Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
239 PM EDT Mon Mar 18 2024

Valid 00Z Tues Mar 18 2024 - 00Z Fri Mar 21 2024

...Eastern Great Lakes into New England...
Days 1-3...

A longwave troughing pattern over southeastern Canada will direct
a series of upper level disturbances at the Great Lakes and
Northeast through much of the work week that will also play a role
in a prolonged period of cyclonic flow in the regions. A 500mb
lobe of vorticity and WNW 850-500mb flow will support periods of
snow; both lake effect and upslope enhanced, will occur down wind
of lakes Erie and Ontario, as well as in parts of the Green and
White Mountains. Snow will decrease in intensity over the White and
Green Mountains by Tuesday morning, but the next 500mb disturbance
approaching from the Upper Midwest will spawn low pressure over
southern Ontario that spurs additional snow showers over the
northern and eastern Great Lakes Tuesday afternoon. WPC PWPF
between 00Z Tues - 00Z Wed sports moderate-to-high chances
(50-80%) for >4" of snowfall in the Tug Hill and highest peaks of
the Green and White Mountains.

By 00Z Wed, the aforementioned 500mb disturbance will accompany a
~90kt 500mb jet streak over the Lower Great Lakes. The diffluent
left-exit region will be located over the Northeast and upper
level divergence will only increase on Wednesday as a second 500mb
jet streak over the Upper Midwest allows the 500mb jet streak to
top 100kts over the Great Lakes Wednesday afternoon. A cold front
will race through the northern Mid-Atlantic and bring about a
renewed surge in 850mb CAA over the Northeast, while a surface
trough over the Upper Great Lakes reinvigorates lake effect snow
showers over the U.P. of Michigan and over central New York. Given
the timing of the frontal passage over northern Pennsylvania and
central New York, steepening lapse rates could result in some
localized snow squalls late Wednesday morning and through late
Wednesday afternoon. WPC PWPF between 00Z Wed - 00Z Thurs does
depict pockets of moderate-to-high chances (50-80%) for snowfall
totals >4" in Michigan's eastern U.P., while similar odds for >6"
of snowfall are in place over the Tug Hill.

Ongoing CAA, 500mb height falls, and diffluent flow aloft at
250-500mb Wednesday night will support additional heavy mountain
snow in the northern Appalachians beyond 00Z Thursday and into
Thursday morning. However, as high pressure quickly builds in from
the Great Lakes, periods of snow will diminish as the pressure
gradient lessens, allowing for just a few lingering snow showers by
Thursday afternoon in northern New England. WPC PWPC depicts
moderate-to-high chances (50-80%) for snowfall totals >4" in
portions of the Adirondack, Green, and White Mountains, as well as
as across northern and central Maine between 00Z Thursday - 00Z
Friday.

...Northern Rockies & Northern Great Plains...
Day 3...

Summarizing the synoptic scale picture around 00Z Wednesday, strong
high pressure over south-central Canada will lead to strengthening
easterly flow along the eastern slopes of the northern Rockies and
a stalled frontal boundary. Meanwhile, a narrow ribbon of 700mb
moisture will reside beneath the divergent right-entrance region
of a 250mb jest streak oriented NW-SE that extends from southern
Alberta all the way to the Mid-Atlantic. By Wednesday morning,
850-700mb frontogenesis over northern Montana and extend as far
east as western North Dakota will hover over the head through
Wednesday night and into Thursday morning. A band of snow will form
over the northern and eastern Plains of Montana, as well as
western North Dakota, while upslope flow leads to accumulating
snowfall along the Lewis Range and as far east as the Big Snowy
Mountains. By 00Z Thursday, a wave of low pressure forming along
the aforementioned stationary front will have a renewed surge of
Pacific moisture to work with thanks to an approaching upper low
along the Pacific Northwest coast. This disturbance will work its
way east across the Northern Rockies by early Thursday morning and
foster a new area of low pressure over central Wyoming. On the
northern flank of the low, renewed 850-700mb frontogenesis will
give rise to a deformation axis of snow over the Dakotas, but there
remains some different iterations from guidance member to guidance
member on where the band of moderate-to-heavy snow sets up
Thursday morning and into Thursday afternoon. At the moment, WPC
PWPF shows a broad swath of low chances (10-30%) for >4" of
snowfall from central North Dakota to as far east as western
Minnesota
between 00Z Thursday - 00Z Friday. These snowfall totals have the
potential to cause hazardous travel conditions, so residents in
these areas should monitor the forecast closely in the next 24-48
hours. There is better consensus across guidance that the far
northern Plains of Montana and the northern most Rockies of
Montana. WPC PWPF shows moderate chances (40-60%) for >4" of
snowfall through 00Z Friday (and still snowing beyond that) in the
northern most Plains of Montana, while there are high
probabilities (>70%) for snowfall totals >8" along the Lewis
Range above 6,000ft.

Mullinax