Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
315 AM EST Wed Nov 09 2022

Valid 12Z Wed Nov 09 2022 - 12Z Sat Nov 12 2022

...Intermountain West...
Day 1...

Anomalous trough characterized by a 500mb closed low will move
across the Great Basin today and then spin into the Central Plains
as a shortwave rotates through the base to reinforce the trough
during this evolution. Height falls, PVA, and downstream
divergence will combine with upper diffluence within the LFQ of a
poleward arcing jet streak to drive deep layer ascent, which could
become quite robust at times. The most impressive omega will
likely occur in a SW to NE axis where low-level CAA helps to
sharpen the baroclinic gradient, aided by the ageostrophic
response to the upper jet position, leading to an enhanced band of
frontogenesis to drive ascent. This will occur in a moist airmass
of Pacific origin, leading to rounds of heavy snowfall across the
region on D1. The guidance has again shifted a bit E-NE /faster/
with this feature, but the forcing also looks a bit stronger
thanks to the better positioning of the upper jet to enhanced the
fgen. While this will result in snowfall across the higher
terrain, especially from the Kaibab Plateau northward through the
Sawtooth and Absarokas, and east as far as the CO Rockies, the
intense ascent could also dynamically cool the column sufficiently
for some lighter accumulations even into the valleys. WPC
probabilities for more than 6 inches are confined to the higher
terrain, and highest in parts of the CO Rockies, Wind Rivers,
Tetons, and Uintas, where locally 12 inches of additional snowfall
on D1 is possible. Where that fgen band advects eastward, the
resulting heavier snow rates could additionally lead to an inch or
two of snowfall as shown by WPC probabilities even into the Snake
River Valley and lower elevations around the Wyoming Basin and
Upper Colorado River Basin in WY/UT/CO.


...Northern High Plains into Upper Mississippi River Valley and
Western Great Lakes...
Days 1-3...

An amplified trough driven by a closed low with 500mb heights
reaching -2 sigma according to the NAEFS ensemble tables will
eject from the Central Rockies and pivot into the Northern Plains
by Thursday aftn. During this time, the low may deepen further as
a lobe of vorticity swinging through the base of the trough lifts
northeast and helps to reinforce the closed low as it moves
towards Ontario by Friday morning. As this occurs, a downstream
jet streak will become increasingly meridional to place favorable
and impressive LFQ ventilation atop the region, helping to produce
a surface low pressure. This low will then lift northeast from CO
through MN, while WAA out ahead of it draws moisture and enhanced
ascent northward. The rich theta-e air will surge northward and
lift cyclonically within the WCB late Thursday into Friday,
producing a robust TROWAL, although guidance has slowed with the
development of this feature. However, impressive 290-295K moist
isentropic ascent into what should become a strongly forced
fgen/deformation band NW of the low, will lead to widespread heavy
snow from eastern MT through northern MN. The model consensus this
morning has shifted a bit NW, which also results in a greater
chance for some freezing rain accretion, as well as sleet
accumulation, southeast of the heaviest snow axis.

On D1, the heaviest snowfall is expected across eastern MT into
western ND where WAA will likely produce some enhanced fgen and a
band of heavy snowfall lifting northward before pivoting to the
east as the surface low deepens. The DGZ during this period is
likely to be quite deep, within which saturation should produce
large aggregates and high SLR. This higher SLR will allow snowfall
to accumulate rapidly, and WPC probabilities for 6+ inches have
increased across NE MT and into the Missouri Plateau of ND where
locally 8 inches is possible on D1. However, the more significant
snowfall and crux of this event is likely during D2. As the low
wraps up, 290-295K isentropic ascent with mixing ratios of 4-6
g/kg will surge cyclonically into a strengthening deformation axis
while the TROWAL strengthens aloft. This supports an extended
period of heavy snowfall rates which the WPC snow band tool
suggests will be 1-2"/hr, and could be at times heavier due to
high SLR within a deep DGZ, and some theta-e lapse rates <0C/km to
support slantwise convection. An impressive band of snow is likely
to develop and move slowly northeastward, with the greatest focus
for snowfall now expected across eastern ND into far northern MN.
WPC probabilities for 6+ inches of snow are now 80-90%, and it is
likely that some areas will receive well in excess of 12" of
snowfall on Thursday. Lighter snows of 4-8" are likely surrounding
this heaviest band, encompassing much of ND, northern MN, and
northern SD on D2. By Friday /D3/ the low will continue to eject
eastward and should be weakening as it occludes to the east.
Lingering snowfall is likely across the Arrowhead early Friday,
with LES developing on intensifying CAA in the wake of this system
across Lake Superior. WPC probabilities Friday are moderate for
4+" across the Bayfields Peninsula and southern Lake Superior
coast.

Additionally, with the more NW track progged by the guidance
today, a stripe of moderate freezing rain accretion is now
appearing more likely, especially across the Coteau des Prairies
of SD and within the James River Valley D1, with a secondary
maxima likely in the Arrowhead of MN D2. Here, the longest
duration of elevated WAA atop low-level cold air is expected which
should result in modest to significant freezing rain. Better
instability surging northward could promote more intense rates,
especially in SD/ND, which could limit accretion efficiency, but
WPC probabilities have increased and now feature a 50% chance for
0.1" and 10-20% chance for 0.25" in SD/ND, with a 20-30% chance
for 0.1" in the Arrowhead. This freezing rain could result in
dangerous travel and minor impacts to infrastructure.


Key Messages for the Northern Plains and Upper Mississippi River
Valley:

-- A winter storm will likely bring a swath of heavy snow, and a
corridor of sleet and freezing rain, to portions of the northern
High Plains, Great Plains, and Upper Mississippi River Valley late
Wednesday through Friday.

-- Moderate to major impacts (top image) may produce considerable
disruptions to daily life. Driving conditions may be hazardous to
dangerous.

-- Bands of heavy snowfall with rates in excess of 1"/hr will
likely produce significant snowfall accumulations in some areas.
Strong gusty winds combined with heavy snow could produce blizzard
conditions across the Dakotas.

-- Freezing rain and sleet are likely to the south of the heaviest
snow which could produce hazardous travel and at least minor
impacts to infrastructure.

-- Strong northeasterly winds over western Lake Superior may lead
to hazardous waves.

Weiss