FOUS11 KWBC 170810
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
310 AM EST Fri Jan 17 2025

Valid 12Z Fri Jan 17 2025 - 12Z Mon Jan 20 2025

...Great Lakes... Days 2-3...

By Friday night, a deepening area of low pressure will escort a
strong Arctic front south that delivers 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 Friday night while 
periods of snow develop along the northern and western coasts of
Michigan's Mitten by Saturday morning. Following an initial WAA
thump of snow Saturday afternoon, 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 Monday, 
WPC probabilities show moderate-to-high chances (50-70%) for 
snowfall >4" and low chances (~10%) for snowfall >8" in the Tug 
Hill Plateau. The Michigan U.P. sports moderate-to-high chance
probabilities (50-70%) for >8" of snow in the Porcupine Mountains
and in parts of the eastern U.P.. Expect these probabilities and 
snowfall totals to rise as LES bands increase in coverage an 
intensity throughout the region as winds accelerate and the heart 
of the bitterly cold temperatures aloft race over the Great Lakes next week.

...Rockies and High Plains... Days 1-2...

The combination of both upper level diffluence ahead of a deep-
layer trough 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 will bring about Snow squalls and mountain
snow to parts of these regions through Saturday. This exceptional 
cold front (a "blue norther" out ahead of the upper trough will 
push south this morning while at the same time, 500mb PVA ahead of 
an approaching upper trough occurs over the northern Rockies/High 
Plains. Snow showers will breakout across Montana this morning and 
make its way south throughout the day into Wyoming, then into 
northern Colorado by Friday afternoon. 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 are also possible today along 
the cold front as it tracks east across the Dakotas Snow squalls 
can 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 beginning near the
start of the evening rush hour in the Denver/Boulder metro area. 
While the snow squall potential may not be as high by this time, 
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. The one notable exception is the 
Denver metro area as Moderate Impacts are now depicted, indicating 
there are likely to be disruptions ot daily life with potential 
closures and hazardous driving conditions.

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

...Confidence is increasing in a disruptive winter storm from the
Mid-Atlantic to the Northeast with locally heavy snowfall possible
along the I-95 corridor...

Model diagnostics: Over the past 24-36 hours, guidance has come 
into better agreement on a deepening 500mb trough over the Ohio 
Valley and higher height rises in the Atlantic south of Bermuda. 
AI-based guidance (ECMWF-AIFS and GFS GraphCast) have been showing 
this increasing potential over this time span while ensembles 
(ECMWF-EPS, GEFS, GEPS) have all gradually trended snowier as 
well. While this is the case, deterministic guidance still shows 
the lingering uncertainties with the 500mb trough's amplitude and 
position 12Z Sunday. The 00Z GFS is slightly deeper vs the 00Z 
ECMWF. The 00Z CMC is similar to the ECMWF 500mb disturbance's 
strength, but with less confluence over the Northeast, allowing 
for a more storm track on the northern side of the track 
distribution. The UKMET also has a similar look to the Canadian, 
albeit with a colder boundary layer setup given stronger confluence
over the Northeast. Overall, the AI-based guidance and the trends 
in the ensembles provide an increased confidence in a synoptic- 
scale setup that is boosting confidence in a winter storm from the
Mid-Atlantic to the Northeast, but more specific details on storm 
track still need to be ironed out over the next 24-36 hours.

Synoptic & mesoscale analysis: The approaching upper-level trough 
Saturday night will amplify over the Ohio Valley and place a 
strengthening 250-500mb layer averaged jet streak over the Mid- 
Atlantic by Sunday morning. NAEFS shows 200mb and 500mb winds over
the Southeast U.S. are above the 90th climatological percentile 
and foster healthy upper level divergence aloft on Sunday. At 
lower levels, a cold front Saturday night will stall out along the
Mid-Atlantic coast and into southern Virginia. The front, residing
beneath the synoptically-favorable jet stream setup, will spawn an
area of low pressure Sunday morning in the Carolinas and gradually
strengthen as it heads for the Mid-Atlantic coast Sunday 
afternoon. The heaviest banding of snow is likely to reside north 
of 850mb frontogenesis, which will become more and more pronounced 
as the 850mb low forms somewhere along the Jersey Coast and tracks 
northeast towards Long Island and the MA Capes Sunday evening. The
storm is a progressive one, but given the sufficient synoptic and 
mesoscale processes, the deformation zone of heavy snow would be 
more than capable of generating ~1"/hr snowfall rates, especially 
for areas from the Delaware Valley on north and east through the 
Tri-State area and into southern New England where they are more 
likely to have better mesoscale forcing as the 850mb low deepens Sunday night.

Timing & snowfall/impact probabilities: As mentioned before, this 
is a quick hitting system that is all but gone by midday Monday as 
it races toward the Canadian Maritimes by Monday afternoon. Snow 
is forecast to be at its heaviest Sunday morning and afternoon for
the DC/Baltimore metros, the Philly/NYC metros Sunday afternoon 
and evening, then across southern New England Sunday night into 
early Monday morning. Latest WPC probabilities show moderate-to- 
high chances (50-70%) for snowfall totals >4" from the Delaware 
Valley on north through the Lower Hudson Valley and across southern
New England. Low-to-moderate chances (30-50%) for >4" extend as 
far north and east as Downeast Maine. WPC probabilities even show 
some spotty low chance probabilities (10-20%) for snowfall totals 
>8" in parts of southern New England. The DC/Baltimore metro areas 
sport low chance probabilities (10-30%) for >4" of snowfall at the
moment, while the Philly and NYC metros are in the moderate range 
(40-60%) for >4" of snow. While exact amounts remain unclear, the 
WSSI-P Minor probabilities are >60% for the full length of I-95 
from DC to Boston, indicating the heightened potential for hazardous
travel conditions Sunday into Monday. WPC has initiated Key 
Messages for this event and can be viewed at the bottom of this discussion.

Mullinax

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

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

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

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