FOUS11 KWBC 210819
QPFHSD
Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
419 AM EDT Thu Mar 21 2024
Valid 12Z Thu Mar 21 2024 - 12Z Sun Mar 24 2024
...Northern Plains through the Great Lakes...
Days 1-2...
A fast moving 500mb disturbance exiting the northern Rockies is
partly responsible for the development of low pressure in eastern
Wyoming later this afternoon. Farther north, a narrow and NW-SE
oriented 700mb FGEN zone stretching from southern Alberta to
eastern Montana. As the upper level disturbance tracks east, a
surge in 700mb WAA and 500mb PVA will coincide with an adequate
plume of 700-300mb moisture aloft. Combined with upslope easterly
flow (thanks in part to high pressure anchored over south- central
Canada) and a boundary layer just enough below freezing, and the
end result is now from the northern Bitterroots and Lewis Range to
as far east as central South Dakota. While the former are largely
due to upslope flow, the developing swath of heavy snow is driven
by the aforementioned 700mb FGEN and the region being located
beneath the divergent right- entrance region of a 250mb jet streak
positioned over the Great Lakes. Meanwhile, the surface low
Thursday evening will deepen with a strengthening LLJ that will
direct a plume of rich low level moisture around the northern flank
of the low and into the sufficiently cold air-mass in the Upper
Midwest. The expectation is for a narrow band of heavy snowfall to
ensue Thursday afternoon and evening over southeast North Dakota
and northeast South Dakota that will track east into central
Minnesota Thursday night.
By early Friday morning, the band of heavy snow will race east
across the Mississippi River and into central Wisconsin. Guidance
continues to show coherent 700mb FGEN banding and synoptically-
forced ascent thanks to the 250mb jet streak over Wisconsin that
will persist over Michigan during the daytime hours. While those
in the Upper Mississippi Valley will have received their snowfall
at night, the bulk of the snow in eastern Wisconsin and Michigan
will come during the daytime hours. While hourly snowfall rates
topping 1"/hr are possible, most accumulations will be confined to
grassy surfaces, sidewalks, and side streets given the strong late
March sun angle in play. Snowfall will also likely compact
quickly, making it a pasty snow in these areas on Friday. Snow will
come to an end by Friday evening as the storm races into northeast
Ohio. WPC PWPF sports high chances (70%) for >4" of snowfall in
southeast South Dakota, east-central Wisconsin, and central
Michigan. WPC PWPF does show low chances (10-30%) for snowfall
totals >8" in localized portions of east-central Wisconsin and
central Michigan. The WSSI-P does depict a swath of moderate-to-
high chances (50-70%) for Minor Impacts throughout most of these
affected areas (particularly in areas with >4" snowfall totals),
suggesting the potential for a few inconveniences to daily life
that are primarily due to possible snow covered roads and reduced
visibilities.
...New England...
Days 1-3
Periods of heavy snow will linger across portions of the northern
Appalachians and northern Maine today as a powerful sub 990mb low
over Downeast Maine tracks into southern Quebec later today.
Moderate impacts, largely due to a combination of heavy snow and
blowing snow, will stick around this morning but conditions will
gradually improve this afternoon before finally concluding Thursday
evening.
While high pressure briefly builds in Thursday night and sticks
around for most of Friday, the storm system responsible for periods
of moderate-to-heavy snow in the Upper Midwest and Great Lakes will
begin to produce snow over northern New York Friday evening.
Similar to the Upper Midwest and Great Lakes, the seemingly endless
swath of 700mb FGEN and upper level divergent flow courtesy of a
250mb jet streak's right-entrance region aloft will be responsible
for the initial round of snow that occurs from the Adirondacks
Friday night to the Northeast Kingdom and both the Green and White
Mountains through early Saturday morning. However, farther south,
a potent 500mb trough in the Southeast will direct low pressure in
the eastern Carolinas north up the Eastern Seaboard. This upper
trough will also stream anomalous moisture with Gulf of Mexico
origins up the East Coast. As snow moves into northern Maine during
the day on Saturday, the 250mb jet streak over southeastern Canada
will strengthen at the same time as another jet streak along the
Mid-Atlantic coast also intensifies. This will support a brief
"kissing jet" setup along the New England coast that will produce
heavy precipitation rates, perhaps as far inland as central and
northern Maine Saturday afternoon and early Saturday evening. The
storm will quickly race northeast Saturday evening and all snow is
likely to conclude after midnight.
Latest WPC PWPF shows high chances (>70%) for snowfall totals >8"
Friday evening through Saturday evening in parts of the
Adirondacks, Green and White Mountains, and through central Maine.
Given the dearth of moisture and exceptional synoptic scale forcing
aloft, there are even moderate-to-high chance probabilities
(50-70%) for >12" from Vermont's Northeast Kingdom on north and
east into north-central Maine. The WSSI-P currently shows moderate-
to-high chances (50-70%) for Moderate Impacts beginning Friday
evening and lasting into Saturday night from southern Vermont and
most of interior New Hampshire to much of inland Maine.
$$
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