wed Feb 12 2025

...Great Lakes and Northeast...
Days 1-2...

A significant winter storm will wind down this morning across New
England as an area of low pressure pulls away from southern New
England leading to rapid drying and renewed CAA. Light snow should
finally come to an end along coastal regions by about 16z or so.
However, the CAA behind the accompanying cold front will setup an
environment favorable for LES E/SE of Lake Ontario and SE Lake
Superior. Shifting winds (more N/NW early, becoming more W D2 and
weakening by D3) will result in a shifting band of LES. Latest
Great Lakes ice analysis shows Lake Ontario remaining mostly ice
free with surface water temperatures around 3 to 4 degrees C. For
D1, the most significant accumulations are likely in the northern
Finger Lakes region, where they have a 70-90% chance of exceeding
4" near the lake shore, with D3 probabilities for 4+ inches
reaching 50-70% focused in the Tug Hill Plateau. Additionally,
probabilities for 4+ inches are 30-50% D1-2 across the Keweenaw
Peninsula and near Whitefish Point in the U.P. of MI.


...Central Plains through the Mid-Atlantic...
Days 2-3...

The next in this parade of systems across the CONUS begins to
develop D2/Monday as a shortwave trough ejects from near Baja and
then moves east into the Southern Plains by Tuesday evening. This
shortwave will remain embedded within a longwave trough axis as it
moves east through its evolution, but 00z guidance has trended
slightly more amplified with the longwave pattern. This subtle
change
can be traced to a more consolidated western trough and faster Pac
NW shortwave that pumps up the eastern U.S. ridge ever so slightly
in order to prompt more southwesterly upper flow as opposed to
straight zonal winds. This trend, should it persist, would give the
impressive IVT (up to the 97.5th climatological percentile per the
12z NAEFS) the ability to bring the precipitation shield a bit
farther north across the Ohio Valley and Mid-Atlantic, where snow
is the dominant ptype. Additionally, upper diffluence will likely
increase with time as a downstream jet streak intensifies towards
150kts and arcs sufficiently poleward while leaving its tail across
the Central Plains and into the Lower MS VLY by Tuesday evening.
This will provide additional deep layer ascent as the system
organizes over the Mid-Atlantic through D3 and promoting a longer
duration moderate to heavy snowfall. Snow ratios for a majority of
the event are expected to remain around climo or slightly below
(8-11:1) due to an elevated DGZ and strong winds through it. So,
the main question remains how far north and how heavy QPF is should
the more amplified trend continue.

Light snow is expected to develop first across the central Plains
and Mid-Mississippi Valley on D2 as the upper jet begins to
intensify over the central U.S., which then begins to quickly
expand eastward into the Mid-Atlantic and central Appalachians on
D3.

WPC probabilities at this time for snowfall are highest for 4+
inches in the higher elevations of WV/VA where they peak above 70%,
and locally 6+ inches of snow is possible. Elsewhere, WPC
probabilities for 4" have increased and reach 40-60% across
central/northern VA, MD, and DE. Probabilities for at least 2" are
low 10-20% across the central Plains and Mid-Mississippi Valley,
but increase to above 70% across eastern KY and western WV.

There also exists the potential for an area of mixed ptype
associated with this system, one over the Ozarks of AR and a more
impressive area extending from western NC to southwest VA. The set
up supports a potentially significant ice storm in the Piedmont
and Foot Hills of the central/southern Appalachians where a
classic CAD setup and overrunning precip combine to create an
environment ripe for freezing rain. Large high pressure spanning
across the northern and eastern U.S. through Tuesday will
consolidate and reposition over southern Quebec by early Wednesday.
As a constant flow of precipitation remains aimed at the southern
Appalachians, cold northerly flow at the surface also continues
along and just east of the Blue Ridge Mts. while modest mid-level
south-southwesterly flow brings the warm nose into central VA. As
a result, WPC probabilities for at least 0.25" of ice have
increased to around 20-40% from northwest NC to much of southwest
VA east of the continental divide. Additional freezing rain is also
likely during the period following D3, with moderate- level WSSI-P
values up to 40-60%. Lower probabilities of 10-30% for at least
0.1" of ice exist across the AR Ozarks.


...Central Rockies and Central Plains...
Day 3...

Another winter storm will quickly develop by the end of D3 (Tuesday
night) across the central Rockies/Plains as the aforementioned long
wave trough over the West begins to lift northeastward due to
interacting shortwaves initially over the Northeast and near
northern Baja California. This interaction reinvigorates the
extended and strong jet streak arching from the Mid-Mississippi
Valley to New England, prompting optimal upper diffluence in the
central Plains. At the surface, large high pressure up to the 99th
climatological percentile per the 12z NAEFS will supply cold air
throughout much of the Plains and a cold front well south to the
Southern Plains. Thus, forcing will be mostly elevated and for the
northern parts of the precipitation shield a very deep DGZ (over
300 mb for parts of KS, NE, and IA Monday morning per the 00z GFS)
and very light winds could support snow ratios well above climo.
WPC probabilities through the end of the forecast period (12z
Wednesday) reach above 70% in the highest terrain of the CO Rockies
and are 40-70% for much of western and central KS, with the
majority of snowfall expected from this event after 12z Wednesday.


Snell