Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
353 PM EST Sun Jan 24 2021

Valid 00Z Mon Jan 25 2021 - 00Z Thu Jan 28 2021

...Central Plains to Midwest...
Days 1-3...

A major winter storm is likely for parts of the Central Plains and
Midwest Monday into Tuesday.

An area of low pressure will begin to consolidate across Oklahoma
Monday morning and then lift slowly northeast into Missouri while
deepening through Tuesday morning. This low will be driven by a
potent negatively tilted shortwave moving out of the Southwest and
closing off over Kansas Monday aftn. This mid-level wave will then
slowly fill and open into Tuesday morning as it ejects into the
westerlies and moves over the Great Lakes late Tuesday. At the
same time, a strengthening jet streak downstream of an amplifying
trough across the West and of Pacific origin will reach 170kts
across the Mid-Atlantic, leaving its diffluent RRQ atop the
Plains, while a secondary jet streak blossoms over the Southwest,
creating a coupled jet structure favorable for rapid cyclone
deepening. These features together will drive deep layer ascent
for the significant winter storm.

As all the features together lift northeast through Monday, there
is likely to be an expanding area of precipitation as isentropic
lift/WAA spread northward ahead of the surface low. This will
cause an expanding shield of precipitation to lift northward. The
antecedent column is marginally cold, so precipitation may briefly
start as snow from northeast Kansas through Indiana, but the
strengthening warm nose will quickly cause a transition to
freezing rain as precipitation lifts northward. Eventually, the
column will be cold enough for all snow, but this should be
confined to Iowa, northern IL, northern IN, and into MI/WI. A
prolonged period of freezing rain is likely in this E-W band, and
WPC probabilities for 0.25" are as high as 40% from northeast
Kansas through northern MO, and striping into far western OH.
While much of the freezing rain is due to the warm nose aloft, it
is also noted that a dry slot will try to work into this area,
especially MO and northeast KS, drying the DGZ and leading to more
freezing rain locally.

The heavy snow is likely to begin early Monday across Kansas on
the NW side of the low, and then gradually blossom northward
through NE/IA and into MO/IL/MI. Across this region, the column
will be marginally colder and supportive of snow. However, the
heaviest snow is likely on the NW side of the low as CAA develops
causing intensifying fgen and a potent deformation band is likely
to setup. At the same time, persistent moist isentropic ascent at
290-300K with mixing ratios of 3-4 g/kg will efficiently intersect
the DGZ, while a TROWAL strengthens aloft. These together suggest
a potent deformation band is likely to develop to dynamically cool
the column and cause heavy snow rates which may reach 2"/hr and
could be accompanied by lightning and thunder. The conceptual
model for this event suggests the band will pivot initially NW of
the low, before laterally translating eastward the latter half of
the event. Where this band pivots is likely to experience the
heaviest snowfall, and at this time that is forecast to be across
extreme SE Nebraska and into southern IA. Here, WPC probabilities
are high for 8", with a narrow corridor of more than 12" likely
Monday into Monday night. Surrounding this max swath, both to the
SW as the band begins to develop, and to the northeast which is
colder but will see the band begin to weaken, WPC probabilities
are high for 4" from central KS all the way into southern MI, with
a secondary maximum possible near Chicago, IL due to some lake
enhanced snowfall.

As this first system pulls away to the northeast D2 into D3, yet
another shortwave ejecting from the Southwest will approach the
Central Plains, aided by a more impressive subtropical jet streak
rotating through the trough base. This second system is likely to
be much weaker than the first, and forcing/moisture is more
transient as well. However, an additional round of moderate to
heavy snowfall is possible from the High Plains of NE/KS eastward
into Missouri during D3. WPC probabilities are low for 4".

...Central Appalachians and Mid Atlantic/Southern NY/Southern New
England...
Days 1-3..

A warm front extending from the surface low across the Central
Plains will arc northeastward towards the Mid-Atlantic beginning
Monday night. S/SW flow out of the Gulf of Mexico will produce
warm/moist advection into the region associated with this front,
and precipitation is likely to spread across the area through
Tuesday, lifting into New England on Wednesday. A shortwave
driving the surface low will weaken as it heads northeast into New
England, and this splitting of energy will somewhat weaken the
potential for heavy precipitation across the area. However, potent
WAA, subtle height falls, and the RRQ of a departing jet streak
will combine to drive deep layer ascent for wintry precipitation.

As moisture spreads northward Monday night into Tuesday, it will
encounter cool high pressure from the northeast wedged into the
Central Appalachians. This will create an overrunning scenario,
and while the initial burst of precipitation is likely to be snow
in most areas, it will quickly change to freezing rain as temps
aloft warm above 0C. There is still uncertainty into how long
freezing rain will persist, mostly due to the potential for
secondary low development offshore to help reinforce the cold air
at the surface. However, models have backed off on this secondary
development so more freezing rain than snow is expected,
especially south of the Mason Dixon line, and much of the
Mid-Atlantic may eventually change to rain. The heaviest freezing
rain is likely in the terrain of the WV Panhandle, northern VA,
western MD, and into the Laurel Highlands where WPC probabilities
are more than 70% for 0.25", and isolated amounts to 0.5" are
possible. Elsewhere, Up to 0.1" of freezing rain is likely,
including the I-95 corridor from DC to Philadelphia.

Further north, the column will be colder so more snow is likely.
Eventually, even much of central and southern PA may change to
sleet/freezing rain, but WPC probabilities indicate a high chance
for at least a few inches of snow. The best chance for significant
accumulations of 4" or more are confined to the Poconos and into
central upstate NY where the column will remain all below
freezing, and the weakening shortwave will provide the best
enhancement of synoptic ascent.


...Pacific Northwest to California...Southern Great
Basin....Southwest...Southern Rockies...
Days 1-2...

The first disturbance to cause locally heavy snow is an impulse
over southern CA that will open and eject eastward towards the
Four Corners tonight, and then progress into the Plains Monday.
This impulse will be followed almost immediately by a second
shortwave that digs down the West Coast before briefly closing off
over Southern CA Monday evening, and then also ejects northeast
towards the Four Corners on Tuesday. These back-to-back features
will spawn modest surface waves that will move across the region,
but both will be accompanied by enhanced moisture from the
Pacific, as well as modest jet level diffluence for ascent.
Combined heavy snow from these impulses will be greatest in the
ranges orthogonal to the 700mb flow, which will provide upslope
enhancement to the snowfall. This is likely in the Mogollon Rim,
the Sierra, and the S CA ranges on D1, spreading into the Mogollon
Rim, White Mountains San Juans, and other ranges of NM/AZ on D2.
Snow levels will be quite low during this time as well, as low as
1000-2000 ft, so light snow is expected in the less common terrain
also. WPC probabilities for 8 inches are high on D1 in the
Mogollon Rim and Sierra, as well as the southern Wasatch and San
Bernadinos, with high probabilities for 8 inches confined to the
Mogollon Rim and White Mtns on D2. Total snowfall may exceed 2 ft
in the higher terrain by Tuesday evening.


Day 3...

On Day 3/Tuesday, a closed low over the northeast Pacific moves
southeast towards the northwest US. Strong onshore flow develops
in OR and crosses CA, with the strong onshore moisture fluxes and
large 300 mb divergence maxima leading to high confidence in heavy
precipitation in northern and central CA.  The position of the
mid-level trough and jet streak support a long duration of moist
advection and intense ascent focused into northern and central CA,
and snowfall could be extreme on D3 even as snow levels begin to
rise from the south. WPC probabilities for  12" are greater than
90 percent along the Sierra and into the Siskiyou/Trinity/Shasta
regions, and with intense ascent directed into these ranges
coincident with anomalous moisture, more than 3 ft of snow is
likely in some places on D3.

Elsewhere, very cold snow levels across WA and OR may permit some
light accumulations even into the lowlands and metro areas around
Portland, OR on D3.

Weiss