Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
351 PM EST Thu Jan 18 2024

Valid 00Z Fri Jan 19 2024 - 00Z Mon Jan 22 2024

...Continuing Great Lake Effect Snow...
Days 1-2...

Broad cyclonic flow encompassing much of the eastern CONUS will
sharply amplify Friday into Saturday as a potent shortwave digs
out of the Hudson Bay and drops almost due south into the Upper
Midwest Friday night. This shortwave will continue to amplify into
a sheared out but strong lobe of energy rotating southeast into
the Mid-Atlantic by Saturday night, leaving a transient but
anomalous closed low in its wake. This entire feature will shift
eastward late D2 into D3, with upstream shortwave ridging
blossoming into the Great Lakes thereafter, bringing an end to the
widespread lake effect snow (LES).

As the pattern evolves D1, the low-level flow over the Lakes will
gradually transition to a more northerly wind direction as CAA
commences. This CAA will follow a cold front shifting southeast
into the Ohio Valley Friday, and reinforced by a robust surface
trough late Friday. 850mb temperatures are progged to fall to as
low as -20 to -25C over Lake Superior, and while temperatures will
moderate farther southeast across the region, 850mb-sfcT
departures will be more than sufficient to support heavy LES
thanks to still warm lake temperatures. Steepening lapse rates
beneath the cold core of this low will drive inversion depths to
10,000 ft or more, highest south of Lakes Superior and Michigan,
where instability may reach 750 J/kg which could support rates in
excess of 2"/hr at times. WPC probabilities on D1 and D2 are above
50% for more than 6 inches near Marquette, MI, Cleveland, OH, and
along the Chautauqua Ridge, but will wind down quickly late D2.
Total snowfall in these areas will likely reach 12 inches in some
places. Additional moderate snow is likely south of Lake Ontario
as well. However, the most intense snowfall is likely in NW IN
south of Lake Michigan where the wind will favorably align along
the long fetch of the lake to produce an intense single band. WPC
probabilities D1 and D2 are above 80% for 6+ inches, and locally 2
feet of snow is possible.


...The West...
Days 1-3...

The West remains active through the weekend as short-wavelength
ridging gives away to more pronounced troughing as a closed low
approaches the CA coast Friday night, and even though it weakens
with time into Saturday, it will result in a broad trough across
the West with lobes of vorticity periodically shedding onshore.
These periodic shortwaves will result in times of enhanced ascent
through height falls/PVA, which will combine with modest upper
diffluence ahead of a Pacific jet streak to drive synoptic ascent
into the West, especially the latter half of the forecast period.
Additionally, the generally zonal or broadly cyclonic flow which
develops across the West will surge moisture from west to east,
noted by PW anomalies reaching +2 to as high as +3 sigma in some
areas, which will support an expansion of precipitation where this
forcing acts upon the greater column moisture. The divergent flow
downstream of the trough will result in WAA and snow levels rising
to 6000-7000 ft, so the heaviest snow should remain above all but
the higher passes, but significant snowfall is likely across many
of the terrain features by D3.

For D1, the heaviest snow is likely in the WA Cascades where WPC
probabilities for more than 6 inches are 40-60%. Potentially more
significant on D1 will be the continued freezing rain threat
across the Columbia Basin and Gorge. Cold Canadian high pressure
will remain entrenched across the interior northwest, with
easterly flow draining sub-freezing air persistently into the
region. As the precipitation overruns this cold air, it will
result in a continuation of freezing rain, with additional
significant accumulations of ice likely on top of areas already
hit hard by icing the past few days. WPC probabilities for ice
accreting above 0.1" are 50-80% in the southern Columbia Basin and
in the Gorge, with lower probabilities extending up into the
Cascades as well as the snow levels rise. Additional ice is likely
across this same area on D3 with the next round of precipitation,
with WPC probabilities for 0.1" as high as 20-30%.

As the more broad PW anomalies spread onshore D2 into D3, snowfall
will expand through much of the West with light to moderate
accumulations encompassing much of the terrain from the Sierra
through the NW WY ranges and southward into the San Juans.
However, the most significant accumulations should be generally
confined to the Sierra and Shasta/Trinity/Siskiyou region where
WPC probabilities D2 and D3 are above 70% for 6+ inches, and storm
total snowfall could reach 2-4 feet in the highest terrain.


...Ohio Valley through Southern New England...
Days 1-2...

Dual shortwaves embedded within the large cyclonic flow across the
eastern CONUS will shed E/SE through Saturday to produce a swath
of snowfall from the Ohio Valley through the Mid-Atlantic and
extreme southern New England. The first of these shortwaves will
rotate eastward through the base of the trough crossing the Ohio
Valley Friday morning and then racing eastward to off the
Mid-Atlantic coast south of New England by Saturday morning.
Immediately in the wake of this first trough, a secondary impulse
will dig out of the Great Lakes and eject across the Mid-Atlantic
again on Saturday. While this secondary trough will encounter
drier air, it will likely have the result of temporally extending
the ascent aloft, and help to develop an inverted trough across
the Mid-Atlantic.

Coincident with the first shortwave, a potent upper jet streak is
progged to intensify towards 170kts as it digs from the Mid MS VLY
through the Mid-Atlantic. This will place favorable LFQ diffluence
atop the best height falls from the lead wave, helping to develop
a low pressure over the Ohio Valley which will then traverse
eastward through Friday. The low should remain south of most of
the area, keeping cold air locked in place, and the downstream WAA
should isentropically ascend the warm front to enhance lift and
work in tandem with the response of the upper jet to drive some
greater 850-700mb fgen. Although overall forcing appears modest,
it will work into a moist column with a deep DGZ noted by SREF
probabilities for 100mb of depth exceeding 50%. This will likely
manifest as a swath of light to at times moderate snowfall, with
the rates the most intense in the presence of the inverted trough.
Where this trough pivots Friday aftn into Friday night will likely
experience the heaviest snowfall accumulations due to the longer
duration of snow. The guidance has trended a bit southward today,
and current WPC probabilities for 4+ inches have increased to
30-50%, highest along the PA/MD border and into southern NJ where
this inverted trough may pivot to the east.

The most significant snowfall from this event is likely across the
Central Appalachians where initial light snow ahead of the wave
and within the region of best synoptic ascent will quickly
transition to a long duration and possible impressive upslope snow
event Friday aftn through Saturday. The guidance varies with the
depiction of how much of the DGZ will remain saturated during this
time due to very dry air in the mid-levels, but it is likely on
the post-frontal CAA on N/NW flow, that any moisture should deepen
due to the favorable pattern and result in a long duration of at
least moderate snow. The most intense snowfall and region of
greatest probability for significant accumulations is across the
terrain of the Alleghenies where WPC probabilities for more than 6
inches of snow are 50-70% D1, and continue with lower
probabilities into D2, but moderate accumulations are possible as
far south as the Blue Ridge of NC.


Weiss


***Key Messages for Jan. 17-19 Northwest Winter Storm***

--Additional Wintry Precipitation Tonight into Friday
Another strong storm system is responsible for additional heavy
precipitation across the Pacific Northwest this afternoon and into
Friday.

--Ongoing Treacherous Ice for Columbia Basin/Gorge
Heavy freezing rain will continue this afternoon and into Friday
for portions of the Cascades, Lower Columbia Gorge, and Lower
Columbia Basin. Additional significant accumulations of ice
(localized ice amounts > 0.25â€) will likely result in dangerous
travel and additional areas of tree damage and power outages
through tonight.

--Heavy Snow Develops Again Late Today
Heavy snow will overspread the Cascades through this evening.
Expect snow rates to peak above 1â€/hr at times and total
snowfall accumulations of 6-12†(localized totals >12â€) are
expected for the Washington Cascades.

--Mountain Pass Dangerous Travel
Heavy snow today will transition to mixed precipitation, including
freezing rain later tonight, at many of the mountain passes. This
will continue to promote dangerous travel through Friday,
especially across the Cascade Passes.