Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
423 AM EDT Sat Mar 28 2020

Valid 12Z Sat Mar 28 2020 - 12Z Tue Mar 31 2020


...Western Great Lakes...
Days 1-2...

Closed upper low will lift from the Central Plains into the Great
Lakes while deepening, aided by increasing upper level support
through the LFQ of a 150+kt sub tropical jet streak. Beneath these
features, a surface low will deepen in conjunction with the upper
support as it lifts northeast as well towards the U.P. of MI.

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, there has been a subtle shift southward in the guidance
this morning, and the highest WPC probabilities for 4 inches are
centered around the shores of Lake Superior. Further to the SW
toward the Twin Cities, accumulation should be light, but briefly
heavy rates of 1"/hr are possible in the deformation which could
enhance snowfall locally.


...Northeast...
Days 2-3...

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, with some sleet possible due to wet bulb
cooling across NH as well. The snow may be heavy for part of
Sunday as ageostrophic drainage of low level cold air prevents the
robust WAA from overwhelming the column, and WPC probabilities on
Sunday are moderate for 6 inches across Northern Maine. On Monday,
the surface low will linger in the Gulf of Maine as jet level
diffluence and height falls persist. This will allow light to
moderate snow to continue across Maine, and WPC probabilities are
10-30% for an additional 4 inches of snow on Monday. 


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

A series of shortwaves digging through broad troughing and
confluent flow into the West Coast will bring periods of moderate
to heavy snow to much of the terrain of the West. The first of
these will lift onshore in an elongated trough from WA to CA on
Sunday morning, with a more potent impulse digging into Washington
late Monday. Moist advection and significant 700mb ascent will
drive snow above 3000-4000 ft in WA/OR and into ID/MT, with snow
levels as high as 7000 ft in CO/UT. The heaviest snow is likely on
D3 in the Cascades of WA/OR and into the Northern Rockies when WPC
probabilities are high for 12 inches. Otherwise, WPC probabilities
on D1 are moderate to high for 6 inches in the Cascades, Olympics,
and into the Sierra. On D2 this shifts further east into the Great
Basin and Wasatch. 3-day totals in the Cascades and Olympics may
exceed 4 feet.


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

Weiss