Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
354 AM EST Fri Jan 01 2021

Valid 12Z Fri Jan 01 2021 - 12Z Mon Jan 04 2021

...Midwest/Great Lakes...
Day 1...

Anomalously strong closed low pivoting atop Oklahoma this morning
will gradually lift northeast while filling through today, before
becoming absorbed into the westerlies Saturday morning and
shifting off towards New England. This closed mid-level low will
be accompanied by a robust subtropical jet streak angling across
TX/Deep South and coupling with a robust zonally oriented jet
streak across Quebec to drive upper ventilation and surface low
strengthening from OK to IN before shifting into New England
Saturday. Deep layer ascent through mid-level divergence
downstream of the closed low, the aforementioned upper divergence,
and convergence invof surface low will produce precipitation, with
strong WAA ahead of the main system lifting isentropically atop a
surface high to expand precip north and east through the day. This
WAA will be the primary driver of precipitation today, and as the
850mb flow increases to 30-50kts from the south, an expansive area
of freezing rain is likely. There is some uncertainty continuing
into how much freezing rain will accrete due to A) wet bulbs that
are currently a bit below guidance, and B) precip rate which can
have an impact on accretion efficiency. Some sleet on the north
side of the expanding precip shield may reduce freezing rain
accretion, however, the strong of the WAA should quickly
transition much of this to freezing rain, so the bigger concern is
precip rate which could become intense at times through weak
elevated instability and pockets of high PV anomaly rotating
around the low. Despite these uncertainties, WPC probabilities for
>0.25" of freezing rain are high for a narrow swath from northeast
MO through northern IL (just south of Chicago) and into southern
MI. Local maxima approaching 0.5" is possible. This echoes the WSE
mean and HREF mean values for this area.

On the far NW side of this system, a band of precipitation is
likely to dynamically cool to produce heavy snowfall across
eastern KS and into far NW MO. While precip will initially be
mixed/freezing rain, a strengthening TROWAL aloft to drive
moisture into the column coincident with intense deformation
should produce a narrow band of heavy snow with rates in excess of
1"/hr. This band is likely to be transient to the NE and will
occur after a period of warmer antecedent conditions, but WPC
probabilities are moderate for 4 inches, with locally 6" possible.


...Mid-Atlantic/Northeast...
Days 1-3...

Two separate systems will impact the Mid-Atlantic and Northeast
through the first weekend of 2021. The first is currently moving
across Oklahoma, and as this lifts northeast it will weaken and
become absorbed into the mean flow to race across the Northeast on
Saturday. Well ahead of this system, which is composed of a strong
but weakening upper trough and a modest surface low, robust WAA
coincident with mid-level divergence and modest diffluence through
a coupled upper jet structure will drive expanding precipitation
northward beginning this aftn. A cold high pressure in place ahead
of this system will be reinforced both through mid-level
confluence downstream of the approaching upper low, and also
through precipitation falling into the high driving a modest wedge
development. As WAA begins in earnest on a LLJ progged to approach
50 kts, precipitation primarily in the form of rain and freezing
rain will expand from the central Appalachians and into the
Northeast. Further north, especially across northern NY state and
into central New England, snow will be the initial p-type. While
the WAA may eventually overwhelm the wedge to turn all precip to
rain across the Mid-Atlantic and SNE, WPC probabilities indicate a
high chance for 0.25" or more of freezing rain across the Laurel
Highlands of PA, into the Poconos, and even the southern Catskills
of NY. Locally freezing rain may exceed 1/3" of an inch before
changeover. Further north and east and into some of the lower
terrain as far east as the Worcester Hills of MA, freezing rain
may accrete to 0.1".

As the surface low moves quickly eastward, the 700mb low will drag
across NNE. Points along and north of this low will remain all
snow, and this could accumulate heavily, especially across parts
of Northern NH and much of central/northern ME where persistent
WAA and intense isentropic ascent will drive a long duration
moderate to heavy snowfall. WPC probabilities for 4 inches are
high on D2 across much of northern NH and ME, and into the terrain
of upstate NY and VT. Isolated locations, especially across Maine,
may reach 10" of accumulation before the low pulls away Saturday
night.

Very quickly behind this first system, another low pressure is
likely to lift northeastward and impact the Northeast with wintry
precipitation again on Sunday. There remains considerable
uncertainty into this second system as its evolution will depend
somewhat on the outcome of the first wave. The first low will
leave a cold front/baroclinic zone across the Mid-Atlantic and
Southeast. A tough emerging from Mexico on Saturday will tilt
negatively and close off, interacting with this baroclinic zone to
drive cyclogenesis across the Southeast. As this low lifts
northeast, mid-level divergence and height falls combined with WAA
will spread precipitation northward, while the upper low
intensifies and may try to pull the surface low further to the NW.
While guidance differs on the track and hence amount of wintry
precipitation that may impact the region on Sunday, WPC
probabilities are already showing a low-mdt risk for 4" of
snowfall, highest in the terrain of the Catskills and Berkshires.


...Pacific Northwest...
Days 1-3...

A series of three distinct shortwaves embedded within confluent
mid-level flow and a modest atmospheric river will drive nearly
continuous moist advection with periods of enhanced ascent through
the weekend. The individual shortwaves are progged to arrive on
the coast of WA/OR tonight, Sunday morning, and Monday morning.
Each of these impulses will be accompanied by modest height falls
and briefly backed mid-level flow to the SW for robust moist
advection, as well as cold front which should decay as they push
inland but will aid in low-level convergence. Additionally, the
persistent westerly and confluent 700-500mb flow will produce IVT
in excess of 500 kg/m/s, highest D3, providing plentiful moisture
for rounds of heavy precipitation, including snow in the terrain
above 6000 ft early D1, falling to 3000 ft behind the first cold
front. Snow levels are progged to climb to around 5000 ft ahead of
each subsequent impulse/cold front, before crashing back to 3000
ft in their wakes. This suggests that the heaviest snow through
the 3 days will remain above 5000 ft, but with moderate to heavy
snow falling to 3000 ft at times. The ranges most impacted will be
from the Shastas/Siskiyous of northern CA, to the WA and OR
Cascades, the Olympics, and eastward through the Bitterroots,
Sawtooth, Northern Rockies, and into the Tetons. Probabilities for
6 inches from WPC increase in coverage each day, and there will
likely be some higher terrain of the Olympics and Cascades that
receive in excess of 1 ft each of the 3 days. 3-day total snowfall
is likely to reach 2-3 ft in the OR Cascades and parts of the
Northern Rockies, with isolated maxima above 5 ft possible in the
WA Cascades.


Weiss