FOUS11 KWBC 031927
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
227 PM EST Fri Jan 3 2025

Valid 00Z Sat Jan 04 2025 - 00Z Tue Jan 07 2025

...Western U.S....=20
Days 1-3...

The busiest stretch of winter weather in the Pacific Northwest and
Northern Rockies occurs over the next 24-48 hours. The region is
already witnessing periods of heavy mountain snow along the spine
of the Cascades and as far east as the Absaroka and Lewis Range.
Some very high elevations snow is occurring as far south as the
Trinity /Shasta and the northern Sierra Nevada. As a robust 500mb
trough moves over the Pacific Northwest this evening, the strongest
synoptic-scale ascent will be over the Northern Rockies as
excellent250-500mb diffluence aloft with >90th percentile=20
500-700mb mean specific humidity values support a swath of heavy=20
snow as far east as the Little Belt and Big Snowy Mountains of=20
Montana, and as far south as the Bear River Range bordering Idaho=20
and Utah. The Lewis Range and Bitterroots in particular also have=20
the added help from a large 1040mb+ high over southern Canada that=20
is prompting anomalous southeasterly flow (<1st percentile at 850mb
on NAEFS tonight) to cause upslope enhancement on the eastern=20
slopes of these ranges.=20

By Saturday morning, the storm continues to head east with snow=20
ending in parts of the Sawtooth and Bitterroots but peaking over=20
the Tetons and Bear River Range. Some measurable snowfall is=20
expected in the Wind River and Big Bear Ranges, including ranges as
far south as the Colorado Rockies. At the same time, the next=20
Pacific storm system arrives Saturday afternoon with most snowfall=20
in the Olympics and Cascades peaking Saturday night and=20
accumulations primarily see above 4,000ft. This plume of moisture=20
will reach the northern Rockies Sunday morning, but mainly lead to=20
lighter snowfall accumulations at the more remote elevations of the
Blue, Bitterroots, and Lewis Range. There is another weak system=20
that moves into Oregon late Sunday night into Monday, but snow=20
accumulations appear minor (<4") in from the Oregon Cascades to=20
eastern Oregon.=20

In total, many of the higher peaks of the Pacific Northwest and
Northern Rockies will see heavy snowfall through Sunday with most
of the heavier amounts confined to the more remote areas of these
aforementioned mountains ranges. Portions of the Cascades, Blue
Mountains, and Lewis Range (primarily above 5,000ft) have high
chances (>70%) for snowfall totals >12". For elevations >8,000ft=20
in the Tetons, Bear River Range, and Absarokas, there are high
chances (>70%) for snowfall >8" according to WPC probabilities.


...Great Lakes and Northeast...=20
Days 1-2...

The strengthening -NAO block over Greenland and the North Atlantic
will keep a large gyre-like 500mb upper low parked over southeast
Canada and the northwest Atlantic through Sunday morning. On its
western flank, it will work in tandem with strong 1040mb+ high=20
pressure over central Canada to keep a tight pressure gradient over
the Great Lakes, resulting in cold NWrly flow over the Lakes. This
will prompt favorable LES on northwesterly flow along with a few=20
upper level disturbances traversing the Great Lakes through Sunday=20
morning. The Great Lakes will gradually cool as a result, but the=20
lapse rates will remain steep enough over and downwind of the lakes
to where lake-induced instability will exceed 500 J/kg on=20
occasion. The snow belts of the Michigan U.P., northern portion of=20
Michigan's Mitten, from northeast Ohio on north east along the=20
Chautauqua Ridge, and from the shores of eastern Lake Ontario to=20
the Tug Hill will be most favored for heavy snow through Sunday AM.
A prolonged WNW-NWrly fetch connecting a single band from lakes=20
Superior, Huron, and Ontario will help produce the more significant
amounts (1-2ft+) in Upstate NY between Syracuse and the Tug Hill.=20
Latest WPC probabilities also show the Chautauqua Ridge with=20
moderate-to-high chances (50-70%) for additional snowfall totals=20
>8" through Sunday AM, while chances for >8" in the far eastern=20
Michigan U.P. are lower chances (10-30%). Snowfall rates in these=20
areas of 1-2"/hr are expected and could come in waves through=20
Sunday. The WSSI shows Major Impacts for these areas through=20
Sunday, indicating there are likely to be dangerous to impossible=20
driving conditions for those impacted by these heavy LES bands.


...Northern High Plains...
Days 1-2...

The impending winter storm in the Northern Plains is a great=20
example of mesoscale banding via strong frontogenetical forcing.=20
Snow gets started this afternoon in parts of central Montana and=20
really increases in intensity by 12Z Saturday. At this time, a=20
700mb low over southern Alberta and an elongated trough axis will=20
prompt healthy mid-level divergence aloft from central Montana to=20
western South Dakota. Closer to the surface, lower pressure over=20
the western U.S. and strong 1040mb+ high pressure over Canada is=20
leading to winds near the surface to be unusually strong out of the
SE that is directing moisture, with origins out of the Gulf of
Mexico, into the Northern High Plains. This is leading to=20
exceptional 850mb WAA emanating out of the Southern High Plains at=20
low level. In addition, this mean SErly flow at low-levels favors=20
upslope enhancement through Saturday as the region becomes=20
favorably placed on the northern flank of the 700mb low by 18Z=20
Saturday. Lastly, SWrly 700mb winds out of the southwestern U.S.=20
are causing their own WAA and FGEN forcing as warm 700mb temps=20
collide with much colder air in the Northern Plains.

The end result is a setup for bands of heavy snowfall setting up
from the heart of "Big Sky country" on south and east into western
South Dakota. The WPC Snowband Probability Tracker shows HREF
guidance sporting 1-2"/hr rates beginning as early as this evening
and continuing through Saturday morning. Areas as far south as
northeast Wyoming also could see these heavy snow rates, primarily
east of the Big Horns where upslope flow can enhance snowfall
rates. As the best upper level dynamics shift southeast Saturday
evening, some lingering easterly flow may keep periods of snow in
the forecast through the overnight hours, but snow should finally
taper off by Sunday morning. WPC probabilities show a wide swatch
of high chance probabilities (>70%) for >4" of snow from northern
Montana to northeast South Dakota. There are also low-chance
probabilities (10-30%) for snowfall >8" in localized portions of=20
central Montana, northeast South Dakota, and northeast Wyoming. The
WSSI shows widespread Minor Impacts through the Northern High
Plains with some spotty Moderate Impact areas in north-central
Montana.=20


...Central Plains through the Mid-Atlantic...
Days 1-3...

...Widespread significant winter storm to develop tonight across
the Central Plains and then move through the Mid-Atlantic early
next week...

A closed low moving into the Pacific Northwest will fill as it
moves towards the interior, and then weaken briefly during its dive
into the Four Corners region. However, as it exits the Central
Rockies Saturday night it will re-amplify, becoming a closed low
once again into the Central High Plains. The models diverge at this
point, with still modest differences in timing and intensity, but=20
the trends in all the 3 global ensemble systems is for a deeper and
slightly slower upper low by Sunday morning, and some clustering
has occurred towards a more uniform solution. If the deeper/slower
solution continues, it would conceptually result in stronger=20
downstream warm advection and a more strong north push of=20
precipitation, but downstream confluence is also quite strong=20
suggesting a sharp northern gradient in precipitation and some=20
subsidence to squelch the northward extent/shift.=20

While the exact track will be crucial to the axis of greatest
impacts, the increased clustering of solutions suggests this axis
is becoming more defined. As the upper low shifts almost due east
through Monday with continued deepening, it will force a
strengthening jet streak aloft to help enhance ascent. This will=20
cause the accompanying surface low to deepen slowly as it moves=20
east, leading to a large swath of heavy snow, sleet, and freezing=20
rain from eastern Kansas through Maryland/Virginia. While=20
confidence is high in an impactful winter storm, the details are=20
still in question.

As the low pivots east, downstream moisture advection will begin to
intensify as reflected by intense 290K isentropic ascent
blossoming across Kansas Saturday afternoon and then continuing
eastward in advance of the low. This will spread PWs that reach as
high as +2 sigma across the Central Plains/Ozarks, with subtly
lower anomalies into the Mid-Atlantic. The accompanying theta-e
ridge is then progged to lift cyclonically into a TROWAL,
especially Sunday when it becomes quite strong from KS into IN,
before weakening in response to the filling of the upper low and
shunting of the moisture to the east. The overlap of this TROWAL
with impressive WAA will help deepen the DGZ (SREF probabilities
for 100mb of depth now reach 70% in KS/MO) to increase the
potential for heavy snow rates. Additionally, cross sections
indicate a threat for CSI/CI where folded theta-e surface intersect
with -EPV, resulting in the potential for thundersnow (through CI)
and snowfall rates of 2+"/hr.

The greatest risk for banded snow appears to be within the WAA from
Missouri to Indianapolis as the setup matches the conceptual model
for a laterally translating band, but then behind the low an
extremely impressive deformation axis is likely which would have
higher SLRs and the greater potential for excessive convective snow
rates. This deformation will likely weaken towards the east as the
upper low begins to fill, but still strong WAA/fgen ahead of the
low within the narrow translating band to the east will support
impressive rates of 1-2"/hr at times. South of this band, the WAA
will overrun the cold surface temps, but forecast soundings
indicate an impressive E/NE dry advective flow beneath the warm
nose to maintain sub-freezing wet bulb temperatures. This will=20
support an axis of heavy sleet and freezing rain, generally from=20
southern MO through northern KY, where sleet accumulations above=20
1", and freezing rain accretions of 0.25-0.5" are likely.=20

As the system moves east and weakens a bit, there is even more
spread in the track, but still a heavy corridor of snow and mixed
precipitation is likely from Ohio to Delaware. The wave is opening
during this time, but the downstream WAA/fgen remains impressive
especially in the 850-700mb layer, and a continuation of the
upstream translating band is likely right through the Mid-Atlantic
states. The intensity and accompanying snowfall is likely to be
reduced from upstream, but could still feature intense rates and
heavy snow from west to east, reflected by the recent EFI reaching
0.9 with a co-located SoT of +5, highest in northern VA. The=20
placement of this band remains in question and guidance has trended
south today, but where it sets up, 1"/hr snows are likely at
times.

With the event just getting started late D1, WPC probabilities D1
are minimal for any significant ice or snow. However, by D2, the
system ramps up, and WPC probabilities for 6+ inches reach above
50% from western KS through the St. Louis metro area, and locally
12+ inches of snow is likely, especially in far NE KS or near
Kansas City, MO. During D3 the 6+ inch probabilities race eastward,
and Sunday night/Monday features a moderate risk (50-70% chance) of
6+ inches from eastern Illinois through the Washington, D.C. metro
area. South of the heavy snow, a swath of significant, even
isolated damaging icing, is likely, with WPC probabilities for
0.25" reaching 50-70% from southern MO through western KY, pushing
into eastern KY and the central Appalachians on D3. Some places,
especially in eastern MO, southern IL, or western KY, could receive
0.5" of ice resulting in damage to trees and power lines.


This system has prompted the issuance of key messages which are=20
linked below.


Mullinax/Weiss



...Winter Storm Key Messages are in effect. Please see current=20
 Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late=
stKeyMessage_1.png__;!!DZ3fjg!-9Qng90GbNE8O2r45hTz67D5vzy0c-veu6dIc4Pg_c0Jm=
YBq9zZk4tmBhZfm2MwXVxpaHjETS4uy5isgeb4F7tfnFjA$=20

$$

=3D =3D =3D
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