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

Valid 12Z Sun Jan 24 2021 - 12Z Wed Jan 27 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.

A potent shortwave will move northeastward Monday, while taking on
a negative tilt across the Plains. This will combine with a
strengthening jet streak and mid level frontogenesis maxima to
produce ascent. This system will likely produce two phases of
wintry precipitation, freezing rain and sleet in the low-mid level
warm advection ahead of the low, and heavy snow in the mid level
deformation/frontogenesis maxima in the cold sector.

For the warm advection phase, precipitation will expand as
isentropic ascent intensifies. This will lift northward as an
expanding area of precipitation from the central Plains into the
mid MS Valley and Midwest. Temperatures 850-700 mb rise above
freezing, even as low-level temperatures remain below freezing,
until warming later.
On Day 1, the stripe of freezing rain/drizzle covers central KS to
northern MO, central/southeast IL, and southern IN to the OH
Valley in KY/OH.
Another swath of freezing rain will develop on Day 2/Monday from
northeast KS across northern MO to the IA border, central IL,
central IN, and 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 good agreement
that a band of heavy snow will develop in the mid level
deformation zone in the cold sector. The combination of isentropic
ascent into the dendritic growth zone 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. 
Heavy snowfall is expected from southeast Nebraska across
northeastern KS, far northwest MO, southern Iowa 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.  WPC probabilities for 8" peak near 80 percent in these
areas, with maxima of 12" expected in parts of southeast Nebraska
to southern Iowa.


...Central Appalachians and Mid Atlantic/Southern NY/Southern New
England...
Days 2-3...

An area of low pressure moving through the Ohio Valley Monday
night will cause warm/moist advection to stream out of the OH
Valley into the central Appalachians. Upper level jet streaks
are forecast to cross from the OH Valley across PA and WV, which
will aid in transporting moisture into the region, and also
produce forcing for ascent within this moistening column.

As the moisture and ascent approach late Monday, precipitation
will spread quickly northeastward into the Central Appalachians,
and then into the Mid-Atlantic. 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 initial cold air damming, surface
cold air in the mountains of western MD and adjacent portions of
WV and northwest PA/southwest PA will be reinforced by
precipitation.  A mixed sleet and freezing rain event is likely,
especially in the terrain and elevated valleys of the Central
Appalachians through the Laurel Highlands of southwest PA. WPC
probabilities in this area are greater than 50% for 0.25" of
freezing rain.

Further north, precipitation will last longer as snow before the
warm nose advects through PA, but ridging to the north in NY acts
to slow north progression of precip until Tue.
Modest low-mid level convergence and deformation leads to light
snow across much of southern and western NY to adjacent PA.
WPC probabilities for 4 inches are moderate across central PA into
southern NY.

For the I-95 corridor from Washington, DC to Philadelphia, 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 off the mid Atlantic coast could
allow the cold air damming to remain in place.


...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 this Sunday evening, moving out on to the high Plains
tonight.  Enhanced moisture and ascent along its path are still
expected to combine with orographically forced ascent in windward
terrain
to spread 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 moving upslope into this higher terrain. WPC
probabilities are moderate for 8 inches in these areas, with
locally more than 12" possible.  4" to 8" of snow is likely D1 in
the San Bernardino range.

A second area of snow on Day 1 is across the OR Cascades down into
the CA Cascades, Shasta/Siskiyous, and northern CA Sierra Nevada.
The 700 mb wave driving ascent and moisture convergence streams
ashore this morning and crosses the WA/OR Cascades today,
continuing down into the CA Sierra Nevada until the 700 mb trough
moves inland into NV.
As the wave crosses southern NV into adjacent UT Monday, the ECMWF
develops a closed low, with focused moisture convergence near the
circulation center driving heavy snow across the ranges of
southern NV/southern UT. 

A prolonged period of moist advection is likely in the mountains
of AZ due to the slow movement of the mid level circulation
center. Around a foot of snow is expected in the Mogollon Rim,
supported by high probabilities for 12" in the Mogollon Rim. 
Total snowfall across the Mogollon Rim may exceed 3 ft by Tuesday.
Moderate snow is likely in the San Gabriels and San Bernardino
Mountains of southern CA.

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 CA.  The strong jet max crossing the
Siskiyous and Shastas/CA Cascades and then the northern CA Sierra
Nevada Mountains leads to high confidence in heavy snow, as
multiple models show 1-2 inches of liquid equivalent of snow, with
higher local maxima.
The probability of a foot of snow peaks in the Siskiyou Mountains,
where is is 80-99 percent for the 24 hours ending 12z Wed.

The probability of a quarter inch of freezing rain is less than 10
percent Day 3.

Petersen