Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
331 AM EST Fri Jan 03 2020

Valid 12Z Fri Jan 03 2020 - 12Z Mon Jan 06 2020

...Central Plains...
Day 1...

Modest clipper type low and associated northern stream impulse
will drop quickly southeast today from ND towards IL. This system
will be fast and predominantly moisture starved. However, high-res
guidance has increased its forecast QPF, and steepening lapse
rates beneath the upper trough combined with robust 925-850mb fgen
should support a narrow swath of light to moderate snow. While
amounts are not forecast to be significant, weak instability and
theta-e lapse rates of 0C or less suggest convective potential,
and HREF v3 probabilities have increased for 1"/hr rates.
Pinpointing exactly where this occur is challenging, but WPC
probabilities indicate pockets of a high risk for 2" of snowfall
in MN/IA, with isolated amounts to 4" possible where the excessive
snow rates occur.


...Pacific Northwest to the Northern Rockies...
Days 1-3...

An amplified upper ridge is forecast to shift east from the West
Coast into the Intermountain West late Friday.  Atop this feature,
a surface low will lift onshore British Columbia, driving a warm
front into the PacNW today, followed closely thereafter by a cold
front early Saturday. Flow atop the ridge will persistently be
angle into the West as well, before diving southeast into the
Rockies, and this will drive anomalous PWATs into the region. The
combination of several mid-level impulses embedded within the
flow, periodic enhanced jet level diffluence, and the surface
fronts, will produce several rounds of snow in the West. The
heaviest snowfall is likely in the Olympics and Cascades of
Washington, where on any given day WPC probabilities for 12 inches
are moderate to high, and 3-day totals could exceed 4 feet.
Otherwise, WPC probabilities are high for 6 inches, especially day
1 and again on day 3, in the Oregon Cascades, and other ranges
spreading as far east as the Northern Rockies and as far south as
the Tetons and Wind Rivers. Snow levels are expected to drop
behind the cold front, and will be 500-1000 ft east of the
Cascades, and 1000-2000 ft to the west.


...Northeast and Central Appalachians...
Days 2-3...

Concern for heavy snowfall continues to revolve around the
potential phasing of northern and southern stream energy somewhere
along the east coast during Saturday. Guidance continues to
feature a variety of solutions in both the amount of phasing, as
well as when this will occur, not to mention the resultant surface
low track. There has been wide variability leading to lowered
confidence in the forecast, and a trend towards the ensemble means
remains prudent.

That leaves a surface low which will lift northeast through
Pennsylvania before transferring to secondary low development
south of Long Island. If northern/southern stream phasing can
occur early on Saturday, this low may track into the Gulf of
Maine, but the trend has been for a more suppressed solution as
the dynamics eject east quickly, allowing the surface low to
remain weak and push off to the east as well. Initially the
thermals will only support rain due to a warm column, with the
exception being northern New England, but as the low pulls away
cold air will wrap back towards the coast causing a p-type
transition from rain to snow. The amount of precipitation
available once the column cools remains in question, and at this
time most of the snowfall is forecast to be light to moderate,
with the highest amounts confined to the terrain. WPC
probabilities for 6 inches are 30-50% in the Adirondacks, Greens,
and Whites, with lesser amounts further to the south and at lower
elevation.

Much further south, into the Appalachians from NC/TN into WV, a
period of upslope slow looks likely. Some of this may be heavy as
NW flow increases behind the departing system, and WPC
probabilities for 4 inches are high in WV, and low along the TN/NC
border.


...Great Lakes...
Day 3...

Fast moving northern stream shortwave moving across the Plains
will combine with the LFQ of an intensifying upper jet streak to
produce weak surface cyclogenesis and a band of snowfall from
Minnesota into Upstate New York. This feature will move quickly as
it remains embedded in rapid flow, but brief moderate to heavy
snowfall is likely, with the highest snow amounts confined north
of the Canadian border. However, robust synoptic ascent and some
enhancement due to LES could produce moderate snow accumulations.
WPC probabilities for 4 inches are low across the U.P. of
Michigan, as well as downwind of Lake Erie.


The probability of significant icing (0.25 inch or greater) is
less than 10 percent.

Weiss