Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
415 AM EDT Sun Mar 27 2022

Valid 12Z Sun Mar 27 2022 - 12Z Wed Mar 30 2022

...Great Lakes...
Day 1...

Continued cyclonic flow around a longwave trough axis slowly
shifting into the western Atlantic will maintain CAA and lake
effect snow in the favored W/NW snow belts today. Although
moisture and forcing should diminish through the day, persistent
inversion heights around 6000 ft and good forcing into the
lowering DGZ will allow LES to persist much of D1 with a slow wane
in coverage and intensity this evening. In the areas that see the
most LES, generally the Chautauqua Ridge and the Tug Hill Plateau,
WPC probabilities for an additional 4+ inches after 12Z this
morning are 30-50%. Tonight moisture should wane more rapidly, but
lingering streamers, especially where the effective fetch includes
Lake Huron or Georgian Bay, could produce additional light
accumulations through early Monday.


...California to the Rockies...
Days 1-3...

An anomalously deep 500mb low will swing onshore the central CA
coast Monday aftn and then slowly fill into an open wave across
the Southwest Tuesday evening. Confluent and fast flow downstream
of this trough will combine with a strengthening subtropical jet
streak of 130kts arcing into southern CA to spread deep column
moistured noted by PWs of +2 standard deviations into the Great
Basin. This will manifest as periods of heavy precipitation, which
above 6000 ft will be snow.

On D2, heavy snow is expected in the Sierra where impressive
moisture, deep layer ascent, and enhanced omega as mid-level flow
upslopes favorably into the terrain will overlap. WPC
probabilities for more than 6 inches are above 90%, and locally
12-15" of snow is possible. Significant pass level snow is likely
D2 as well, including Donner Pass. While the best overlap of
ascent and moisture is likely to be in the Sierra, as the entire
trough shifts eastward in tandem with the upper jet streak, heavy
snow will spread into the Great Basin and Central Rockies on D3.
WPC probabilities for more than 6 inches of snow reach as high as
50% Tuesday in the White Mountains of AZ, the San Juans, the
Uintas, the Wind Rivers, and much of the CO Rockies.


...Upper Midwest through the Mid-Atlantic...
Day 3...

A potent southern stream shortwave will eject out of the Southwest
on Tuesday and likely deepen into a closed low across the Central
Plains by the end of the forecast period. At the same time,
northern stream energy digging out of Alberta will drop rapidly
into the Dakotas, and these features will try to phase late D3 and
beyond. While the guidance has backed off slightly on the
interaction of these features, at least some phasing combined with
intensifying diffluence within the LFQ of a subtropical jet streak
will cause a deepening surface low to move out of the Rockies and
towards the Great Lakes D3. The guidance continues to feature a
wide variation in low placement and speed, leading to lowered
confidence in the evolution as the system moves northeast Tuesday.
However, impressive warm and moist advection noted by 850mb
southerly flow approaching 50 kts out of the Gulf of Mexico and
IVT approaching +4 standard deviations above the climo mean will
drive copious moisture northward. Where the column is cold enough,
this will manifest as moderate to heavy snow, sleet, and freezing
rain. With the surface low expected to track into the U.P. of MI,
heavy snow on D3 should be confined well NW of that track, and WPC
probabilities for more than 4 inches of snow are currently just
20-30% and along the far northern MN border and the Arrowhead.
This snow is likely to be of the heavy and wet variety with low
SLR, so even a few inches could cause impacts, and more heavy snow
is likely into D4. Southeast of the snow area, a band of sleet and
freezing rain is likely, and while the NBM has backed off a bit on
its accretion, there is good consensus that a stripe of freezing
rain exceeding 0.1" is likely from northern WI across the L.P. of
MI and as far east as the Laurel Highlands of PA. Additional
freezing rain is likely into D4 as well, and it is possible that
notable impacts will occur from this freezing rain later this week.

Weiss