Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
338 PM EST Wed Nov 20 2019

Valid 00Z Thu Nov 21 2019 - 00Z Sun Nov 24 2019

...Great Basin, Central Rockies, Central Plains...
Days 1-2...

Deep closed low dropping across California will spin towards the
Four Corners Thursday, lowering snow levels to around 6000 ft
across the Mountain West. Significant and anomalous moisture will
stream beneath this feature and spread across the West, while jet
level diffluence and robust 700mb southerly flow produces
widespread ascent. This will create heavy snow, with the highest
amounts focused in the upslope favored areas within the ranges of
the San Juans, Sangre De Cristos, Wasatch, and along the Mogollon
Rim. In these ranges above 6000 ft, WPC probabilities indicate a
high chance for 8 inches through Thursday evening, with additional
moderate snowfall possible on Friday before the low ejects
eastward.

Additionally, a stripe of moderate to briefly heavy snow may
develop NW of a developing low pressure into the High Plains of CO
and NE. This will be in response to subtle increase in thickness
gradient leading to a fgen band which may lift northeast tonight.
The DGZ is elevated and HREF probabilities for heavy snow are
modest, but a quick burst of snow is possible, and WPC
probabilities for 2" have increased to 10-20%.


...Minnesota and Michigan...
Day 1...
Surface low pressure and associated southern stream shortwave
ejecting from the Southwest will interact tonight with a northern
stream shortwave and cold front falling out of Canada. Between
these two features, mid-level deformation will intensify and a
stripe of heavy snow is likely, focused in the vicinity of the
Arrowhead of MN. In this region, an intense band of frontogenesis
is likely to develop, collocated with the RRQ of a departing jet
streak. Forecast soundings indicate a warm nose approaching 0C
from the south which may lead to a sharp NW to SE gradient in
amounts, but intense ascent within the DGZ and an isothermal layer
near the surface favorable for aggregate maintenance should
support heavy snow rates which may reach 2"/hr at times. Modest
SLRs may limit accumulation somewhat, but WPC probabilities for 8
inches have climbed to 30%, and some amounts over 10" are
possible. Just to south over the U.P. of Michigan, moderate snow
is likely as well as the column cools in response to the surface
low pulling off to the northeast Thursday afternoon. WPC
probabilities suggest a moderate risk for 4 inches during Day 1.


...New England...
Day 2...
The surface low moving out of the Great Lakes will cross into
Ontario Canada and then lift northeast through Saturday. As this
occurs, warm southerly flow will lift atop a cold surface high
pressure, initially leading to light freezing rain across portions
of the Adirondacks and into northern New England. Southerly flow
should quickly erode the cold dome by Friday morning, so total
accretion should be light, and WPC probabilities for 0.1" are less
than 5%. As the cold front crosses New England later on Friday,
upslope flow and some modest lake effect off Lake Ontario may
produce light snows from the Tug Hill eastward through northern
Maine. WPC probabilities for 2 inches are less than 50%, highest
in northern Maine.


Weiss