Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
330 AM EST Sun Feb 21 2021

Valid 12Z Sun Feb 21 2021 - 12Z Wed Feb 24 2021

...Midwest through the Mid-Atlantic and Northeast...
Days 1-3...

A closed 850 mb low traversing across Iowa to WI guidance will
cause downstream warm advection with precip continuing to increase
in coverage today.  Low level frontogenesis along the
circulation's path is expected to lead to a band of enhanced
precip.  The high res guidance has nudged amounts up this cycle
with several inches of snow expected in western Iowa, with a
general 4-6 inches extending into northeast Iowa.  Amounts are
slightly lower in WI and MI as the circulation moves steadily east
northeast, limiting the duration  of snow and thus amounts.

Further east on Monday/D2, low-mid level warm advection will
spread precipitation northward through the Mid-Atlantic and into
the Northeast. The column will initially be cold enough for snow
in the northern mid Atlantic, but warming low-level temperatures
should quickly change p-type to rain for the Mid-Atlantic and
Southern New England.  This suggests that any significant
accumulations should be confined to interior locations like
northeast PA, northward into the Catskills, and central New
England where WPC probabilities for 4 inches are around 30%.
Higher probabilities exist int he southern Adirondacks as south to
southwest boundary layer winds turn upslope in windward terrain,
enhancing QPF and thus snow amounts. The GFS has higher amounts
and looks overdone based on the short duration of ascent and prior
performance.

Following the wave departing New England, the 00z ECMWF shows a
secondary 850 mb wave crossing the Great Lakes Monday night and
into the lower Lakes early Tue. A period of enhanced snow is
expected in the Tug Hill Tue., but shifting wind directions leads
to shifting locations of bands of snow.


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

Low-mid level confluent flow channels moisture onshore across the
Pacific northwest and into the northern Rockies. At least one
shortwave trough will drop into WA state on Monday evening, while
persistent upper level jet energy and divergence maxima drives
ascent across the region. The combination of high Pacific moisture
and the strong jet will produce an atmospheric river (AR) that
supports heavy precipitation where moisture and lift occur.  A 700
mb jet peaking near 50 kt provides strong bands of convergence
across WA, ID, and western MT, where the flow leads to strong
upslope flow in windward terrain.  The couplets of upper
divergence/lower convergence, mid level frontogenesis maxima, and
upslope flow lead to high rates of snow, with the long duration
leading to several feet of snow expected across the WA Cascades
into the ranges of northern ID.

The strongest 300 mb jet moves south from British Columbia across
WA, ID, and MT Monday night, then departing east across the
northern Plains on Tue.  Consequently, QPF and snow amounts are
not as high on Tue., with aid provided in the Pacific northwest as
the trough passage is accompanied by drying aloft.  As the upper
trough shifts east onto the Plains, snow develops in the ranges of
eastern WY and the Black Hills Tue-Wed.

The probability of a quarter inch of freezing rain is less than 10
percent Days 1-3.

Petersen