Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
402 AM EST Mon Jan 21 2019

Valid 12Z Mon Jan 21 2019 - 12Z Thu Jan 24 2019


...Central Rockies...

A southern stream shortwave will continue to amplify as it moves
across the Great Basin, with heavy mountain snows becoming likely
from western Utah into the central Rockies on Monday.  Heaviest
accumulations are expected to center along the Wasatch into the
Uintas, with WPC Day 1 probabilities indicating a Moderate Risk
(greater than 40 percent probability) for accumulations of a foot
across portions of the region.  Meanwhile, upslope flow behind a
low to mid level front dropping southeast across the northern
Rockies and High Plains, will promote heavy mountain snows farther
northeast, with WPC probabilities indicating the potential for
locally heavy accumulations from south-central Montana to
north-central Colorado on Monday into early Tuesday.

...Northern and Central Plains to the Upper Midwest...

Models continue to show light to moderate snows developing ahead
of a northern steam trough as it moves from the Dakotas toward the
upper Mississippi valley late Monday into Tuesday.  By Tuesday
morning, as the previously noted southern stream trough begins to
move east of the central Rockies into the High Plains, low
pressure developing east of the Rockies is forecast to track
east-northeast from the central Plains into the lower Missouri and
mid Mississippi valley region on Tuesday, before continuing across
the upper Great Lakes region Tue night.  Strong deformation aloft
will support light to moderate snows north of the low track, with
WPC Day 2 probabilities indicating a greater than 40 percent
chance for accumulations of 6-inches or more from the eastern
Iowa-Minnesota border into southern Wisconsin. 

...Northeast to the Ohio Valley and Central and Southern
Appalachians...

Low pressure over the upper Great Lakes Tuesday morning is
expected to track farther to the northeast into eastern Ontario
and southern Quebec.  Precipitation ahead of the low is forecast
to begin as snow, resulting in some light accumulations before
changing over to rain across many locations across Upstate New
York and northern New England on Wednesday.  A trailing cold front
will push east from the Ohio valley into the Mid Atlantic
Wednesday night into Thursday morning.  A well-defined surface
wave lifting north along the front is expected to send moisture
onto backside the front, supporting a change from rain to a wintry
mix, resulting in some light snow and ice accumulations before
precipitation comes to an end across the Ohio valley into the
central and southern Appalachians Wednesday night into Thursday
morning.

...Pacific Northwest to the Northern Rockies...

A strong upper ridge is forecast to support dry conditions across
much of the region into early Tuesday.  By late Tuesday, warm
advection precipitation ahead of shortwave trough moving across
the top of the ridge will spread across the region; with models
showing the wave dropping into the Pacific Northwest early
Wednesday.  This may support some locally heavy mountain snows,
especially across portions of the northern Cascades and northern
Idaho ranges by Wednesday morning.  Models show the system
continuing to drop southeast Wednesday into early Thursday.  This
wave along with a frontal band dropping south through the northern
Rockies will promote some light to moderate snows across the
northern Rockies Wednesday into early Thursday.

Pereira