Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
536 PM EDT Fri Mar 27 2020

Valid 00Z Sat Mar 28 2020 - 00Z Tue Mar 31 2020


...Southwestern U.S. to the Western Great Lakes...
Days 1-2...

Deep positively tilted trough will begin to tilt neutrally, and
eventually close off as it lifts northeast from the Four Corners
into the Central Plains. This mid-level low will deepen quickly
Saturday as it moves towards the Great Lakes, aided by increasing
upper level support in the form of upper-level difluence and
upper-level divergence of a 150+kt subtropical jet streak. Beneath
this features, surface low is expected to develop in the lee of
the Rockies and then lift northeast towards the U.P. of Michigan
while also deepening.

Before the surface low has a chance to exit the Rockies, low- and
mid-level flow will the complex terrain...which will result
periods of heavy organized mountain snows from Colorado into
southeast Wyoming and adjacent portions of the Nebraska panhandle
and Kansas. 

As the system ejects northeastward, ascent will become intense
along and north of the path of this low. In addition to the height
falls and jet level diffluence, WAA will become strong, spreading
precipitation northward into the Northern Plains, and a pronounced
theta-e ridge is forecast to wrap cyclonically around the low as a
robust TROWAL develops from far eastern South Dakota into into
Minnesota. Collocated with this TROWAL, a potent deformation band
is likely to develop and pivot across Minnesota/Wisconsin and the
Upper Peninsula of Michigan. While the low-level thermal structure
of the atmosphere will be marginal for snow, the strongly forced
ascent should dynamically cool the column to cause p-type
transition during periods of heaviest precipitation from rain to
snow.

While uncertainty still remains as to where this band may setup,
the non-NAM consensus is for a swath of heavy snow from central
Minnesota into northern Wisconsin and the Minnesota Arrowhead. The
heaviest snowfall rates may reach 1"/hr at times within this band.


...Northeast...
Late Day 2 into Day 3...

A warm front extending from the significant low pressure near the
Great Lakes will lift northeastward through the Mid-Atlantic and
into New England. This warm front will briefly stall as a wedge of
high pressure extends down from Maine, before resuming its
northeastward trek during the day on Sunday.  As this occurs,
strong WAA will spread precipitation northward, with isentropic
ascent further enhancing the coverage and intensity of the
precipitation. At the same time, a wave of low pressure is likely
to develop along the boundary near Cape Cod, and strengthen as it
moves northeastward late in the forecast period. As precipitation
expands into New England, it will initially be snow across Central
and Northern New England.  There is the potential for some sleet
to mix in due to wet bulb cooling. 

It is in this area where accumulations are expected to become
moderate to heavy, and WPC probabilities for 4 inches are high
across the northern half of Maine.


...Pacific Northwest to the Northern Rockies...
Days 2-3...

Two distinct shortwaves embedded within a deepening trough
approaching the west coast, combined with persistent confluent
mid-level flow will provide ascent and moisture into much of the
terrain of the Western U.S. on Saturday and Sunday. This will
spread periods of snow across much of the terrain of the West,
with snow levels varying from around 4000 ft in the Washington
Cascades, to as high as 7000 ft in UT/CO.

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

Bann