Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
207 PM EST Thu Feb 02 2023

Valid 00Z Fri Feb 03 2023 - 00Z Mon Feb 06 2023


...Great Lakes and Northeast...
Day 1...

A potent shortwave rotating through an anomalously strong longwave
trough across the eastern CONUS will dive through the Great Lakes
this aftn and then move off the New England coast Friday morning.
This shortwave will drive an arctic cold front rapidly
southeastward, with strong CAA developing in its wake. The passage
of this front will result in two areas of heavy snow.

The most significant accumulations are likely in the favored NW
snow belts downstream of the Great Lakes where lake effect snow
(LES) will result in high SLR and rapid fluffy accumulations. The
greatest potential for heavy snow from LES is likely across the
eastern U.P., northwest L.P., Tug Hill Plateau, and southeast of
Lake Erie along the Chautauqua Ridge. In these areas, WPC
probabilities for more than 4 inches are moderate, but local 6-8
inch snowfall is likely, especially where any effective fetch from
upstream lakes can contribute.

The second threat from this frontal passage will be what could be
widespread significant snow squalls beginning in the L.P. of MI
this aftn, and becoming more intense across Upstate NY, northern
New England, and western/central PA. Here, the overlap of ascent
through frontal convergence combined with sharpening 0-2km fgen,
increasingly negative theta-e lapse rates and 100-200 J/kg of
SBCape should result in a band or multiple bands of fast moving
snow squalls. The strongest squalls will likely have >1"/hr
snowfall with strong winds of 30mph or greater leading to
extremely reduced visibility. While farther downstream towards
I-95 the instability will wane, but scattered snow showers and
isolated squalls will be possible through Friday morning all the
way to the Atlantic Coast.


...Pacific Northwest through California...
Days 2-3...

Shortwave ridging across the West Saturday morning will quickly
erode as a closed mid-level low dives out of the Gulf of Alaska
towards the Pacific Northwest coast by Sunday morning, and then
lifts onshore the CA coast into the Great Basin by the end of the
forecast period /Sunday evening./ Height falls and PVA associated
with this impulse will overlap with increasing upper diffluence
within the LFQ of an approaching Pacific jet streak to produce
deep layer ascent, aided by low-level convergence as a surface
cold front gets pushed onshore. Moisture advection within the jet
streak and within confluent flow downstream of this trough axis
will spread IVT in excess of 250 kg/m/s coincident with PW
anomalies reaching +1 sigma across the west as a modest AR surges
into the region, highest across CA. Snow levels within this
Pacific sourced airmass will generally be 3500-5000 ft, but surge
to around 7000 ft in the core of the AR into CA. Even as the cold
front collapses southeast and the trough digs into the Great Basin
late in the forecast, snow levels do not vary too much, so much of
the heaviest snow should remain in the highest terrain. For D2,
the highest probability for more than 6 inches of snow will be
confined to the WA Cascades and Olympics where they reach 50-70%.
These probabilities expand considerably on D3 as the better
overlap of moisture and forcing shift onshore. WPC probabilities
for more than 6 inches of snow above 30% extend from the WA and OR
Cascades through the northern CA ranges, along most of the Sierra
and into the Sawtooth and Salmon River ranges of ID. The heaviest
snow will likely be focused into the Sierra however, where locally
1-2 feet of snow is likely, but generally above 7000 ft.


Weiss


Key Messages for Northeast Snow Squalls:
--A strong Arctic front will race across the Great Lakes today and
then push through the Northeast tonight.

--Snow squalls will accompany the Arctic front, causing heavy
bursts of snow and gusty winds. Sudden whiteout conditions within
snow squalls will create very dangerous driving conditions,
particularly on highways.

--In the frontâ€s wake, some heavy lake effect snow bands will be
possible downwind of the Great Lakes.

--A combination of bitterly cold temperatures and gusty winds will
lead to dangerous cold wind chills in the Northeast from Friday
into Saturday. Wind chills in northern New England are likely to
fall well below minus 30 degrees in many locations, which the area
has not experienced for decades.

--Limit time outside and dress in layers as frostbite and
hypothermia can occur in a matter of minutes.