Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
350 AM EST Wed Feb 14 2024

Valid 12Z Wed Feb 14 2024 - 12Z Sat Feb 17 2024

...Northwest and northern California to the northern Rockies...
Days 1-3...

An upper level trough taking on a negative tilt over the northeast
Pacific will spawn a potent storm system off the Pacific Northwest
coast and direct a healthy current of 850-500mb moisture from
northern California on north along the WA/OR coast today. The
upper low, emerging from the southwest flank of an omega block
over the Yukon, will track to the south of an upper trough over
Vancouver Island Thursday afternoon and lead to a persistent fetch
of Pacific moisture through Thursday. At the same time, a cold
dome of Canadian high pressure will be anchored over British
Columbia and Alberta, providing a sufficiently cold air-mass for
snow to fall not only over the usual mountain ranges of the
Northwest, but at lower elevations as well. Heavy snow will ensue
over the higher terrain of northern California and the Oregon
Cascades this morning, then spread north into the Washington
Cascades and east into the Blue Mountains of Oregon and the
Northern Rockies Wednesday night into Thursday. A steady diet of
500-700mb moisture flux will be channeled from the West Coast into
the heart of the Intermountain West on Thursday at the same time
as a stalled frontal boundary banked up along the Northern Rockies
and the Big Horns also helps in easterly upslope flow into western
Montana and northwest Wyoming. Snow will come to an end on Friday
for the Pacific Northwest mountain ranges, but linger in parts of
Tetons and along the Wind River Range in western Wyoming.

WPC 48-hour PWPF sports highs chances >70%) for >18" of snowfall
in the northern Sierra Nevada and the Salmon/Trinity Mountains of
northern California, the Oregon and southern Washington Cascades,
the Blue Mountains of northeast Oregon, the Boise/Sawtooth of
Idaho, and both the Tetons and Wind River Range of Wyoming.
Moderate-to-high chances (50-80%) for >8" are depicted in the
Olympics, the Bitterroots, and Absaroka. The Probabilistic WSSI
(WSSI-P) features moderate-to-high odds (50-80%) for Moderate
Impacts in the northern Sierra Nevada late Wednesday into
Thursday, but based on the forecast totals, there is higher
confidence in Minor Impacts (>70% probabilities) in all of the
mountain ranges listed above.

...Central Great Plains, Great Lakes, & Interior Northeast...
Days 1-3...

A frontal wave over the central Great Plains will strengthen as a
weak 500mb trough over the Central Plains and a more potent upper
trough over the southern Canada will create a strengthening 500mb
jet streak oriented NW-SE from eastern Montana to central Iowa
Thursday night. The divergent left-exit region of the 500mb jet
streak will provide plenty of upper level support to spawn an
organized surface low along the frontal boundary draped over the
Midwest Wednesday afternoon. A swath of snow will develop on the
northern flank of the surface low where 850mb frontogenesis will
be strongest starting Wednesday afternoon in southern South Dakota
and tracking into the Upper Mississippi Valley Wednesday night. By
the Wednesday evening, the 850-700mb FGEN signal is so robust over
the Upper Mississippi Valley that the 00Z HREF shows
moderate-to-high chances (50-70%) for at least 1"/hr snowfall
rates along the MN/IA border Wednesday evening that heads into
southern Wisconsin early Thursday morning. There could also be a
secondary band over southern Minnesota and central Wisconsin that
aligns better with the aforementioned left-exit region of the
500mb jet streak and supports a narrow area of 500-700mb
frontogenesis. Latest WPC PWPF shows high chances (>70%) for
snowfall totals >4" across eastern South Dakota with
low-to-moderate chances (20-50%) for >4" of snowfall across
southern Minnesota and central Wisconsin.

By Thursday morning, the left-exit region of the 500mb jet streak
will continue to escort the surface low east and further
strengthen the low as it moves into the heart of the Great Lakes.
The same swath of heavy snow will track through central Michigan
Thursday morning, where the 00Z HREF is even more bullish on
exceptionally heavy snowfall rates. The 00Z HREF probabilities of
>2"/hr snowfall rates across the northern tier of Michigan's
Mitten is between 20-40%, but >1"/hr rates are fairly high
confidence, supported by probabilities as high as 70-90%. By
Thursday evening, the storm will continue intensifying and head
for northern New York Thursday evening. Not only will their be
snow along the 850-700mb FGEN, but colder air spilling in behind
the storm will trigger some lake-enhanced snow bands Thursday
night and into Friday morning. Latest WPC PWPF now shows high
chances (>70%) for snowfall totals >6" in parts of the Adirondacks
and over the Tug Hill. Low chances (10-40%) for snowfall totals
>4" are also depicted in parts of western New York. The snow
associated with the strong WAA and 850-700mb FGEN will also
trigger mountain snow in parts of New England Thursday morning
into Friday with WPC PWPF showing moderate chances (40-60%) for
snowfall totals >4" in parts of the Green Mountains and the
Berkshires.

...Central Rockies & Central Great Plains...
Days 2-3...

A fast moving 500mb vorticity maximum will track into the Great
Basin early Thursday morning and direct a conveyor belt of 700mb
moisture into the heart of the Intermountain West. Mean 850-300mb
winds within the column out of the west and sufficient PVA will
provide ample lift (both synoptically-forced and via
topographically-favored terrain) to support periods of snow in the
northern Colorado Rockies Thursday afternoon and into Thursday
night. Latest WPC PWPF shows moderate-to-high odds (50-70%) for
>8" of snowfall along the Park Range of northern Colorado. Farther
east, broad 200-500mb troughing over the Intermountain West and
downsloping winds into the central Great Plains will lead to lee
cyclogenesis over eastern Colorado on Thursday. This is occurring
at the same time as a cold front dives south from the northern
Great Plains and southerly 850mb moisture flux streams north from
the southern Great Plains. What unfolds is 850-700mb frontogenesis
over western Nebraska and southern South Dakota Thursday afternoon
that moves south and east into the Middle Mississippi Valley by
Friday morning. Global guidance remains unsure as to whether this
850mb frontogenesis sets up farther north or south and its
intensity as it heads for the Middle Mississippi Valley, but there
is growing consensus that it will lead to a swath of
moderate-to-heavy snow in parts of the central Great Plains.
Latest WPC PWPF shows low chances (10-30%) for >4" of snowfall
from the Black Hills to southern South Dakota Thursday into early
Friday morning. Meanwhile, in northeast Nebraska and northwest
Iowa, PWPF show moderate odds (40-60%) for >4" Thursday night into
Friday. This setup, given the impressive mesoscale signal, could
lead to localized amounts surpassing 8" should the combination of
vertical ascent within the column, modest DGZ saturation, and
sufficiently cold boundary layer temperatures come to fruition.

...Central Appalachians...
Day 3...

By Friday morning, the storm system responsible for the heavy snow
Thursday night in South Dakota will make its way through the
Middle Mississippi Valley early Friday and into the OH/TN Valleys
by Friday evening. Precipitation will blossom over the Ohio Valley
Friday afternoon and evening as a robust 200kt 250mb jet streak
forms over the Ohio Valley. The precipitation shield will be
co-located beneath the divergent left-exit region of the 250mb jet
streak and be driven aloft by not just an established 925-700mb
frontogenesis layer at lower levels, but also strong 300-400mb
frontogenesis via vertical cross-sections from the GFS over the
Mid-Atlantic 00Z Saturday. While this event may lack rich Gulf of
Mexico moisture and will be fast moving, excellent synoptic scale
ascent can still support heavy snowfall rates from the Ohio Valley
to the Mid-Atlantic. In addition, mean 850-300mb winds out of the
west will support strong upslope flow into the central
Appalachians. This is likely to be a fast moving storm, but with
the help of upslope flow, latest WPC PWPF shows low chances
(10-30%) for snowfall totals >4" along the Potomac and Laurel
Highlands. The storm system is forecast to track through the
Tennessee Valley and quickly track over the southeast Virginia
Tidewater by early Saturday morning with the axis of snow falling
just north of the storm's track. Given the exceptional synoptic
and mesoscale forcings at play and sufficiently cold boundary
layer conditions, there is also a chance for measurable snow
farther east over northern Virginia, northern Maryland, and from
central Pennsylvania on east to the Delaware Valley and Jersey
Shore. Latest WPC PWPF shows low-to-moderate chances (20-50%) for
>1" in these areas Friday night into Saturday morning with
communities along the Mason-Dixon line, across the Lower
Susquehanna Valley, and around the Philadelphia metro area
currently on the higher side of that probabilistic range. This
could make for slick travel conditions in some parts of the
Mid-Atlantic Saturday morning.


Mullinax