Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
249 PM EST Wed Mar 02 2022

Valid 00Z Thu Mar 03 2022 - 00Z Sun Mar 06 2022

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

An increasingly active period of winter weather is expected
through the weekend as a series of shortwaves drop across the West
Coast and amplify into a longwave trough centered over the Great
Basin. D1 into D2, as split shortwave energy shifts into WA state
with a secondary vorticity lobe spinning down the CA coast. WAA
downstream of these features combined with subtle RRQ upper
diffluence will spread moisture onshore, into which modest
synoptic ascent will produce snowfall across the Northern Rockies
D1, with WPC probabilities for more than 6 inches above 50%
focused near Glacier NP. By D2, the southern energy shifting
onshore CA becomes dominant, and WPC snowfall probabilities for 6+
inches become centered across the Sierra and reach as high as 40%.

By D3 the mid-level evolution becomes quite complex with the
northern stream shortwave shifting into Saskatchewan, the lead
southern stream moving across the Four Corners, and a third,
powerful, closed low dropping once again along the CA coast and
into the Great Basin. Exactly how all of these features interact
remains in question and will likely take another day or so to
resolve, but impressive moist advection, a strengthening upper jet
streak, and robust deep layer ascent should produce widespread
moderate to heavy snowfall across much of the West. WPC
probabilities for 6+ inches are currently above 30% from the
Sierra, across many of the Great Basin ranges, into the southern
Wasatch, the San Juans, and northward into the Central Rockies and
ranges of NW WY. Snow levels early should be 4000-5000 ft, but
will likely fall through D3 to as low as 2000-3000 ft, lowest in
CA/NV, and locally 12" of snow is possible in the higher terrain.


...Northeast...
Day 1...

A shortwave embedded within broad troughing across the northeast
CONUS will advect eastward and drive a clipper type surface low
across Upstate New York and New England. Ascent in the vicinity of
this clipper will be impressive through height falls and LFQ
diffluence ahead of an approaching upper jet streak, but also fast
moving to limit the snowfall potential. Brief moist advection
ahead of this wave will help drive PWs to slightly above the climo
mean, and the column is likely to be cold enough for snow across
Upstate NY, and central/northern New England. LES and orographic
enhancement is likely, especially as CAA develops in the wake of
the clipper, leading to the highest WPC probabilities for 4 inches
being confined to the Chautauqua Ridge, the Tug Hill Plateau, and
the western slopes of the Adirondacks. Despite accumulation
elsewhere being generally less than 4 inches, a brief period of
moderate snowfall is likely across much of central and northern
New England with light accumulations expected in Boston and
Portland, ME.


...Central High Plains through the Upper Midwest...
Day 3...

A messy two-phase winter event is likely beginning Friday night
and persisting beyond this forecast period into the medium range.

This event will be driven by a complex trough developing across
the Intermountain West through the interaction of several
shortwaves, producing an amplified longwave trough by D3.
Downstream of this trough, a vorticity lobe will be shed eastward
into the Upper Midwest Friday night, with a more impressive
shortwave closing off over the Central Plains Saturday aftn. Each
of these impulses will be accompanied by modest to strong LFQ
diffluence as jet streaks downstream of the primary trough shift
eastward, and moisture increasing to +2 standard deviations above
the climo mean with respect to PWs as 850mb flow intensifies out
of the Gulf of Mexico.

The lead shortwave will have little surface reflection, but
increase isentropic ascent atop a retreating high pressure will
likely lead to moist overrunning and mixed precipitation from
eastern MN through WI, and both the U.P. and L.P. of MI. The
column is likely too warm for snowfall where the heaviest
precipitation will occur, but a period of moderate freezing rain
is becoming more likely, especially in WI and the L.P. of MI. WPC
probabilities for 0.1" of freezing rain D3 are above 50%.

As that lead wave shifts eastward, the trailing but more potent
shortwave will close off immediately in its wake. This will be
accompanied by more intense ascent, and a surface low is likely to
develop in the lee of the Rockies and shift northeast into the
Northern Plains by the end of D3. This low is likely to deepen as
its shifts northeast in response to pronounced synoptic ascent,
and some of the stronger guidance indicates the theta-e ridge will
wrap into the cyclone as a TROWAL. While there are still placement
differences in the surface low and the resultant deformation axis
which is likely to form NW of the low, there is increasing
confidence that a stripe of heavy snow will occur from the Front
Range of WY eastward towards the Pine Ridge of NE and Black Hills
of SD, likely due to some upslope enhancement, and then northeast
into central SD as the deformation axis intensifies. WPC
probabilities are currently above 50% for 4 inches within this
swath, but locally much higher totals are possible.


Weiss