Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
302 PM EDT Sat Mar 19 2022

Valid 00Z Sun Mar 20 2022 - 00Z Wed Mar 23 2022

...Northern Maine...
Day 1...

A shortwave lifting north across New England will combine with
upper diffluence to maintain deep layer ascent across the
Northeast through Sunday morning. Downstream, a wave of low
pressure will cross through Ontario and Quebec, lifting a warm
front and associated WAA northward, continuing to surge moisture
from the Gulf of Mexico into Maine. As this occurs, Canadian high
pressure will continue to retreat to the east, leaving cold
surface air in place even as the warm nose drives 850mb temps
above 0C. This will produce periods of freezing rain continuing
across primarily Aroostook County Maine through early Sunday, with
WPC probabilities indicating a 30-50% chance for 0.1" of accretion
before precipitation winds down late D1.


...Northern Plains...
Day 3...

A northern stream shortwave dropping out of Alberta will move
along the Canada/U.S. border Tuesday, and at least marginally
interact with a powerful southern stream closed low over the
Southern Plains. Robust moist advection will drive an impressive
theta-e ridge northward downstream of the southern stream low,
upon which height falls and PVA from the northern impulse will
produce rain and snow. The guidance features quite a bit of
uncertainty in placement and intensity of these features by D3,
leading to lower than usual confidence in precip type and amounts.
However, at least light to moderate snowfall is likely, and WPC
probabilities are 10-20% for 4 inches in eastern ND and far NW MN
near International Falls.


...The West to the High Plains...
Days 1-3...

Amplifying longwave trough moving onshore the Pacific Northwest
tonight will shift into the Great Basin Sunday night and then
close off over the Four Corners on Monday. This mid-level low is
progged to become quite intense, noted by 500mb height departures
approaching -3 standard deviations according to the NAEFS
situational awareness table. This low will continue to deepen as
it shifts into the Southern Plains on Tuesday, with only slow
filling occurring on D3. As the low pivots eastward, the
amplification of this trough will produce intensification of
downstream upper jet streaks, with additional coupling leading to
pronounced upper diffluence. The overlap of impressive height
falls and upper diffluence will lead to lee cyclogenesis Monday
morning, with this low then deepening and shifting northeast
through Tuesday. While there still are considerable placement and
timing differences of this low, the trend has been subtly faster
and more suppressed, leading to more snow across the Southern
Rockies, but less to points further north.

On D1, PWs of +1 to +1.5 standard deviations will provide
sufficient moisture for heavy snow in parts of the Sawtooth,
Northern Rockies, Absarokas, Tetons, and Wind Rivers, where WPC
probabilities are high for 6 inches or more. Additional moderate
snow is likely in the Olympics and WA Cascades on D1 as well. By
D2, the heavy snow becomes more significant in the WA Cascades due
to a secondary jet streak pivoting onshore to produce enhanced
moisture. WPC probabilities D2 for this area are high for 6+
inches, but as this jet pushes southeast and onshore D3, it will
shunt forcing to the east and shut off the heavy snow.

More notably, the development of the lee cyclone in eastern NM/CO
will likely produce heavy snow in the Southern Rockies D2 and D3.
The pronounced synoptic ascent will be aided by upslope NE flow
around the developing surface low, while impressive moist
advection will wrap cyclonically into a TROWAL across southern CO.
This setup supports heavy snow focused into the Raton Mesa where
WPC probabilities peak above 80% for 6 inches, and locally up to
12 inches is possible as noted by the WSE 90th% and NBMv4.1
probabilities of greater than 30% for 12 inches. Heavy snow is
also likely along the Palmer Divide, the Sangre De Cristos, and
into the Front Range, but amounts in these areas, while still
significant, should be less than the Raton Mesa.


Weiss