Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
344 PM EDT Fri Nov 04 2022

Valid 00Z Sat Nov 05 2022 - 00Z Tue Nov 08 2022

...Western U.S....
Days 1-3...

An impressive atmospheric river (AR) will inundate the mountainous
terrain of the Pacific Northwest and northern Rockies with
anomalous quantities of atmospheric moisture. 00Z 11/4 NAEFS
showed 700-500mb moisture at or above the 99th climatological
percentile Saturday morning from northern California to as far
inland as the central Great Basin. Integrated Vapor Transport
(IVT) values off the Oregon coast reach as high as 1,000 kg/m/s,
which is roughly +5 sigma above the climatological norm via NAEFS.
It is from this slug of rich Pacific moisture that the heaviest
rounds of precipitation arrive tonight and into Saturday morning
from Oregon and northern California on east into central Idaho and
western Wyoming. A cold front pushing south through the Northwest
on Saturday will steadily bring snow levels down through western
Wyoming and into northern Utah and Colorado, resulting in periods
of heavy snow. Latest WPC PWPF indicates a 50-70% chance for
snowfall totals >12" in the Sawtooth, Bitterroot, and Teton Ranges
from this evening to Saturday evening.

Meanwhile, the next strong Pacific storm system tracks into the
Pacific Northwest late Saturday night and into Sunday. In
addition, a strengthening and highly anomalous Alaskan high
pressure system (1050+ mb Monday morning in the Yukon) will bleed
south through western Canada and introduce a fresh injection of
anomalously cold air on by Monday (record cold max temperatures
are possible from Washington to central California on Monday).
This will lead to crashing snow levels throughout these regions as
early as Sunday night. Northern Washington has the best odds on
Day 2 to see >12" snowfall according to the latest WPC PWPF with
probabilities as high as 60%. The upper trough responsible for
this northwestern U.S. storm system continues to dive south Monday
morning and direct an IVT of ~600-800 kg/m/s at California. 500mb
heights off the California coast fall as low as 3-4 sigma below
normal by 18Z Monday. The corresponding height falls and intense
upslope flow into the Sierra Nevada, Shastas, and Salmon Ranges
sets the stage for periods of heavy snowfall Monday. The latest
experimental pWSSI shows Major impact potential as high as 40-60%
in portions of the Salmons and up and down the Sierra Nevada on
Monday. Snow load and snowfall rate are the primary driving
factors in weighting the Major impacts for the pWSSI. The WPC PWPF
does shows anywhere from 50-80% probabilities for >12" of
snowfall, as well as 20-40% probabilities for >18" of snowfall
between 00Z Mon - 00Z Tues. While not as heavy of snow is expected
farther north, portions of the Boise, Sawtooth, and Teton Ranges
will receive another round of heavy snow where PWPF probabilities
for >8" of snowfall are as high as 40-60%.

...Central Plains and Upper Midwest...
Days 1-2...

A sharp and vigorous upper level trough tracking through the
Central Plains will be the catalyst for cyclogenesis over the
Mississippi Valley tonight and into Saturday morning. As the 500mb
trough takes on a negative tilt Friday evening, robust vertical
ascent from strong WAA at low-mid levels will result in a TROWAL
on the western flank of the 850mb low. 1000-500mb RH values within
the TROWAL are likely to be >90% and the intense vertical
velocities (VVs) within the 700-500mb layer will aid in enhancing
the dynamic cooling effect within the lower levels of the column.
With such strong mesoscale drivers in place, global guidance is
tending to be too warm in the boundary layer where this banding
sets up. Looking at CAM guidance, a few (HRRR and ARW most
notably) show the potential for ~2"/hr snowfall rates within the
deformation zone. The 12Z HREF does show a corridor of 30-40%
probabilities for 3"/3-hr snowfall totals from north-central
Kansas on northeast into southeast Nebraska. Given the marginal
boundary layer conditions, snowfall type will be a heavy/wet snow
that could pile up quickly on trees, grassy surfaces, and should
2"/hr rates come to fruition, bridges and overpasses. One initial
limiting factor is recent anomalous warmth that has led to warm
soil/road temperatures. This likely means The most impacted areas
will not occur within a large swath, but more likely in a narrow,
concentrated zone where the strongest VVs aloft reside within an
overly saturated DGZ aloft.

It is possible for 1-2"/hr rates to extend as far north as western
Iowa and southern Minnesota Saturday morning, but by this point,
the cyclone will have matured and VVs will not be as robust as
they were in the Central Plains. This means less dynamic cooling
within the atmospheric column, which will make it tougher for
snowfall to overcome the marginal boundary layer temperatures
during the daytime hours in the Midwest. Still, latest PWPF shows
a 10-20% chance of snowfall totals >2" as far north as the
Minnesota Arrowhead, so some lighter snowfall totals cannot be
ruled out even as the storm makes its way over Lake Superior
Saturday evening.

The probability of significant icing greater than 0.25" is less
than 10 percent.

Mullinax