Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
344 PM EST Thu Jan 21 2021

Valid 00Z Fri Jan 22 2021 - 00Z Mon Jan 25 2021

...Great Lakes and Northeast...
Days 1-2...

A continuation of moderate to heavy lake effect snow is likely
downwind of Lake Ontario and Lake Erie into the weekend. Broad
cyclonic flow across the northeast persists, with at least 2
embedded shortwaves moving through the flow - one tonight and
another Friday evening. While each shortwave will have the
potential to produce some light WAA snow ahead of a cold front due
to some modest ascent, the primary heavy snow producer will be
again due to LES as flow becomes aligned to Ontario and Erie
behind each front on CAA. For Ontario, heavy LES should begin
early Friday morning and then persist into the evening, with a
slow SE push of the trajectories likely into early D2. This should
produce bursts of heavy snow rates of >1"/hr at times, and WPC
probabilities for 6" are high on D1 in the Tug Hill Plateau, with
high probabilities for 4" on D2 shifting to a more SE trajectory
off the Lake. For Lake Erie, the heaviest snow is expected D1 on
W/SW flow beneath the inversion, producing long fetch across this
lake as well. Total snowfall may be less here than east of Ontario
due to slightly less favorable fetch and upslope, but WPC
probabilities also indicate a high chance for 4" on D1 here with
some Huron moisture likely aiding in this snow band.

While not LES, there is potential for scattered snow squalls or
heavy snow showers across Upstate New York and Northern/Central
New England on D1, potentially lingering into early D2. Increasing
instability behind the cold front with continued mid-level ascent
could produce bursts of snowfall with rates >1"/hr for a short
duration. Accumulations within any of these showers/squalls should
be light, but could produce brief periods of limited visibility
and difficult travel.


...Central Rockies to Great Basin and Southwest...
Days 2-3...

A closed low deepening over Southern California will drop
southeast Saturday and then gradually begin to open as it moves
atop the Four Corners region on Sunday. This feature will produce
height falls and mid-level divergence, with modest upper
diffluence also present for ascent. Downwind of this trough, a
Pacific jet streak will intensify, while 700-500mb flow becomes
oriented to the SW. These features together will cause an increase
in PW evident by saturation in the 700-500mb RH fields, creating
an environment favorable for heavy precipitation. While the
mid-level low opens and begins to eject quickly, pronounced
synoptic ascent in the presence of this deeper moisture will allow
for heavy snowfall to spread from the Sierra eastward through the
Utah ranges, and into the CO Rockies and San Juans D2, shifting
further south towards the Mogollon Rim and White Mtns of Arizona
on D3. WPC probabilities are high for 8 inches on D2 in the Uintas
and San Juans, as well as some of the higher isolated peaks in the
Great Basin. By D3, the remaining high probabilities for 8 inches
are confined to the Mogollon Rim, White Mtns, and continue into
the San Juans. While less impressive, snow levels which may drop
as low as 3000 ft into Southern CA will produce moderate snow as
far south as the San Gabriels and San Bernadinos, as well as light
accums possible even into the somewhat lower elevations east of
Los Angeles and east of Tucson.


...Northern Plains to Upper Midwest...
Days 2-3...

A piece of energy shedding from a closed low over the Great Basin
will eject northeast and embed within confluent mid-level flow as
it traverses into the Plains and Midwest this weekend. As this
wave moves northeast, it will be accompanied by moist advection
within the aforementioned confluent flow from the Pacific, while
low-level southerly flow intensifies out of the Gulf of Mexico
driving warm and moist advection northward. At the same time,
modest diffluence from the LFQ of an upper jet streak will aid in
ascent, and a fast moving but potent swath of precipitation is
likely to spread eastward from the High Plains of SD eastward
through the Western Great Lakes. While this system is fast moving,
it does have plentiful moisture with which to work, and forcing is
briefly robust. Additionally, regional soundings indicate an
environment favorable for very high SLRs, possibly approaching
20:1, due to a deepening DGZ, a nearly isothermal layer within the
WAA, and a cold overall column with temps in the teens near the
surface. These high SLRs should squeeze out heavy snow despite
modest QPF, and WPC probabilities for 4" are 40-60% from central
SD eastward across southern MN and into Wisconsin. Local maxima in
excess of 6" are likely in southern MN which has the best overlap
of high SLR, moisture, and forcing.


...Pacific Northwest...
Day 3...

A shortwave dropping out of the Gulf of Alaska will be accompanied
by the LFQ of a modest Pacific jet streak to drive moisture and
ascent into the Pacific Northwest the latter half of D3.
Increasing moisture and ascent will spread precipitation down the
coast from WA through northern CA. An elongated trough across the
West ahead of this feature will lead to lowering snow levels, with
further reduction likely as the shortwave drops along the coast.
Snow levels may fall below 1000 ft as far south as Portland, OR,
and between 1000-2000 ft into northern CA. The heaviest
precipitation is likely into the OR Cascades with this impulse,
but WPC probabilities for 4 inches are high from the Siskiyous
northward along the Cascades of OR and WA and into the Olympics.
With the very low snow levels and the potential for easterly winds
due to the pressure gradient across OR/WA, light snow accums are
possible into the Columbia Gorge and some of the other surrounding
lowlands on Sunday, but looks to remain just outside the major
metro areas of Seattle and Portland at this time.


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

Weiss