Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
331 PM EDT Fri Mar 18 2022

Valid 00Z Sat Mar 19 2022 - 00Z Tue Mar 22 2022

...Northern New England...
Days 1-2...

Mid-level shortwave will slowly eject out of the MS VLY tonight
and then spin into the Canadian Maritimes Sunday aftn. Downstream
of this trough, modestly coupled jet streaks will produce some
enhanced upper diffluence, and the overlap of this ventilation
with height falls/PVA will drive a surface low pressure from
Missouri, into Michigan, and then into Ontario by Saturday night.
Ahead of this low, a warm front and associated WAA will surge
northward on S/SW flow out of the Gulf of Mexico to expand a
precipitation shield across the Northeast. As this occurs, a
surface high pressure will gradually retreat in response to the
increased mid-level divergence ahead of the shortwave, and this
setup will support a period of snow to mixed precipitation,
generally for the terrain of northern NH and northern/central ME.
While some light snow accumulations are likely in northern ME, the
most impactful winter weather is likely to be freezing rain as the
warm nose lifts atop sub-freezing surface air locked into the
terrain. WPC probabilities for 0.1" of freezing rain are above 20%
in the white Mountain of ME D1, extending into far northern ME D2.
Total freezing rain may approach 0.25" in parts of Aroostook
County, ME.

...The West...
Days 1-3...

A shortwave will amplify as it moves onshore the Pacific Northwest
Saturday aftn, and then dig into a potent closed low over the Four
Corners Sunday and then maintain intensity as it shifts slowly
into the Southern Plains Monday. The amplification of this trough
will lead to intensifying jet streaks both upstream and downstream
of the primary longwave axis, leading to enhanced upper
diffluence, most notably D3 across the High Plains, but with still
impressive upper level ascent Saturday and Sunday. While initially
moisture will be somewhat limited, increasing WAA ahead of a
surface cold front combined with rising IVT embedded within the
upper jet streak will drive PWs to +1.5 standard deviations above
the climo mean by Sunday aftn across the Great Basin and spilling
into the High Plains D3. The combination of impressive upper
diffluence and height falls/mid-level divergence will drive lee
cyclogenesis Monday morning across CO, with this surface low
deepening while it drifts slowly eastward by the end of the
forecast period.

During D1 and D2, Friday night through Sunday night, most of the
snow fall will be driven by jet level dynamics and confined above
5000 ft on D1, but dropping to as low as 1500 ft D2 behind the
front, lowest in the Cascades. WPC probabilities for more than 6
inches of snow on D1 are above 30% only in the Olympics and WA
Cascades, with a secondary maximum in the higher terrain of the
Central Sierra. By D2, snowfall becomes more widespread as the
trough elongates and shifts inland, with heavy snow exceeding 6
inches becoming more than 30% likely in the NW WY ranges and parts
of the Northern Rockies.

During D3, the setup supports more widespread and significant snow
across the Central and Southern Rockies from the Laramie Range
southward through the Sangre De Cristos. The intensifying lee
surface low will help wrap moisture westward into the terrain and
enhance ascent through upslope flow, and a significant snow event
is becoming more likely for this region. Snow levels are generally
expected to remain above 4500 ft on D3, but the slow moving system
will produce persistent ascent and high column moisture through
Tuesday morning. WPC probabilities for 6+ inches are high along
the Front Range, Palmer Divide, Raton Mesa, and Sangre De Cristos,
with locally 12 inches of snow possible. At this time it appears
the I-25 corridor will experience just light accumulations through
D3, but more moderate snowfall is possible just beyond this
forecast period.


Weiss