Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
500 AM EDT Tue Mar 19 2024

Valid 12Z Tue Mar 19 2024 - 12Z Fri Mar 22 2024

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

Continued WSWly to Wly flow under troughing over Lake Ontario
today leads to more LES into the Tug Hill plateau where Day 1 PWPF
for >6" additional is 40-80%.

The next shortwave trough is currently over northern Manitoba and
will swing through the upper Great Lakes tonight before
closing into a mid-level low over Lake Ontario on Wednesday before
crossing northern New England Wednesday night. This low tracks on
the left exit region of a WNWly jet over the Great Lakes. A cold
front will race through the Northeast on Wednesday reinforcing cold
air over the Northeast, with developing low pressure on the north
end of the cold front over New England late Wednesday. Given the
timing of the frontal passage over northern Pennsylvania and
central New York, steepening lapse rates should result in some snow
squalls during the diurnal maxima of midday/afternoon Wednesday.

There is a concern for more significant snowfall totals and rates
Wednesday afternoon into Thursday over northern New York/New
England. This depends on the low strength which is still uncertain,
but the CMC continues to be most bullish in terms of precip
magnitude, though the 00Z ECMWF has continued its trend of heavier
precip shifting north, focusing on the White Mtns in NH through
Northwest Maine. Gulf-stream moisture wraps around the low making
for a risk of heavier snow rates. Day 2 PWPF for >6" is 30-70% over
northern portions of the Adirondacks and across the Greens/Whites
which expands over northern Maine for Day 2.5. Wrap around snow
should linger over northern Maine into Thursday.


...Northern Rockies, Northern Great Plains, through Upper
Midwest...
Days 2/3...

Strengthening easterly flow along the eastern slopes of the
MT Rockies, as high pressure shifts south from the Canadian
Prairies, 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 draped in a positively-tilted trough 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 across the Dakotas and
then
extends through MN/WI Thursday night.

Day 2.5 PWPF for >6" is 50-80% around Glacier NP/the Lewis Range
(snow levels around 6000ft on the western slopes and at the
surface east) and for border locations in northeast MT up along the
border.

Day 3 PWPF for >6" is 10-30% over southern MN and north-central
WI.



Jackson/Mullinax