Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
450 AM EDT Sat Oct 28 2023

Valid 12Z Sat Oct 28 2023 - 12Z Tue Oct 31 2023

...Central & Southern Rockies to the Central High Plains...
Days 1-2...

A positively tilted trough over the western U.S., containing a
pair of shortwave disturbances, will work in tandem with an
expansive ridge of high pressure over the Southeast to create an
impressive 200mb jet streak over the Midwest and Great Lakes on
Saturday. In fact, NAEFS shows 200mb wind speeds from the Midwest
to the Great Lakes Saturday afternoon surpassing the 99.5
climatological percentile, showcasing how impressive the 200mb jet
streak is for late October. Strong vertical ascent beneath the
right entrance region of this 200mb jet streak will coincide with
strong positive vorticity advection and a steady stream of
adequate 700-300mb moisture to generate periods of heavy snow from
the Central Rockies to the Central Plains on Saturday and into
Saturday night. The other factor supporting periods of snow is a
large dome of high pressure to the north that is helping to lock
in sub-freezing temperatures throughout the impacted regions. An
enlarging DGZ aloft Saturday evening and into the overnight will
also result in rising SLRs that foster heavier snowfall rates
across CO, southern WY, and western NE. Sunday will feature
additional periods of snow in CO, but a "Blue Norther" racing
south through the southern High Plains will result in a classic
"overrunning" setup from West TX to central OK and even northwest
AR.

WPC snow probabilities for >8" of snowfall this weekend are
highest in the CO Rockies (70-90%) but there are also notable
probabilities (40-60%) for >6" of snowfall along the Sangre De
Cristo in southern CO and northern NM. Closer to the Denver metro
area, WPC snowfall probabilities for >8" of snowfall for the
duration of the storm are around 40-50% in Boulder with slightly
lesser odds (30-40%) throughout the Denver metro. At their peak,
snowfall rates could range between 1-2"/hr, but the WSSI is
indicating Snowfall Amount is the primary driver for expected
impacts. The latest iteration of the WSSI does show Major Impact
potential across the Denver metro area, as well as some of the
higher peaks of the CO Rockies. The Major Impact zones are favored
to witness the more hazardous travel impacts with potential delays
and closures. There is also an expansive Moderate Impact area that
stretches N-S from Castle Rock to Fort Collins along I-25.
Hazardous travel conditions are possible in these areas Saturday
and into Sunday. Quite the significant event unfolding for the
area that also just so happens to be the Denver metro area's first
winter storm of the season.

Farther east, northeast CO and west-central NE feature medium
chances (30-50%) for snowfall totals >4" on Saturday. This is due
to a narrow ribbon of 700mb frontogenesis giving rise to a narrow
band of heavy snow over the region. Localized amounts could top 6"
in areas where the heaviest snow bands take shape. Farther south,
a burgeoning warm nose aloft and sub-freezing temps near the
surface will prompt precipitation to fall as an icy wintry mix
over western KS, the OK/TX Panhandles, and as far south as the
Edwards Plateau and Davis Mountains of western Texas. Most ice
accumulations will be <0.1", but a light glaze from sleet/freezing
rain in these areas could lead to slick conditions on untreated
surfaces. Light sleet/freezing rain accumulations could be
observed as far east as the Ozarks in northern AR and southern MO.

...Northern New England...
Day 3...

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. There will also be a
conveyor belt of 700-300mb moisture available but SLRs will be
rather low given DGZ layers will not be overly deep aloft. There
is a bubble of high pressure over southern Quebec that accompanies
an air-mass that is cold enough to support some snowfall, but
given it is a marginally cold temperature regime, this will likely
limit accumulations to the higher terrain >1,000ft in northern New
England. WPC probabilities are higher in northern Maine with
20-40% odds of receiving >4" of snow between Sunday evening and
Monday evening. There are similar probabilities for >2" of
snowfall in far northern NH during the same time.

...Upper Great Lakes...
Day 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 westerly 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 approaching the Upper Great Lakes will track towards the
region Monday night. Model guidance remains at odds with the speed
of the upper trough (ECMWF slower, GEFS faster) and the amplitude
(ECMWF deeper, GEFS flatter) and is leading to discrepancies in
QPF over Michigan's Upper Peninsula. Given the northerly flow on
the western flank of the upper low, additional lake effect snow
showers are possible with locally heavier amounts. Latest WPC
probabilities showed as much as a 50-60% chance for snowfall
totals >4" along the western coast of the Keweenaw Peninsula on
Monday. Similar probabilities are depicted in far northern MN
along the shores of Lake of the Woods.

Mullinax