Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
419 PM EDT Sun Apr 17 2022

Valid 00Z Mon Apr 18 2022 - 00Z Thu Apr 21 2022

...West Coast to the Northern Rockies...
Days 1-3...

A series of shortwaves lifting onshore the Pacific Northwest
embedded in otherwise zonal but confluence flow will bring a
series of cold fronts and precipitation to the Northwest through
Thursday. The first of these is expected to surge onto the WA
coast Monday night, preceded by robust WAA ahead of the
accompanying cold front, to combine with the LFQ of an approaching
Pacific jet streak to drive enhanced ascent and moisture into the
region. PWs are progged to reach +1 sigma above the climo mean
Monday aftn, at the same time snow levels climb to around
3000-4000 ft within the WAA. This will spread heavy precipitation
onshore, with heavy snow likely accumulating above 6 inches as
shown by WPC probabilities exceeding 50% in the higher terrain of
the Olympics and WA Cascades on D1. This pattern continues into
D2, and while the front will be well inland D2, lowered heights
beneath the upper trough and continued moist confluent flow
onshore will expand precipitation, and snowfall, on Tuesday. WPC
probabilities for more than 6 inches reach 60-80% in the WA and OR
Cascades, with lower probabilities expanding into the Sierra,
northern CA ranges, the Olympics, and Northern Rockies.

By D3, after a brief respite in precipitation due to shortwave
ridging, the second shortwave will dig from the Gulf of Alaska in
conjunction with another Pacific jet streak surging onshore. The
best moisture and ascent will likely be displaced south of the
first event by D3, driving the greatest probabilities for heavy
snowfall across the Shasta/Trinity area where WPC probabilities
for more than 6 inches reach 40%.


...Northern Plains and Upper Midwest...
Days 1 & 3...

A compact surface low moving across ND will shift eastward into MN
tonight and then dive through the Great Lakes Monday night. This
low is being maintained by a potent closed mid-level low moving
across Manitoba and Ontario, while spokes of vorticity pivot
cyclonically around it to drive intense ascent in the Northern
Plains. Impressive deformation on the N side of this low will
continue to push a strong band of snowfall eastward, which is
likely to result in 1"/hr snowfall rates accumulating to more than
4 inches after 00Z tonight in an east-west stripe across the
northern 1/3 of MN. More impressive snowfall is forecast to
develop along the northern shore of Lake Superior along the Iron
Ranges of the Arrowhead where more substantial ascent will occur
as moist flow impinges into the terrain and efficiently upslopes.
Despite modest SLR, WPC probabilities along the Arrowhead are high
for more than 4 inches, and locally more than 8 inches is forecast
as shown by WSE and NBM probabilities.

This first low will exit eastward by D2, and a single day without
snowfall is likely before another wave of low pressure approaches
on D3. This second wave will follow a similar track to the first,
with the primary low forecast to occlude over southern
Saskatchewan but extend an occluded front with secondary low
pressure development into MN. Downstream of this feature, strong
WAA surging northward on a strengthening LLJ will drive a PW plume
exceeding +1 sigma above the climo mean into the Upper Midwest.
While the column may be marginally sufficient for heavy snowfall
due to warm low-levels, at least moderate snow is likely on the
northern edge of this WAA, and WPC probabilities are as high as
30% for 4+ inches in northern MN. The exception is likely to,
again, be in the Arrowhead where moist upslope flow into the Iron
Ranges will lead to enhanced snowfall, which WPC probabilities
suggest has a greater than 40% chance to reach at least 6 inches.


...Eastern Great Lakes and Central Appalachians to the Northeast...
Days 1-3...

Northern stream and southern stream shortwaves will likely phase
across the northern OH Valley Monday night and amplify into a
strong closed 500mb low over New York which will then eject slowly
northward into Canada Tuesday night. The result of this phasing
will be strengthening cyclogenesis off the Mid-Atlantic coast, and
this low is likely to deepen quickly (at least half a bergeron) as
it lifts across Southern New England Monday night and Tuesday. The
guidance has trended further NW with today's suite, and while the
NAM continues to be the NW outlier, the stronger southern push of
the closing 500mb low does support pulling the surface low back to
the NW, and the most likely track is now from Long Island to
Vermont and into Canada. This further NW solution brings more warm
air on strong WAA into New England, and it is likely now that most
of Southern and eastern New England will be primarily rain.
However, NW of the 850-700mb low, generally PA to Upstate NY and
northern VT, precipitation is likely to fall as snow, with heavy
snow accumulating in the terrain.

As the low pivots northward, 290-295K isentropic upglide will
intensity, and become quite saturated as noted by mixing ratios of
3-4 g/kg. This surge of moisture will rotate cyclonically around
the low into a pronounced TROWAL pivoting into Upstate NY, and
coincident with a sharpening deformation axis noted on after
guidance. At the same time, model cross sections indicate elevated
theta-e surfaces folding into themselves, and while EPV is modest,
there is a strong signal for CSI banding just NW of 850mb low.
While there is some uncertainty still into where this will track,
the consensus is for a WAA band of heavy snow to lift from near
Cleveland, OH northeast through western PA and western NY late D1
into D2, with the secondary but more impressive deformation band
pivoting from the Poconos northeast through Upstate NY and VT,
including the Mohawk, northern Hudson, and Champlain Valleys, as
well as the surrounding terrain. Marginal thermal profiles
indicate the heaviest snowfall will occur above 1500-2000 ft in
the Catskills, Adirondacks, and Greens, but the intense ascent
could dynamically cool the column enough for at least light
accumulations down into the Valleys as snowfall rates exceed
1"/hr. The WSE and NBM probabilities have come up today, and
although some melting is likely during periods of lighter
snowfall, WPC probabilities for 48-hr snowfall of more than 8
inches are above 50% in the terrain, and lighter accumulations of
more than 2 inches encompassing much of the region from Cleveland,
OH through the Central Appalachians, and into much of interior New
England. While most of this snowfall will occur along and with the
low pressure system, as it pulls away into D3, NW flow in its wake
should lead to some lake enhanced and upslope snowfall along the
Chautauqua Ridge and into the NW Adirondacks and NE Kingdom of
Vermont.

Additionally, some light freezing rain accretion is likely in the
Central Appalachians D1 as WAA encounters sub-freezing surface
temperatures in the terrain of WV, and WPC probabilities for 0.1"
are 10-20%. Further northeast,light accretions of less than 0.1"
are possible in the Laurel Highlands, Catskills, Berkshires,
Greens, and Adirondacks as a pronounced dry slot pivots northward
on Tuesday causing some drying of the DGZ.


Weiss