FOUS11 KWBC 162023
QPFHSD

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

Valid 00Z Fri Jan 17 2025 - 00Z Mon Jan 20 2025

...Great Lakes & Central Appalachians...
Days 1 & 3...

Day 1...

Continued snowfall across the Buffalo South towns down
into the proxy of the Chautauqua Ridge will be ongoing for another
few hours with multi-hour accumulations approaching 4+" within the
most persistent bands downwind of Lake Erie. This setup will cease
with the loss of sufficient forcing, bringing an end to the threat
by the beginning of the 00z Friday time frame.

Over the Central Appalachians, the previously identified favorable
forecast for heavy snowfall remains steadfast as the overall
synoptic pattern maintained continuity within the 12z suite of NWP.
Heavy snowfall totals are anticipated 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. This has been highlighted within the
latest hi- res window with the recent 18z RAP/HRRR output
indicating a healthy uptick in snowfall beginning ~20z, lingering
through portions of the overnight period before decaying prior to
sunrise as dry air advection and decreased mid-level forcing ushers
an end to the potential. Snow looks to fall heaviest between now
and 06z Friday with rates topping 0.5"/hr in some cases. Steepening
lapse rates have been analyzed via Mesoanalysis over the Mid-
Atlantic over the past few hours along and east of the Appalachian
front, likely providing residents in the region to be on the
lookout for possible snow squalls given the favorable time of day
and more than adequate upper-level divergence aloft. With the
heaviest snowfall still relegated to maintain the Central
Appalachians, WPC probabilities continue to depict locally elevated
chances (>40%) for additional snowfall totals >4" near the MD/WV
border down through the Highlands in West Virginia. Some of the
taller peaks of eastern West Virginia could see snow totals
between 8-12" by Friday morning from the cumulative nature of the
event, especially within Tucker, Randolph, and western Pocahontas
counties.


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.

Across the Central Appalachians, a secondary round of heavy snow
within the 72 hour period will take shape as the Arctic front
pushes across the terrain with prevailing westerly flow advecting
bitter cold air within the primary cold air advection regime post-
Arctic front. A period of upslope snowfall will begin Saturday
behind expected fropa, weakening as we step into early Sunday
laying the ground work for the next round of expected snowfall as
surface cyclogenesis is forecast across the Southeast thanks to an
amplifying trailing wave tracking towards the Mid Atlantic coast.
Ensemble spread has tightened as of the recent 12z suite, however
there is still some discrepancy on the magnitude of the snowfall
potential within the region. A lot is contingent on the
amplification regime of the trailing wave with most of the
ensembles now trending towards a more robust surface reflection and
regional ascent pattern across the entire Mid Atlantic extending
west into the Appalachian Front. Latest probabilities have
increased substantially run-to-run given the trends with the >4"
signal now within the moderate-high category (40-70%), even
carrying a small region of (40-60%) for >6" within the favored
areas of West Virginia, primarily the zones capable of both the
upslope component combined with the synoptic impacts anticipated
over the course of Sunday.


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

There has been little change with the impending snowfall in these
regions, 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 (25-50%) 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.

...Mid Atlantic and Northeast...
Day 3...

A trailing wave along the southern periphery of the Arctic front
will exit out of the southeastern U.S with an expectation for the
wave to amplify as it approaches the Southern Mid Atlantic with
attendant surface cyclogenesis across the Deep South, moving
northeast. Recent ensemble and deterministic output via the 12z
suite of NWP has become increasingly aggressive with the signal for
a low end moderate event for heavy snow prospects, mainly from
southwestern VA up through the Central Mid Atlantic, eventually
bleeding into the periods following as the pattern shifts northeast
up through the Northern Mid Atlantic (PA/NY) and Southern New
England. There has been some discrepancy within the suite on the
exact placement of the heaviest precip potential with a split on
two camps favoring a more amplified setup (UKMET/CMC) and a less
amplified evolution (ECMWF) with the GFS now solidly in the middle
of the two camps. ECMWF AIFS ML output has been very steady over
the course of the last 3+ days of output with the latest run
showing great consistency when assessing run-to-run variability.
This lends credence to a slightly better confidence with regards to
snowfall potential across the area with the probabilities of >4"
going from close to 0% overnight towards (10-30%) within a span of
one run. It's a period to monitor as heavy snowfall threat would
impact the population centers of the DMV up through Philadelphia
and points northeast in the periods beyond.

Mullinax/Kleebauer


$$

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