Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
314 PM EST Wed Jan 20 2021

Valid 00Z Thu Jan 21 2021 - 00Z Sun Jan 24 2021

...Great Lakes...
Days 1-3...
Shortwave ridging early D1 will quickly be replaced by renewed
troughing as a shortwave and associated vorticity impulse moves
across the Lakes by Thursday aftn. Once this shortwave moves east,
continuous and broad cyclonic flow will persist across the Lakes,
with reinforcing shortwaves and cold fronts moving across the
region into the weekend. The heaviest snow through the period is
likely east of Lake Ontario into the Tug Hill Plateau late D1
through D2. This is when the best overlap of forcing is expected,
with westerly low-level winds becoming aligned to the longer fetch
of the Lake. 850mb temps only fall to around -10C so lapse rates
are steep but not excessive, and inversion heights are modest.
However, enough instability across the Lake combined with
prolonged convergence, enhanced synoptic ascent, and the potential
for an effective fetch with moisture from Lake Huron, should lead
to snowfall rates which may at times eclipse 1"/hr. WPC
probabilities are high both on D1 and D2 for 4 inches, with local
totals exceeding 12" possible combined over the two days. Renewed
shortwave ridging D3 will cause LES to wind down on Friday. In
addition to the heavy snow downwind of Ontario, moderate LES is
likely east of Erie D1.5-2, and across the U.P. south of Superior
D1-2 where WPC probabilities for 4" are generally 10-20%.

...Central Rockies...
Days 2-3...

A potent upper low will drop from the OR coast Thursday night to
onshore northern CA Friday, and then open into a large positively
tilted trough across the Great Basin by Saturday. Height falls and
modest jet diffluence will provide ascent into the Rockies, while
moisture streams northeastward from the Pacific Ocean downwind of
the opening trough. This is likely to produce widespread snow from
the Sierra eastward to the San Juans and north as far as the Wind
River range of WY. Snow levels are generally expected to be
3000-5000 ft, but may lower to 2000 ft as the cold core of the
upper low sinks southward. Periodic vorticity lobes swinging
through the flow may enhance ascent and snowfall at times, and
there is likely to be a 36 hour window of off-and-on snow across
the region. WPC probabilities for 6" are high D3 across many of
the ranges of Utah including the Wasatch and Uintas, as well as
into the CO Rockies and San Juans. Local snowfall may exceed 12
inches in the highest peaks. Lower but still significant
accumulations are possible along the Mogollon Rim and the Sierra
of CA.


...Northern Plains to Upper Midwest...
Day 3...

A weakening shortwave ejecting from the Great Basin will shear and
move northeast towards the Plains late Friday. Modest height falls
and weak upper diffluence will precede this feature, with
confluent mid-level flow spreading Pacific moisture to the east.
At the same time, a low-level trough beneath this shortwave will
move eastward, driving increasing southerly WAA ahead of it. While
the forcing for ascent is likely to be transient from west to
east, it could be intense, and forecast profiles indicate a
deepening saturated DGZ with a near isothermal layer on the WAA to
promote snow growth. A period of moderate to at times heavy snow
is becoming more likely Friday, although there is still a large
spread in potential amounts. WPC probabilities for 4" are as high
as 60% across eastern SD into southern MN D3.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Weiss