Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
304 PM EST Fri Dec 23 2022

Valid 00Z Sat Dec 24 2022 - 00Z Tue Dec 27 2022

...Great Lakes through the Interior Northeast...
Days 1-3...

...Dynamic system to slowly lift away from the Northeast but with
lingering effects into the weekend...

Rapidly-deepening cyclone over southeastern Ontario will reach
peak intensity by Saturday morning with a broad circulation around
it over the Great Lakes, anchored by an anomalously deep upper
low. With a strong high pressure over the northern Plains, the
pressure gradient will support strong winds continuing early this
weekend over the Great Lakes, supporting a prolonged lake-effect
snow outbreak that will be accompanied by strong winds and
blizzard conditions at times. With 850mb temperatures -12C to -24C
(core of the cold across the Ohio Valley), plenty of delta T
between the still mild lakes and 850mb will maintain multi-bands
over the U.P. and Lower Michigan but intense single bands off
Erie/Ontario on SW flow. Such strong winds may limit SLRs despite
the strong lift (though the column is mostly colder than the DGZ),
but three-day amounts could still top several feet in favored
locations including very near Buffalo on rates of 2-3"/hr. WPC
probabilities for at least 12 inches over the next three days are
high over the western U.P, northwestern Lower Michigan, and over
western NY and near the Tug Hill farther north than climo due to
the flow. Snowfall will wind down starting late Sunday and into
Monday from the Upper Lakes initially and into Ontario last.


...Pacific Northwest...
Days 1-3...

...Significant to damaging freezing rain event continues...

Arctic air remains in place over much of the Pac NW as a
slow-moving warm front continues to advance inland. This front
will be driven to the east ahead of a modest mid-level shortwave
today, but will encounter shortwave ridging to the east and
in-between mid-level confluence will reinforce the surface high.
This indicates that while moisture will steadily stream eastward
into the region, the high pressure will be extremely slow to
retreat, especially as precipitation falling into it helps to
reinforce the cold surface air. However, robust WAA in the
vicinity of the elevated warm front will surge more quickly on
30-40 kts of 850-700mb wind, driving a warm nose above +0C inland.
This will help raise snow levels, and continue a p-type transition
from snow to sleet to freezing rain, and eventually rain near the
coast. Moisture will be plentiful as an atmospheric river with IVT
>250 kg/m/s shifts onshore coincident with the WAA, resulting in a
long duration of precipitation. With the warm nose surging inland
but surface temps remaining quite cold (teens and low 20s),
significant freezing rain will continue on Day 1. Feeding of cold
air through the Columbia Gorge will maintain this freezing rain
threat, at least in the Gorge itself and in the Columbia Basin,
where amounts may exceed 0.25-0.50 inches. WPC probabilities for
at least 0.25" icing are moderate (>40%) here. A much larger area
of appreciable freezing rain is likely over much of eastern
Washington through Saturday. By Sunday, milder Pacific flow will
encompass the area, raising snow levels along and west of the
Cascades well above pass level. Areas east of the Cascades will be
slower to moderate, with a lingering light icing threat through
Sunday. Farther east, moisture will wring out over northern Idaho
and northwestern Montana deep into the cold air, with three-day
amounts over 6 inches likely in the mountains.

...Midwest...
Days 2-3...

Clipper system out of Canada tied to the incoming Pac NW system
Day 2 will dive southeastward through the central Plains into the
Mid-MS Valley. Modest push of milder air atop the cold surface
layer will support a chance of light icing, but otherwise a band
of snow is likely from North Dakota southeastward into the Midwest
on the order of a few inches. WPC probabilities of at least 4
inches of snow are low, generally <20%, over eastern ND and south
central IL.


Fracasso


Key Messages for Blizzard
--Blizzard Conditions from Strong Winds and Heavy Snow
A powerful winter storm will continue to produce snow through the
Great Lakes and interior Northeast. The heaviest snowfall is
likely in the Great Lakes where Lake Effect Snow will aid in
producing 2-4 foot totals. Even where lesser accumulations occur,
heavy snow rates combined with wind gusts of 40 MPH will result in
blizzard conditions.

--Widespread Potentially Damaging Winds & Ground Blizzards
Even away from the heavy snow, this powerful storm will bring
widespread wind gusts over 40 MPH to a large portion of the
Central and Eastern U.S. into Sunday. In some spots, the wind
gusts could approach or exceed 60 MPH resulting in damage and
power outages. These winds atop existing snow cover will produce
ground blizzards.

--Extremely Dangerous Travel with Whiteout Conditions
Where blizzard conditions occur, expect periodic whiteouts with
near zero visibility and considerable blowing and drifting of
snow. Traveling in these conditions will be extremely dangerous,
to at times impossible.

--Life-threatening Cold Temperatures
Dangerous wind chills will create a potentially life-threatening
hazard for travelers that become stranded. If you must travel,
prepare for extreme cold. In some areas, being outdoors could lead
to frostbite in minutes. Ensure outdoor animals and livestock have
sufficient shelter.

--Coastal and Inland Flooding Threat for the Northeast
Heavy rain falling onto a melting snowpack is forecast to cause
flooding impacts in urban areas, and along rivers and streams
today. Minor to isolated major coastal flooding is possible due to
onshore winds.


Key Messages for Arctic Blast
--Dangerous Cold for Much of the U.S.
An Arctic cold front will continue to push through the remainder
of the eastern U.S. tonight, with frigid temperatures and gusty
winds expected to produce dangerously cold wind chills across much
of the central and eastern U.S. this holiday weekend. 

--Increased Danger if Power Outages Occur
The danger of the cold will be increased if any power outages
occur due to the widespread gusty winds around the winter storm
system.

--Cold Weather Safety
In the event of a power outage...
----Candles and space heaters are a fire risk. Stay warm by
bundling up and keeping doors closed, and placing towels in cracks.
----Practice portable generator safety, and donâ€t use camp
stoves or unvented heating devices to heat indoors. Only use them
outdoors at least 20 feet away from doors, windows, and garages to
avoid carbon monoxide poisoning.
----Bring pets inside and check on your neighbors and the
vulnerable to make sure they are safe.
----Protect exposed plumbing and pipes.

--Moderating Temperatures Next Week
Temperatures are expected to gradually warm early next week, with
seasonal to above-normal temperatures likely by midweek.