Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
330 AM EST Wed Feb 06 2019

Valid 12Z Wed Feb 06 2019 - 12Z Sat Feb 09 2019


...Western U.S....
Day 1 and 3...

Deep mid-level trough will eject eastward from the Four Corners
while filling into Thursday. This will be accompanied by height
falls and jet level diffluence atop the mountains of UT/CO/AZ/NM
and into WY. Widespread snow is likely in this area, with enhanced
snowfall likely across Colorado due to upslope 700mb flow, longer
duration of high 1000-500mb RH, and an increasing baroclinic
gradient /frontogenesis/ along an arctic front digging into the
area. WPC probabilities are high for 4 inches from the Wasatch of
Utah into the ranges of Colorado, with 8 inches possible in the
higher terrain. Lesser amounts are likely elsewhere.

On Day 3, Friday, a surface low will dive down the California
coast, while an anomalously deep upper trough digs into the PNW
from British Columbia. A potent Pacific Jet will transport ample
moisture into the coast, while snow levels fall to near sea level
in Washington State, with a gradient upward from there into
California. Snow is likely to develop across the Olympics and
Cascades, as well as into the California ranges as far south as
the Sierra. In the ranges, WPC probabilities are high for 4
inches, with lighter accumulations becoming likely down to the
valley floors, including Seattle, by the end of the forecast
period.


...Upper Plains and Great Lakes...
Days 1-2...

Trough lifting out of the Four Corners will phase with a weak
northern stream trough digging through Saskatchewan, Canada, to
drive intensifying surface cyclogenesis this evening through
Friday morning. The surface low will track from near Oklahoma
towards Lake Huron, with wintry precipitation likely NW of the
track.

Well NW, from western NE, through the Dakotas, and into MN/WI/UP
of MI, heavy snow will blossom and spread northeast. Initially,
snow will be driven by synoptic lift due to height falls, PVA, and
at least subtly coupled jets. However, the heaviest snow is likely
in MN/WI/MI where frontogenesis is likely to become strong and
combine with mid-level deformation along the 700mb trough to
produce heavy snowfall. Intense snow rates are likely during
Thursday where omega becomes focused into a deep DGZ and forecast
theta-e profiles show negative lapse rates, suggesting the
potential for convective elements and snowfall of over 1"/hr. As
the upper trough begins to tilt negatively Thursday evening, the
surface low will slow down, and a longer duration of heavy snow is
likely from WI into the U.P. Here, WPC probabilities are moderate
for 8 inches, with 12 inches possible. Points to the SW have a
high chance for 4 inches or more.

South of the heavy snow, a stripe of freezing rain is likely from
eastern KS through eastern IA, and arcing through southern WI and
central/northern MI. Strongly forced ascent though an anomalously
moist column suggests heavy precipitation rates in this area,
which will limit accretion efficiency despite temps in the mid and
upper 20s ahead of the surface low. Significant accretions are
possible, and although the signal is limited for 0.25 inches, WPC
probabilities are high for 0.1-0.2 inches in the band of heaviest
freezing rain. Colder air will flood in on strong CAA behind the
surface low, but the column should dry out too quickly for any
changeover to snow before the precipitation ends Thursday night.



...Northeast...
Days 1-2...

A warm front arched eastward from a low pressure in the OH VLY,
along which a secondary surface low will develop and move eastward
south of New England. Warm and moist advection will lead to
increasing isentropic lift atop a cold surface high pressure. This
high will retreat, but wet-bulb affects will maintain sub-freezing
wet-bulb temperatures across the terrain of Massachusetts, upstate
New York, and points northward through much of the event as a
mid-level wave moves eastward. A sharp inversion suggests 850mb
temps will climb above 0C to the Canadian border, so only light
snow accumulations will occur with this system, confined to the
White mountains of NH and elevations in Maine. Further south,
significant freezing rain is likely, and the guidance has
increased its accretion with the overnight model runs. There is a
good multi-model signal for more than 0.25" of freezing rain in
the Adirondacks, as well as the Berkshires, Southern Greens, and
Worcester Hills. Although rain rates may be moderate at times,
limited instability and modest forcing will allow for relatively
efficient accretion even without a strong source of dry/cold
advection to maintain freezing rain which is typically self
limiting. WPC probabilities feature a moderate risk for 0.25" of
accretion in the terrain, less elsewhere below 1000 feet, and a
few locations may reach 1/3 of an inch of freezing rain.

A secondary system will move through the Great Lakes driving
another round of warm/moist upglide into the region on Friday. The
warm air on D2 will eventually overwhelm the cold surface air, so
a brief period of freezing rain may produce light accretions, most
likely in the terrain of NH and ME, before p-type transitions to
plain rain later on Day 2.


Weiss