Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
405 PM EDT Sat Oct 30 2021

Valid 00Z Sun Oct 31 2021 - 00Z Wed Nov 03 2021


...Central Rockies to the central High Plains...
Days 2-3...

A deepening upper low moving across northern Ontario and into
Quebec will drive longwave troughing into the Midwest, leading to
broad cyclonic flow across much of the CONUS. Within this cyclonic
flow, pieces of shortwave energy will periodically dig southeast
from western Canada and the Pacific Northwest, leading to locally
enhanced ascent through modest height falls and PVA. Beneath this
upper trough, a surface cold front will dive through the Plains
and into the East while getting hung up into the Front Range of
the Rockies Monday. Along this front and beneath any of these
shortwaves, local enhancements of ascent atop increased moisture
from return flow out of the Gulf will lead to areas of moderate to
heavy snow.

The most significant snowfall is likely D2 /Sunday night and
Monday/ across the CO Rockies and stretching into the High Plains
of WY and the Panhandle of NE. Here, an overlap will occur between
the low-level baroclinic zone along the front, the favorable RRQ
of an intensifying upper jet streak pivoting across the Northern
Plains into the Upper Midwest, and a shortwave exiting the Pacific
Northwest. These features together will drive intense ascent for a
plume of moderate to heavy precipitation. The trends in the WSE
and the NBM have shown an increase in snowfall potential here as
guidance depicts an area of theta-e lapse rates less than zero
within which -EPV exists in the lower portion of the saturated
DGZ. This implies not only a potential east-west band of heavy
snowfall, but potentially convective rates through CSI, as shown
by approximately 50% probability for 1"/hr rates on the WPC
prototype snowband tool. The heaviest accumulations are likely in
the terrain above 6000 ft in the Park Range and Front Range where
WPC probabilities are 20-30% for 4 inches. Although probabilities
within this band for 4" are less than 5%, it is worth nothing that
a few inches of snowfall is becoming more likely, and if
convective rates can occur to overcome the marginal low-level
thermal structure, especially overnight into Monday morning,
locally higher snowfall is possible.

On D3, a secondary strengthening of the upper jet streak will
occur across the Midwest while a shortwave ejects out of the Great
Basin and into the Central Rockies. The combination of modest
height falls/PVA and RRQ diffluence will produce periods of
moderate snow above 7000 ft across northern CO, where WPC
probabilities for 4 inches are as high as 20%.


...Lake Superior...
Days 2-3...

Lake effect precip begins behind the cold frontal passage off Lake
Superior Sunday with higher elevations having an accumulating risk
starting Sunday night /Day 2/, lowering closer to lake level late
Monday. Water temperatures near +10C will drive steepening lapse
rates and a deepening inversion to above 9000 ft. This should
allow for lake enhanced precipitation to expand across the U.P. of
Michigan, but marginal low-level thermals may lead to a rain/snow
mix outside of the higher terrain. WPC probabilities for 4 inches
peak on D3 just south of the Keweenaw Peninsula, but are generally
just 1-5%.

For Days 1-3, the probability of significant icing is less than 10
percent.

Weiss