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

Valid 12Z Sat Jan 23 2021 - 12Z Tue Jan 26 2021


...Great Basin to the Central and Southern Rockies/California...

Days 1-3...

On D1, a 700 mb wave ejects from northern UT across southern WY,
with convergence along the wave's path supporting heavy snow in
windward terrain of northern UT and southern WY.
South of the CO, upper divergence crossing the ranges of western
CO supports several inches of snow there. A mid-level low will
pivot down the coast of CA and become closed Saturday evening
before opening up and ejecting northeast through the Four Corners
on Sunday. A persistent and slowly intensifying jet streak
downstream of this trough will keep a the upper jet streak
crossing AZ into northwest NM and southern CO,  providing moisture
and aiding ascent across the region. As this trough expands
eastward, snow levels will crash. Where upper-level divergence
occurs atop the moisture stream, heavy snow is likely. WPC
probabilities for 6" are high on D1 across the San Juans. Isolated
maxima above 12" are possible in the highest terrain of the San
Juans. D2/Sunday, WPC probabilities are highest for 6 inches in
the Mogollon Rim of AZ, with a secondary max in the San Juans once
again, as well as spreading across the White Mountains of AZ.

As the first system exits to the east, a more powerful shortwave
will dive along the coast from WA to CA. This will have the effect
of significantly lowering snow levels, while also transporting
Pacific moisture onshore and then inland towards the Four Corners
once again.  Upper divergence and mid level
deformation/frontogenesis will be the primary driver of ascent
this period as the jet energy stays mostly west of the trough and
over the Pacific Ocean. However, pronounced increase in mid-level
RH and 700 mb ascent suggests moderate to heavy snow will spread
down into lower elevations areas in WA/OR.  While the heaviest
snow will once again occur in the terrain, there is potential for
lowland snow accumulating above 1" in WA/OR, including the
Columbia Gorge. In the OR Cascades, WPC probabilities are high for
4 inches. Snow spreads into the northern CA Sierra Nevada
Mountains Sun night, with several inches expected.

On Monday, the amplifying trough lowers snow level so snow grows
in coverage in the ranges of Southern CA inland across the
southern Great Basin and southern Rockies concurrent with
intensifying ascent as upper divergence maxima cross this region
in the left exit region of a jet max crossing southern AZ and NM. 
It is becoming increasingly likely that heavy snow will accumulate
in these ranges, and WPC probabilities are high for 8 inches in
the Mogollon Rim and southern UT Wasatch Monday.  Totals of 12" to
18" are likely in the ranges of AZ.


...Northern/Central Plains to Upper MS Valley/Great
Lakes/Midwest...
Days 1-3...

The forecast for Saturday features a wave ejecting from UT and WY
across the Plains and merging with a northern stream wave. While
the amplitude of this feature is modest, it will be embedded in
moist flow,  diffluence aloft providing ascent, aided by
intensifying isentropic lift from low level warm advection.
A cold column noted on regional soundings will allow for high snow
to liquid ratios, aided by a deepening dendritic growth zone.  The
lift intersects this saturated dendritic growth zone, so that
favors higher snow to liquid ratios. WPC probabilities for 4
inches are 60% across southern MN, with local maxima above 6"
possible. The greatest snow accumulation is likely 00Z-12Z Sunday,
with rapid waning occurring thereafter as the system deamplifies,
with 700 mb ascent decreasing and snow amounts/coverage declining
as a result. On Day 2, light snow is indicated over northeast WI
across Lake Michigan to the UP of MI. Modest lake enhancement
could bring 3 or 4 inches to portions of the UP near northern Lake
Michigan.

On Monday/Day 3, a closed 700 mb low is forecast by the
ECMWF/GFS/UKMET/Canadian global to develop and move across KS to
northern IL by Tuesday morning. This shortwave will be accompanied
by an intense jet-level divergence maxima, and moisture wrapping
around the cyclonic circulation to produce snow in a mid level
frontogenesis band from northern KS across southeast NE, southern
IA, and northern IL.  The last 2 runs of the ECMWF have shown
potential for a foot of snow in southeast NE and southern IA.  WPC
probabilities currently indicate a 60-80% chance for 4" of snow
from northern KS into northwest MO and southeast IA.

South of the 700 mb low, warm air advection is likely to drive a
warm nose above 0C, creating a mixed p-type scenario with sleet
and freezing rain likely across a large area from northeast KS
across northern MO, central IL, northern IN and OH. The guidance
still features considerable spread in the intensity and placement
of this low. WPC probabilities on D3 are as 45% for 0.1" of
freezing rain accretion in central IN to west central OH. 


...Mid-Atlantic/Central Appalachians...
Day 3...

A mid level wave/850 mb circulation moves into the Ohio Valley on
Day 3. Moisture from the Gulf of Mexico will spread northward on
warm/moist advection, expanding precipitation northeastward into
the Central Appalachians and Mid-Atlantic Monday. As this
precipitation expands, a wedge of high pressure will be in place
over northeast. Despite a brief burst of snow at precip onset,
most of the precipitation will be in the form of sleet or freezing
rain from western VA northward into the Laurel Highlands of PA and
much of western MD. There is an increasing signal for a period of
freezing rain which could become impactful. WPC probabilities for
0.1 inches are as high as 80% in the Panhandle of WV and western
MD, into the southern Laurel Highlands, with 50-70  probabilities
for 0.25". With differences in low-level thermal structure
persisting, it is possible that the freezing rain is mixed with
sleet. Future updates will hopefully narrow the spread and
increase confidence in the exact evolution.  In central PA,
uncertainty relates also to the QPF gradient, as well as precip
type.  Several inches of snow are possible where a long duration
snow event occurs, but the models are not in agreement where this
could occur yet.

Petersen