Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
359 AM EST Tue Feb 23 2021

Valid 12Z Tue Feb 23 2021 - 12Z Fri Feb 26 2021


...Northern Plains to the Northeast...
Days 1-2...

A well-defined 850-700 mb shortwave is forecast to move quickly
east across the northern Plains to the upper MS Valley and Upper
Great Lakes by Wed morning. The NAM/GFS both show a prominent
banding signature with overlap of the 850-700 mb
convergence/theta-e advection and frontogenesis.  In addition to
favorable upper forcing, strong low-mid-level frontogenesis will
likely be supportive of a period of moderate to heavy snow
shifting east across North Dakota into northern Minnesota on
Tuesday, and then across northern Minnesota into northern
Wisconsin and the Upper Peninsula of Michigan Tuesday night into
Wednesday morning. 

The axis/orientation of snow is similar in ND but starts to split
in MN/WI/MI as the NCEP models and convective allowing models have
a more northern axis of QPF/snow than the global
Canadian/UKMET/ECMWF in WI and MI.

As the system moves to the east late Wednesday, there is some
indication that northwesterly flow on the backside of the system
may produce lake-enhanced snows across portions of the eastern
U.P. and northern Lower Michigan. 

This system is expected to move across southern Ontario and Quebec
Wednesday evening, before brushing northern New York and New
England with a period of snow Wednesday night into early Thursday.
 With the primary wave going north of the border, the enhancements
of QPF/snow are expected to be in the Adirondack
Mountains/northern Green Mountains/White Mountains, where several
inches of snow are possible.

...Pacific Northwest to the Northern Rockies...
Days 1-3...

Two rounds of heavy snow are expected across the ranges of WA to
ID through Thu.  The first is today and tonight, as the NAM shows
a coupled upper jet maxima crossing WA into ID, with the coupled
region having a well defined divergence maxima to trigger ascent.
The vertical profiles show deep layer ascent peaking in the
850-700 mb layer, corresponding with the Dendritic growth zone
favorable for snow and high snow to liquid ratios. Heavy snows are
expected across the northern Cascades, the Blue Mountains and the
Rockies from northern Idaho and western Montana to western
Wyoming. The upper trough progresses inland and an upper ridge
approaches from the eastern Pacific on Wed., so snow amounts are
light. 

The next system Thu consists of the upper trough moving onshore
and inland, with a lingering upper jet crossing WA and ID that has
sustained couplets of upper divergence/lower convergence maxima
and sustained ascent in the 850-700 mb layer that results in a
long duration snow.  The strength and endurance of ascent plus
integrated vapor transport leads to the likelihood of heavy snow
in the WA Cascades into the northern OR Cascades, extending inland
into the OR blue Mntns and then the ranges of northern ID to
northwest MT. A couple of feet of snow are expected in windward
terrain of the WA Cascades, with amounts gradually tapering
heading inland. The probability of a foot of snow is high is the
northern WA cascades.

...Central-Southern Rockies/High Plains...
Day 2/early Day 3...
Snows are expected to develop across the CO Rockies/foothills as
the shortwave trough digging across the Northwest on Wednesday
continues to dig southeast and approach the region Wednesday
evening into the overnight.  Low-level convergent, upslope flow is
expected to support snow developing over the Colorado High Plains,
foothills, and front range Wednesday night into early Thursday,
with several inches of snow expected in favored windward terrain
along the low level frontogenesis/convergence axis.  The low level
convergence axis shifts south and weakens on Thu in NM, so snow
amounts taper then.  WPC PWPF shows 60-70 percent or greater
probabilities for accumulations of 4 inches or more centered on
the foothills, front range, and Sangre DeCristo Mountains of CO.

For Days 1-3, the probability of significant icing (0.25-inch or
more) is less than 10 percent.

Petersen