Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
357 AM EST Sat Feb 27 2021

Valid 12Z Sat Feb 27 2021 - 12Z Tue Mar 02 2021

...Northern Plains to the Western Great Lakes...
Days 1-2...

A compact and fast moving but potent low pressure system will race
from eastern Nebraska Saturday evening to Ontario, Canada Sunday
evening while deepening. This low will deepen in response to
height falls as a shortwave embedded within an otherwise
positively tilted longwave trough lifts northeast, coincident with
intensifying jet-level diffluence as a poleward arcing jet streak
reached 170kts. This deep layer synoptic ascent will be aided by
locally intense mesoscale forcing as strong mid-level fgen
develops on the NW side of this surface low. Guidance has
continued its trend towards a stronger low again overnight, and
this has led to an increase in snowfall. Additionally, there is
increasing evidence that some CSI banding with possible upright
convection /thundersnow/ is possible especially from near the
Buffalo Ridge of MN northeast into the western U.P. of Michigan
due to negative theta-e lapse rates and -EPV intersecting the
saturated DGZ. HREF snowfall probabilities for 1"/hr have
increased, and there is likely to be a narrow band of 1-2"/hr
snowfall rates late Saturday night through Sunday afternoon. WPC
probabilities for 4 inches are highest in the Black Hills due to
elevation and some upslope enhancement, and the western U.P. of MI
which will experience the most intense banding due to the more
thoroughly developed low. However, anywhere in between these areas
WPC probabilities for 4 inches are as high as 30%, and locally 6+
inches of snowfall is possible from central SD through southern MN
and into WI and the U.P. of MI.


...Northeast...
Days 1 & 3...

Weak southern stream energy lifting from the southeast, along with
a more defined northern stream trough lifting across the Great
Lakes will support precipitation spreading north from the central
Appalachians and Mid-Atlantic into the Northeast on Saturday, with
precipitation continuing across northern New England into late in
the day.  As high pressure retreats, a wintry mix at the onset is
expected to change over to rain across most locations.  Exceptions
are likely to include the higher elevations of northern New
Hampshire into central and northern Maine, where precipitation may
remain mostly snow and the probabilities for accumulations of
4-inches of snow or more are as high as 60%, generally above 3000
ft.

On Monday night, a potent trough will dig across the Great Lakes
reinforcing the broad cyclonic flow across the eastern CONUS. This
trough will be accompanied by rapid and potent height falls along
with PVA to drive an area of precipitation, mostly as snowfall. A
surface low will pass east beneath the shortwave, driving a strong
cold front off the coast late Monday night. While snowfall
accumulations with this feature are expected to generally be
modest, WPC probabilities for 4 inches are as high as 30% in the
favored upslope terrain of the Adirondacks and Greens.
Additionally, while accumulation outside of the terrain should be
light, there appears to be an increasing potential for snow
squalls as low-level CAPE climbs to 50-150 J/kg coincident with
negative 0-2km theta-e lapse rates. These squalls could race
across New England and Upstate New York/Pennsylvania producing
briefly intense snowfall rates and lowered visibility late Monday
and Monday night.


...Pacific Northwest...
Day 3...

A closed low off the coast of CA will extend a lobe of vorticity
into Canada, with embedded weak height falls dropping into WA
state Monday night. Ahead of this feature, briefly backed
mid-level flow will drive some WAA precipitation into the Olympics
and Cascades of WA producing periods of moderate snowfall. This
feature will drop southeast quickly leaving generally zonal flow
in its wake, so forcing for ascent is likely to be modest and of
short duration. WPC probabilities for 6 inches are generally less
than 40%, highest in the far northern Cascades.


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

Weiss