Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
416 AM EDT Sun Oct 29 2023

Valid 12Z Sun Oct 29 2023 - 12Z Wed Nov 01 2023

...Central & Southern Rockies to the Central & Southern High
Plains...
Day 1...

A positively-tilted upper trough over the Four Corners region will
continue to provide strong vertical ascent atop the atmosphere
while modest 700mb moisture flux streams in over the southern
Rockies and south-central Plains. Meanwhile, southerly 850mb flow
will deliver added low-level moisture flux north of the cold front
tracking south through Texas. The heaviest snowfall totals will
reside in to Rockies of southern CO and northern NM. The Sangre De
Cristo Range in particular sports as high as 50-70% probabilities
for >6" of snowfall on Sunday. Ice accumulations >0.1" are <10%,
but a light glaze of ice from a combination of sleet/freezing rain
could still lead to slick spots in eastern NM, northwest TX, and
west-central OK. Some instances of patchy, light ice accumulations
in southern KS, southern MO, and as far south as the Davis
Mountains and Edwards Plateau of Texas cannot be ruled out.

...Northern New England...
Day 2...

A wave of low pressure in the Mid-Atlantic Sunday evening will
track north and east along the Northeast coast by Monday morning,
then continue to strengthen throughout the day as it tracks off
the coast of Nova Scotia by Monday evening. The surface low is
able to deepen because of exceptional upper level diffluence via
positive vorticity advection out ahead of a positively tilted
500-200mb mean trough over the Upper Midwest. A conveyor belt of
700-300mb moisture aloft tracking into a sufficiently cold
air-mass will give rise to periods of moderate-to-heavy snow in
northern Maine Monday morning. WPC probabilities of receiving >4"
of snow have risen to 70-80%, making it increasingly likely
portions of north-central ME and the North Woods could even see
totals surpass 6" in localized areas. The WSSI-P shows a high
chance (60-80%) of Minor Impacts across northern ME on Monday with
Snow Amount and Snow Rate the primary drivers. Hourly snowfall
rates could approach 1"/hr within the heaviest bursts of snow
Monday morning.

...Upper Great Lakes...
Days 2-3...

An exceptionally cold air-mass for late October (NAEFS shows <10th
climatological percentile temps at both 850mb and 700mb) will
coincide with a persistent WNW flow throughout the day on Monday.
Periods of lake effect snow showers may bring about some light
snow accumulations during the day, but a more potent upper trough
passing over the Midwest will track towards the region Monday
night. The ECMWF and GFS have come into a little better agreement
on timing through Monday, but the ECMWF remains more amplified and
slightly slower with the trough's progression than the GFS. While
the exact details on the evolution of the surface low are not
clear, the tightening pressure gradient of high pressure to the
north and the low to the south will result in strengthening
northerly flow on Tuesday. NAM guidance sfc-850mb delta-Ts over
Lake Superior on Tuesday are as large as 32-36C and results in
steep surface-850mb lapse rates. In addition, there is a pivoting
850mb trough axis early Tuesday morning that could be the focus of
a heavier band of snow before the lake effect streamers take
shape. Latest WPC probabilities show as much as a 40-50% chance
for snowfall totals >6" along the western coast of the Keweenaw
Peninsula on Monday evening into Tuesday. Similar probabilities
are depicted in far northern MN along the southern shores of Lake
of the Woods.

Mullinax