Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
400 PM EST Sat Jan 23 2021

Valid 00Z Sun Jan 24 2021 - 00Z Wed Jan 27 2021

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

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

A potent shortwave shedding from energy in the Southwest will
advect northeastward Monday, while taking on a negative tilt
across the Plains. This will combine with a strengthening jet
streak to the northeast and downstream of the longwave trough to
produce ascent, and surface cyclogenesis is likely early Monday
over OK before lifting northeast while rapidly deepening. This
system will likely produce two phases of wintry precipitation,
freezing rain in the WAA ahead of the low, and heavy snow in the
CAA/deformation as it departs.

For the WAA phase, precipitation will expand as isentropic ascent
intensifies, tapping Gulf of Mexico moisture over the Southern
tier. This will lift northward as an expanding area of
precipitation from eastern Kansas through Ohio the first half of
Monday. This WAA is likely to be strong, so a warm nose of >0C
will spread northward, even as low-level temperatures remain below
freezing, at least initially. This suggests a swath of freezing
rain will develop along the leading edge of precipitation Monday.
There still exists uncertainty into how far northward the mixed
precipitation shield will extend, but a non-NAM consensus suggests
the heaviest freezing rain will occur along a line from the MO/IA
border eastward through central IL, IN, and into far western OH.
WPC probabilities in this area are 30-50% for 0.1", with locally
up to 0.25" possible.

As the low deepens and moves eastward, guidance has become more
aggressive that a band of heavy snow will develop on the NW side
and slide eastward along the northern periphery of the
precipitation into Tuesday. As the low strengthens, isentropic
upglide on the 290K and 300K surface will intensify from the
south, which correlates well with the height of the DGZ. This will
occur concurrently with a developing TROWAL, and the combination
of isentropic ascent into the DGZ combined with theta-e advection
aloft will produce a saturated column with intensifying mesoscale
ascent. While the column will initially be marginal for snow,
these intense dynamics are likely to cool the column sufficiently
such that precipitation will rapidly change to heavy snowfall
within this deformation band. The conceptual model for this event
suggests this band will initially pivot NW of the surface low near
the 700mb low, and then gradually translate eastward, driving
heavy snowfall from eastern KS into northern IL. Within this band,
snowfall rates may peak at 2"/hr, and cross sections indicate some
modest -EPV favorable for possible thunder-snow. While there is
still some uncertainty in the exact placement of this band, of
which can cause significant accumulation differences due to the
intense rates, the consensus is that the band will pivot near the
KS/NE/MO/IA conjunction, and it is here where WPC probabilities
are highest for 8", with isolated maxima over 12" possible. As
this band shifts eastward, it should begin to weaken and move more
rapidly, but WPC probabilities for 6" are 40-60% from eastern KS
through northern IL, including Chicago.


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

An area of low pressure moving through the MS VLY Monday will
drape a warm front well east towards the Atlantic Coast. As this
warm front lifts northward, secondary low pressure may develop off
the VA Coast Tuesday, and the combination of warm/moist advection
with the warm front and secondary low pressure development
offshore will create a challenging mixed-precipitation event late
Monday through Tuesday.

As the primary surface low moves northeast towards the Great
Lakes, it will be accompanied by a slowly filling shortwave, with
downstream ridging expanding across the east. This strengthening
SW mid-level flow will help drive warm Pacific moisture across the
CONUS, aided by a robust jet streak rising to 170kts over the
Northeast. This jet streak will aid in transporting moisture into
the region, with low-level WAA associated with the warm front
transporting Gulf of Mexico moisture northward as well. Forcing
for ascent within this moistening column will become robust
through a strong upper divergence maxima, and isentropic ascent
within the WAA regime atop a wedge of high pressure anchored at
the surface. As the moisture and ascent approach late Monday,
precipitation will spread quickly northeastward into the Central
Appalachians, and then into the Mid-Atlantic and Northeast through
Tuesday. Strong ascent initially will likely lead to a burst of
snowfall in most areas, however, rapid warming aloft will quickly
transition snow to sleet, freezing rain, and eventually rain south
of the Mason Dixon line and outside of any terrain. With the wedge
likely being reinforced by precipitation, a long duration freezing
rain event is becoming more likely, especially in the terrain and
elevated valleys of the Central Appalachians through the Laurel
Highlands. WPC probabilities in this area are greater than 50% for
0.25", with significantly more possible as shown by WSE 90th
percentile plots. However, uncertainty with respect to
rates/runoff and whether the wedge will erode or be reinforced
preclude higher probabilities at this time. Further north,
precipitation will last much longer as snow before the warm nose
advects through PA, and WPC probabilities for 4 inches are
moderate across central PA into southern NY. For the I-95 corridor
from DC to Philadelphia, a burst of snow late Monday is likely
before a rapid transition occurs to sleet, freezing rain, and then
rain. There remains uncertainty here, as the secondary low
development could isallobarically cool the column enough that snow
and/or freezing rain will be able to last longer than current
progs suggest. However, at this time, the total wintry
accumulations for the I-95 corridor due to snow and freezing rain
are forecast to be light.


...The West...
Days 1-3...

A series of impulses in this wave-train will advect into the West
through early next week, bringing repeated rounds of precipitation
and gradually lowering snow levels. Heavy snow is likely in
virtually all of the Pacific Northwest, CA, Great Basin, and Four
Corners ranges through the period.

The first impulse is a closed low over southern CA that will open
and eject eastward towards the Four Corners on Sunday evening.
This 500-700mb wave will gradually lose amplitude as it shifts
eastward, but pronounced mid-level divergence and heights fall
will drive ascent across the area. The downstream jet streak ahead
of this trough will maintain intensity, but the overall forcing
will begin to wane as the entire system shunts east as it gets
replaced by the next impulse dropping down from the Gulf of
Alaska. Backed flow ahead of the trough axis combined with Pacific
moisture advected through the upper jet will provide plenty of
moisture to spread heavy snowfall from the southern CA ranges
through the Mogollon Rim and into the San Juans. The heaviest snow
on D1 is likely in the Mogollon Rim and San Juans due to the more
orthogonal moist flow upsloping into this higher terrain. WPC
probabilities are high for 6 inches in these areas, with locally
more than 12" possible. More than 4" of snow is likely D1 in the
San Bernadino range with snow levels generally in the 4000-5000 ft
range.

As this first system kicks out to the east, it will be replaced
almost immediately by another vorticity streamer coming down the
West Coast, and tightening into a closed low over Southern CA,
while yet a third system drops into the Pacific Northwest. While
these two impulses deepen, heavy snow will again spread across the
West. For the lagging shortwave into the Pacific Northwest,
moisture is a little more limited due to the angle of approach of
this shortwave, with a more northerly mid-level flow developing
and weak LFQ jet diffluence. Despite the relative lack of moisture
(PWs below climo), strong height falls and modest upslope will
squeeze out all available column moisture as snowfall. Snow levels
crash even further with this system, becoming as low as 500 ft in
WA/OR. The heaviest snow is expected in the Cascades of WA and OR
on D1.5-2, and WPC probabilities are high for 6 inches, but
snowfall is not expected to spread inland beyond the Cascades.
While not heavy, light accumulations up to 1" are possible as low
as the Columbia Gorge on D2.

The other impulse, the closed low redeveloping across Southern CA,
will be much more significant. In addition to rapid snow level
drops, forcing through height falls and pronounced jet level
diffluence will spread eastward, with moisture wrapping from the
Pacific southward into CA and the Great Basin/Four Corners. A
prolonged period of moist advection is likely on long
trajectories, which when combined with the snow levels falling to
as low as 1000-2000 ft, supports exceptional snow potential in the
Mogollon Rim and Southern/Central Sierra which will be orthogonal
to the mid-level flow for ideal upslope enhancement. WPC
probabilities are high for 12" D2 in the Sierra and Mogollon Rim,
continuing into D3 in the Mogollon Rim. Additionally, heavy snow
is likely in the San Gabriels and San Bernadinos, with lighters
snows likely even into the lower terrain outside of Los Angeles,
San Francisco, San Diego, and Tucson. Total snowfall across the
Mogollon Rim may exceed 3 ft by Tuesday.


Weiss