Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
401 PM EDT Fri Oct 21 2022

Valid 00Z Sat Oct 22 2022 - 00Z Tue Oct 25 2022

...Pacific Northwest through the Central Rockies and Northern High
Plains...
Days 1-3...

A significant early season winter storm is set to deliver copious
amounts of heavy/wet snowfall to the higher terrain the Pacific
Northwest, the Rockies, and portions of the northern High Plains
this weekend and into the start of next week. A highly anomalous
upper trough digging south through the northwestern U.S. will
cause snow levels to crash to as low as ~4,000 ft in parts of the
northwest while precipitation gradually changes over to snow
behind a passing cold front. The most influential variable driving
this multi-day heavy snow is a steady onslaught of rich mid-level
moisture. Supported by large scale ascent aloft from bouts of
positive vorticity advection and strong jet stream dynamics
beneath the right entrance region a strengthening 250mb jet streak
over the northern Rockies, ECMWF EFI depicted highly anomalous QPF
for both forecast days 1-2, as well as some far reaching Shift of
Tails (SoTs) that indicate the potential for significant snowfall
that stretches from the central Great Basin to the northern High
Plains. Both ECMWF/NAEFS situational awareness tables depicted >90
climatological percentile values in the 500mb mean specific
humidity fields, which also reside within the majority of where
most DGZ layers will be. Combined with the strong synoptic scale
ascent aloft and driven more by orographically favored upslope
areas resulting in stronger vertical velocities, snowfall rates
will be heavy at times from the Cascades and Wasatch on east to
the northern and central Rockies.

SLRs will not be overly impressive due to marginal thermals,
particularly below 5,000' AGL in the Northwest. SLRs are likely to
average 8-10:1 in the 4,000-6,000 ft elevations, but the intense
cooling from robust vertical velocities along N-S oriented terrain
(Tetons, Wasatch) could reach as high as 15:1. Eventually, the
storm system will strengthen farther east courtesy of lee
cyclogenesis and strong jet dynamics downwind of phasing polar and
subtropical jets. This is where model guidance diverges as the
handling of this strengthening surface low and its elongated
700-500mb circulation is forcing global guidance to differ on
location of the deformation axis over eastern Wyoming and western
North Dakota. In terms of probabilistic snowfall, the latest WPC
PWPF features >50% probabilities for >8" of snowfall for the
entire 3-day period in the Wasatch, Tetons, Absarokas, Big Horns,
Wind River Range, Colorado's San Juans, and on north to the Lewis
Range of Montana. Some of these areas also feature up to 50%
probabilities of >12" in their tallest peaks.

Latest experimental pWSSI values for Moderate Impacts >60% reside
in the Boise, Teton, and Absaroka through Saturday morning, on
south into the Wasatch and highest elevations of the central Great
Basin into Saturday night. By Sunday, these same probabilistic
thresholds reside in the higher elevations of west-central and
southwest Colorado, as well as in the Big Horns of Wyoming. The
primary driver in the WSSI is "Snow Load" which is calculated via
snow water equivalent. This implies a heavy/wet snow type could
result in major impacts across parts of the highest elevations.
This type of snowfall will not only lead to numerous road closures
and treacherous travel, but also but a strain on infrastructure in
the worst hit areas.

The probability of significant icing is less than 10 percent.

Mullinax