Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
401 AM EST Sat Jan 21 2023

Valid 12Z Sat Jan 21 2023 - 12Z Tue Jan 24 2023

...Central Rockies and High Plains...
Days 1-3...

As of this morning, a winter storm is unfolding across the Central
Plains as an upper low tracks through Kansas later today. The
heaviest banding of snow, set to occur beneath of the TROWAL, is
expected to setup from far southeast Colorado early Saturday
morning and make its way across western and central Kansas. This
band will be fueled by a surge of 850mb moisture flux from Texas
and wrapping around the 700mb low, allowing for a classic warm
conveyor belt to produce snowfall rates as heavy as 1"/hr. The
WSSI beyond 12Z shows the Moderate imapcts area to stretch from
southeast Colorado to west-central Kansas, including areas south
of Goodland and north of Garden City. Expect hazardous travel
conditions in these areas with heavy snow rates and wind gusts up
to 30 mph contributing to near zero visibility in some cases. Snow
will gradually taper off through the afternoon as the 700mb opens
up into a trough and the previously supportive 850mb transport
wains. Still, expect 1-3 inches of snow across northern Kansas
Saturday afternoon with some slick travel conditions possible.

High pressure building in Saturday night in wake of the winter
storm will provide the Central Rockies and High Plains with a
break through Sunday. The break is short lived, however, as the
next and more anomalous upper low dives south through the
Intermountain West on Sunday and dives all the way into Arizona
and New Mexico by Monday. NAEFS shows 500-700mb heights by 06-12Z
Monday over Arizona and New Mexico that are as low as the 1%
climatological percentile. Periods of snow in the central Rockies
and central High Plains will break out Sunday evening as Pacific
moisture associated with the upper trough and diffluent flow
within the 250-500mb layer supports a broad swath of precipitation
from Wyoming on south into Utah and Colorado. Meanwhile, in the
Southern and Central Plains, an 850mb jet will strengthen over the
TX Panhandle and far eastern New Mexico, eventually becoming as
strong as 35-45 knots in West Texas. This LLJ will act to provide
an increase in low level moisture, as well as an upslope component
into the Sangre De Cristo of northern New Mexico and southern
Colorado, the Palmer Divide, and into the Front Range of the
Colorado Rockies. Combined with the nose of the 500mb jet
providing an excellent source of lift at the upper levels of the
atmosphere, heavy snowfall is likely to develop in central
Colorado late Sunday night and into Monday morning. This includes
metro areas such as Denver and Colorado Springs. Latest WPC PWPF
shows 70-80% probabilities of >6" of snow in the Denver metro area
and along the Palmer Divide. The Day 3 WSSI does features Moderate
impacts in these areas. It is worth noting some southerly tracks
in 00Z deterministic guidance tonight that could lead to a more
southerly push in snowfall totals in future forecasts. Still
weighting more of the ensembles for now, but these trends are
worth monitoring in future forecast cycles.

By Monday night, the upper low continues its track into New Mexico
while a developing wave of low pressure forms in west Texas. there
remains a good amount of spread in guidance regarding the track
and evolution of the storm system as it enters Texas by Tuesday
morning. The latest WSO for Day 3 does show >50% probabilities of
snowfall totals exceeding warning criteria in the OK/TX
Panhandles, but given some ensemble members have shown some
additional clustering of tracks farther south, there could be
additional adjustments in future forecast cycles. Regardless, the
setup is ripe for heavy snow in parts of West Texas that could
eventually work its way into north Texas and/or central Oklahoma
by Tuesday.

...Northeast...
Day 2-3...

The upper trough responsible for heavy snow in the central High
Plains will make its way east into the Middle Mississippi Valley
by Sunday afternoon. Strong WAA at low-mid levels out ahead of the
trough and a rich reservoir of moisture will stream from the Gulf
Coast up the East Coast Sunday afternoon. PWs by 00-06Z Monday
range between 0.5-0.75" from southern PA up the East Coast and
into portions of southern New England, which is right around the
90th climatological percentile according to NAEFS. In fact, mean
specific humidity values in the 500-700mb layers are above the
90th climatological percentile from northern PA to coastal ME,
showing there is also anomalous moisture within the DGZ across a
good portion of the Northeast Sunday night. High pressure over the
Northeast Sunday morning will be retreating east into the
northwest Atlantic, leaving the lingering area of sub-freezing
temperatures without much of an anchor to keep any of the already
marginally cold temperatures in place. Given the unusual amount of
moisture and meager thermodynamic profiles, the setup for snowfall
across the Northeast feels more like what one would see in March,
let alone in late January. The calendar is still correct though,
so most lower level temperatures should support snow well north
and west of I-95, but just like in March, it will be the more
elevated and mountainous terrain that feature the best odds of
receiving heavy snow with valleys and snow shadowed areas
receiving less.

Recent 00Z guidance has shown a little stronger confluence over
Quebec than previous runs that is limiting the northerly extent of
the precipitation field. Guidance does maintain a potent upper
trough over the Mid-South on approach, but the aforementioned
confluent flow is forcing the trough to remain more
positively-tilted in the 500mb layers by 06-12Z Monday. This is
leading to the best frontogenetical forcing and strongest vertical
ascent beneath the right-entrance region of the 250mb jet streak
to become positioned more offshore rather than over central New
England, rather than a more robust, intense FGEN setup over
northern New England. Think the latest PWSSI is showing a
reasonable swath of >40% probabilities of Moderate impacts from
the Catskills and southern Adirondacks to the Green and White
Mountains of VT and NH respectively, and across central ME. These
areas can still witness some banding on the northern flank of the
850mb low as it tracks south of Long Island and off the
Massachusetts Capes by early Monday morning.

Latest WPC PWPF displayed 50-70% probabilities for >6" of snow in
the southern Adirondacks with as much as 70-80% chances for much
of VT, central and northern NH, and central ME. There were also
some 30-50% probabilities for >12" of snow in central ME, where
the strongest banding could occur as the coastal low intensifies
in the Gulf of Maine Monday morning. However, do note that given
recent trends, the axis of highest probabilities has the potential
to shift roughly 50 miles south in future forecast cycles. At this
time, the WSSI shows a large Moderate impact areas across central
ME, suggesting hazardous travel and disruptions to daily life may
occur.

...Pacific Northwest to the Northern Rockies...
Days 1-3...

An potent but fast-moving disturbance just off the coast of
British Columbia this morning is set to bring more mountain snow
into the Olympics and northern Cascades today into Saturday night.
Locally heavy accumulations will be confined to the higher peaks
of these ranges.  Model consensus is in agreement on the trough
amplifying as it moves southeast into the interior Northwest
Saturday night and into Utah by Sunday. Still not anticipating any
widespread heavy amounts, but locally heavy accumulations are
possible from northern Idaho to the western Montana ranges.
Northerly winds in the wake of a low level frontal passage moving
east from the Rockies into the High Plains will provide some
upslope enhancement and cause some higher snowfall totals in the
mountainous terrain of central Montana Sunday afternoon and into
Sunday night. 


Mullinax