Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
330 AM EST Thu Dec 15 2022

Valid 12Z Thu Dec 15 2022 - 12Z Sun Dec 18 2022

...Significant winter storm with blizzard conditions and
treacherous icing to transition from portions of the High
Plains/Upper Midwest to the interior Mid-Atlantic/Northeast...


...Northern/Central High Plains through the Upper Midwest...
Days 1-2...

A powerful winter storm continues to unfold across the Northern
Plains and Upper Midwest today as a strong area of low pressure
dawdles over the region. The storm system, from the surface to the
upper levels, will become "vertically stacked" or aligned
throughout the column of the troposphere by this afternoon. Once
this happens, the storm will gradually weaken heading into the
overnight hours Thursday. Until then, bands of heavy snow and
occasional whiteout conditions from a combination of 1-2"/hr
snowfall rates and whipping wind gusts will continue to cause
significant impacts to travel. Latest PWSSI shows a 50-60% chance
for "Moderate" impacts from eastern ND and northeast SD to
northwest MN. WPC PWPF showed 60-80% probabilities for >6" of
snowfall in these same areas. One other area that has similar odds
for >8" of snowfall on Thursday is the MN Arrowhead where there is
a sliver of 40-60% probabilities over an area that has already
picked up over a foot of snow. The combination of strong vertical
forcing along the approaching occluded front, as well as on shore
winds off Lake Superior, will support heavy snowfall from Duluth
on north and east. The same occluded front will also foster >1"/hr
snowfall rates over Michigan's U.P. and northern Michigan. The
burst of heavy snow will be only last a couple hours, but could
still fall heavily enough to warrant hazardous travel conditions
(PWSSI shows 50-80% probabilities for Minor impacts in northern MI
and the U.P. of MI). Lastly, the Black Hills of SD can also expect
heavy snow with the latest WPC PWPF displaying up to a 40-50%
chance for >8" of snowfall on Thursday.

The storm system remains cut-off from the mean flow pattern
through Thursday night and will remain and nearly the same spot in
the Upper MS Valley by early Friday morning. Despite snowfall
rates gradually decreasing, gusty winds could may still generate
blowing snow in parts of the Northern Plains. In fact, the
experimental PWSSI does show 30-40% chances for Minor impacts due
to blowing snow in central North Dakota. The next upper trough
diving into southwest Canada will finally help to push the storm
system into the Greta Lakes by Saturday, but through cold air
advection on the storm's backside, lake effect snow bands may
develop in the favored downwind areas of Michigan's U.P., western
MI, and across northwest PA and western NY by Saturday evening.
WPC PWPF does show some areas of 50-70% probabilities of snowfall
>4" through both Friday and Saturday.


...Central Appalachians through New England...
Days 1-3...

...Ice...

Downstream of the large winter storm in the Midwest is its
associated slug of moisture that is tracking into a sub-freezing
air-mass over the Northeast. This column of sub-freezing air is
anchored in at low levels by the combination of a dome of high
pressure in Quebec and a slow moving cyclone southeast of Nova
Scotia. Not only are temperatures sub-freezing, but dew points are
quite dry as well (low-mid 20s from northern VA and central MD on
north to New England). While the atmosphere will take time to
moisten up, eventually the atmosphere will cool to its wet bulb
temperature which, just along and west of I-95, will drop to, or
below freezing. By early Thursday AM, 290K isentropic ascent will
be strong throughout the Mid-Atlantic as the steady push of moist,
southerly air overruns the cold air damming signature at the
surface in the Mid-Atlantic. The result is periods of
sleet/freezing rain and significant ice accumulations,
particularly from the Blue Ridge and central Appalachians on north
through the Laurel Highlands, where sub-freezing surface
temperatures are to be locked in the longest. Latest WPC PWPF for
>0.25" of freezing rain were as high as 20-30% in the central
Appalachians of northeast WV and Garrett County, MD.

It is also worth noting the expansive foot print of >0.1" of ice
with probabilities ranging from 50-70% from the Shenandoah Valley
of western VA on north through the Laurel Highlands and into
western NY. There are also 50-70% probabilities for >0.01" of ice
along and west of I-95 from the Delaware Valley on south through
the VA Piedmont and into southwest VA, so there could be slick
travel conditions for these areas during the Thursday AM commute.
Eventually, the warm nose at low levels will be too strong and
change precipitation over to plain rain for all western I-95
northern Mid-Atlantic areas by Thursday afternoon.

...Snow...

Farther north, the same source of lift will and resulting
precipitation falling as snow across north-central PA and into
much of interior NY. WPC PWPF shows probabilities of snowfall
totals >6" between 50-70% in far northern PA and in the Catskills
on Thursday. However, unlike their neighbors to the south, the
warm nose via strong southerly flow at 850mb will begin to pivot
more out of the ESE by Thursday night. This is due to a developing
surface low along the Mid-Atlantic coast which is co-located
beneath the diffluent left-exit region of a 150kt jet streak over
the South Central U.S.. This favored area of vertical ascent aloft
makes its way over the Northeast on Friday, while at the same
time, easterly 850mb moisture transport will direct a conveyor
belt of Atlantic moisture towards the northwest side of the 850mb
low along the Jersey shore. With the lingering dome of high
pressure still anchored over southeast Canada, it will be the
elevated terrain of the northern Appalachians and other mountain
ranges closer to the I-95 corridor (Catskills, Poconos, Greens)
that receive the heaviest snowfall. Due to the coastal and 850mb
low's close track along the coast, it will be too mild due to
oceanic influence for I-95 cities from Boston on south to receive
snowfall. Heavy snow will continue in the interior Northeast
Friday night into Saturday as the low tucks into the Gulf of
Maine, where stronger winds could result in whiteout conditions
across northern New England at times.

WPC's WSSI over the next 3 days shows "Major" impacts from the
Adirondacks and across much of southern VT/NH/ME, with some spotty
"Extreme" areas in VT's Green Mountains and the Catskills. The
primary drivers in the WSSI are snow amount and snow load, which
considering the NAEFS tool shows mean specific humidity levels in
the 500-700mb layer above the 90th climatological percentile,
shows the amount of anomalous moisture available within the DGZ.
Travel will be treacherous, if not impossible, in these areas
Thursday night into Friday over the interior northern
Mid-Atlantic, then into interior New England by Friday afternoon
into the day on Saturday. WPC PWPF indicates high probabilities
(>70%) in the Catskills, Adirondacks, Green, and White Mountain
ranges with 40-60% odds of seeing >12" of snowfall.

Key Messages for December 9-17 Winter Storm:
--Blizzard conditions will continue today for portions of the
Northern Plains and Minnesota Arrowhead, where 1-2"/hr snow rates
and winds gusting 45-60 mph create near zero visibility and snow
covered roads. Expect difficult to impossible travel, scattered
power outages, and harsh livestock conditions.

--Heavy snow and gusty winds will also push across the today. 
Blowing and drifting snow will create difficult travel due to near
zero visibility and snow-covered roads.

--Farther east, moisture out ahead of this storm system is
streaming north into the Mid-Atlantic this morning, before
reaching the interior Northeast Thursday night and through Friday.

--An icy wintry mix, including sleet and freezing rain is expected
for parts of the Central Appalachians and northern Mid-Atlantic.
Localized ice accretions may reach 0.25†to 0.50â€, causing
dangerous travel conditions, tree damage and scattered power
outages.

--Heavy snow accumulations will occur across the interior
Northeast, first by this initial wave of moisture today and
tonight, then from a developing coastal storm on Friday. Expect
significant travel disruptions and impacts to infrastructure for
Friday and Saturday.

Mullinax