Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
419 AM EDT Tue Oct 12 2021

Valid 12Z Tue Oct 12 2021 - 12Z Fri Oct 15 2021

...First potent winter storm to bring heavy snow to portions of
the Rockies and parts of the High Plains through Wednesday...

Northern and Central Rockies...
Days 1-2...

A closed mid-level low moving through the Four Corners region will
lift into the Northern Plains Wednesday before ejecting into
Canada on Thursday. 700/500mb heights will be near the CFSR
minimum (1979-2009) for this time of year and will be accompanied
by an intensifying and phasing subtropical jet streak from the
Pacific to drive robust deep layer ascent across the region. An
attendant cold front continues to push eastward ahead of the upper
low and its northeastern extension was draped across Wyoming as a
stationary boundary, already producing snow for the area. Surface
low pressure will move out of northeastern Colorado this evening
and central South Dakota on Wednesday while its frontal structure
occludes as the upper features become vertically stacked. This
cyclone will be accompanied by intense moist advection as rich
theta-e air surges northward from the Gulf of Mexico in
conjunction with Pacific air rotating up from the southwest within
the upper jet streak. This will produce PWs that are likely to
reach 2 standard deviations above the climo mean toward the triple
point, indicative of the precipitation potential accompanying this
system.

Deep layer synoptic ascent and impressive moisture will lead to
continued widespread precipitation, exiting the Great Basin
Tuesday through the Northern Plains, with heavy snow likely in the
terrain, generally above 7000 ft. The guidance is quite
well-clustered with the progression of the surface low and 700mb
low through North Dakota by Thursday morning and lied within the
expected errors for the given time range. Southerly to
southwesterly flow will favor the San Juans in southwestern
Colorado today where over a foot of snow is possible.

As low pressure lifts through the Dakotas, CAM guidance continues
to advertise a strong deformation band pivoting northward later
today into Wednesday a couple hundred files to the NW of the low
center. In this region, impressive upper divergence within the LFQ
of the jet streak will help drive low-level frontogenesis
collocated with the best axis of deformation. At the same time,
rich theta-e air will be lifted cyclonically within a TROWAL north
and west of the 700mb low, providing additional ascent, with
low-level easterly upslope flow also driving strong vertical
motion. Where these features overlap, a band of intense snowfall
is likely and amounts were adjusted upwards over eastern Montana
and over the Black Hills. The column will be thermally marginal
for heavy snow so accumulations will likely be rate-dependent, but
strong ascent within the DGZ atop an isothermal layer supports
intense snow rates, which the WPC prototype snowband and 00Z HREF
probabilities (40-50%) could reach 1-2"/hr.

Day 1 WPC probabilities for 12 inches of snow are 30-60% over the
Bighorns and above 10% for greater than 4 inches of snow for much
of central and eastern Wyoming. Total snowfall may reach 2 feet
along the upwind slopes of the Big Horns. More than 4" of snow is
likely in the San Juans and much of the other terrain of UT early
in the period as the system moves through.

By Day 2 the heaviest snow will slowly exit eastern Montana and
the western Dakotas as the system maintains its strength during
the day. WPC probabilities for 4 inches are high in the Black
Hills and points north-northwestward.


...Pacific Northwest...
Day 2...

A shortwave diving out of the northeastern Pacific will move
onshore WA/OR late Tuesday into Wednesday accompanied by the LFQ
of a 120kt jet streak. A swath of generally light precipitation is
forecast ahead of a weakening cold front and associated area of
low pressure. Snow levels are forecast to rise towards 5000 ft
ahead of the cold front, and then fall back to 3000 ft in its
wake, although most of the accumulating snow should remain above
5000 ft. Snowfall across the highest peaks of the Cascades and
Olympics could exceed 6 inches, but in general WPC probabilities
for 4 inches are less than 30%. Pass level snow may reach and inch
or so.

...Colorado...
Day 3...

Shortwave out of the Pacific Northwest on Day 2 will dig a bit
into the Great Basin as the upper jet expands eastward across the
Four Corners. Westerly flow will capitalize on upslope enhancement
over western Colorado where WPC probabilities of at least 4 inches
of snow are around 30 percent.


The probability for significant icing (0.25 inches or more) is
less than 10 percent Days 1-3.

Fracasso