FOUS11 KWBC 160813
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
313 AM EST Thu Jan 16 2025

Valid 12Z Thu Jan 16 2025 - 12Z Sun Jan 19 2025

...Great Lakes & Central Appalachians... Days 1...

A potent 500mb trough diving south through the Great Lakes this
morning will foster healthy upper level divergence over the
Northeast and Central Appalachians today while also directing 
plume of 700-300mb layer-averaged moisture through these regions. 
In addition, the brief shot of low-level CAA will trigger some 
lake-effect snow showers today with the Chautauqua Ridge sporting
moderate-to-high chances (50-70%) for snowfall totals >4" through 
Thursday evening. The Central Appalachians are most favored for 
heavy snowfall totals given their favorable position beneath the 
diffluent left-exit region of a 100kt 500mb jet streak and upwards 
of 40kt westerly winds at 850mb aiding in healthy upslope snowfall.
Snow looks to fall heaviest beginning late morning and lasting 
through the afternoon with rates topping 0.5"/hr in some cases. In 
fact, given the steepening lapse rates Thursday afternoon over the
Mid-Atlantic, residents in the region should be on the lookout for 
possible snow squalls given the favorable time of day and more than
adequate upper-level divergence aloft. Snow in the Central 
Appalachians looks to continue Thursday night and finally tapers 
off by Friday morning. WPC probabilities depict high chances 
(>70%) for snowfall totals >6" from the Laurel Highlands of 
southern Pennsylvania and near the MD/WV border to the Allegheny 
Highlands in West Virginia. Some of the taller peaks of eastern 
West Virginia could see totals around a foot of snow by Friday morning.

Day 3...

By Friday night, a deepening area of low pressure will escort a
strong Arctic front south that will deliver a frigid air-mass that
is the coldest and most dangerous of the season this weekend and
into next week (see Key Messages linked below). The Arctic front
will turn on the LES machine over Lake Superior by early Friday
morning while periods of snow develop along and in wake of the
Arctic frontal passage over the Great Lakes Saturday afternoon, 
then over the eastern Great Lakes and central Appalachians Saturday
evening. LES bands look to form over parts of central and western 
New York Saturday night that likely persist through the remainder 
of the weekend. Through 12Z Sunday, WPC probabilities show 
moderate-to-high chances (50-70%) for snowfall >4" and low chances
(10-30%) for snowfall >6" in the Tug Hill Plateau, but expect 
these probabilities and snowfall totals to rise as the LES bands 
that linger in the medium range enter the short range over the next
24-48 hours. 

...Rockies and High Plains... Days 2-3...

The impending snowfall in these region, starting late Thursday
night in the northern Rockies/High Plains, is driven by a
combination of both upper level disturbances and the arrival of a
bitterly cold Arctic air-mass that is destined to be the coldest
air-mass of the season to infiltrate the Lower 48. This
exceptional cold front (a "blue norther" out ahead of will push 
south Thursday night at the same time as 500mb PVA ahead of an 
approaching upper trough occurs over the northern Rockies/High 
Plains. Snow showers will breakout across Montana early Friday 
morning and make its way south throughout the day into Wyoming, 
then into Colorado by Friday night. Southern Montana and into 
Wyoming, in particular, sport notably higher snow squall parameters
given the stronger surface- based heating that will cause steeper 
low-level lapse rates. Snow squalls would cause rapid reductions in
visibility due to a combination of heavy snow rates and whipping 
wind gusts, as well as rapid accumulations on roads as temperatures
plummet well below freezing. Motorists in these areas should keep 
a close eye on potential snow squalls as it can cause dangerous 
travel conditions in a matter of seconds.

As the front plunges south Friday evening, the snow potential
shifts south into Colorado with periods of snow getting started
near the start of the evening rush hour in the Denver/Boulder metro
area. While the snow squall potential may not be as high, the
easterly upslope-enhancement of snowfall rates will make for
occasional periods of heavy snow Friday evening. Snowfall rates
will weaken as the front works its way south of the Palmer Divide
Friday night and down the spine of the Sangre De Cristo early
Saturday morning with some snow along the Front Range lingering
into the day on Saturday as a dome of ~1050mb high pressure builds
in from the Canadian Prairies. WPC probabilities are keying in on
the Front Range as the focus for the heaviest snowfall with
moderate-to-high chances (50-70%) for snowfall >4" from the Laramie
Range on south to parts of the Sangre De Cristo. The I-25 corridor
from Fort Collins on south to the Colorado Springs area Friday 
have similar probabilities for >4" of snowfall with even low 
chances (10-30%) for greater than 6" between Friday evening and 
Saturday morning in spots. Some of the higher peaks (>10,000ft) 
have moderate chances (40-60%) for snowfall >8" through Saturday 
morning. The WSSI is currently depicting Minor Impacts for just 
about all of these referenced areas, suggesting the potential for 
winter weather conditions that would require enhanced caution while
driving in these affected areas.

Mullinax

...Winter Storm Key Messages are in effect. Please see current 
 Key Messages below...

https://www.wpc.ncep.noaa.gov/key_messages/La...

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