Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
338 AM EDT Sat Oct 22 2022

Valid 12Z Sat Oct 22 2022 - 12Z Tue Oct 25 2022

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

A significant pattern change is underway across the western CONUS
which will result in a winter storm bringing widespread moderate
to heavy snow across much of the area. The shortwave which will
amplify into a full latitude trough to drive this event is moving
onshore the Washington coast this morning, and will rapidly
amplify into a closed mid-level low over the eastern Great Basin
Sunday morning, with the associated trough digging as far south as
the Gulf of California. As this trough continues to move slowly
eastward into early next week, most guidance has the trough
splitting into northern and southern stream closed lows as potent
vorticity lobes wrap around the parent trough. This will result in
dual closed features, one shifting across the Northern Plains
Monday, the other diving into TX by the end of the forecast
period. While the southern stream low will help drive some
snowfall in the higher terrain of NM by D3, most of the heavy
snowfall will be associated with the northern energy.

Aloft, a complex jet structure will help drive increasingly robust
deep layer ascent, while also providing ample moisture on a modest
AR surging PWs to more than +2 standard deviations across the
inter-mountain West according to the NAEFS ensemble tables.
Phasing of southern and northern stream jet energy, resulting in
an impressive coupled jet structure will pair with the deep trough
and closed northern low to produce lee cyclogenesis across WY this
weekend, with a slowly deepening surface low tracking into the
Dakotas early next week. This low will drive a cold front
southeastward which will combine with height falls to lower snow
levels to as low as 3000 ft in the Rockies and to around 1000 ft
in the High Plains, with coincident but modest fgen and
deformation NW of the low driving some locally enhanced mesoscale
ascent. While any intense UVVs should be generally transient,
locally more intense omega is possible within this deformation
across the Northern High Plains, with additional locally stronger
lift likely within upslope enhanced terrain features. Where the
most intense overlap of ascent and moisture lie, especially in the
terrain from the Northern Rockies southward through the NW WY
ranges and into the Uintas, Wasatch, and CO Rockies, snowfall
rates could be intense, exceeding 1"/hr according to the WPC snow
band probability tool, and this is reflected by pWSSI
probabilities for moderate impacts being driven most impressively
by snow rate.

Additionally, SLRs will generally be near some of the lower Baxter
climatological percentiles for the event due to marginally
favorable thermal structures from the Pacific sourced airmass.
However, strong 700-500 CAA surging southward behind the primary
trough axis will help to steepen lapse rates while concurrently
deepening the DGZ. This suggests SLRs will increase, especially
D2, and have incorporated some of the higher SLR guidance into the
blend to raise SLRs in upslope terrain during periods of CAA.
However, outside of mesoscale ascent through upslope, total UVV
through the DGZ looks modest during this time which will somewhat
limit SLR. Still, above 5000 ft or so SLRs could reach 13-15:1,
especially in the Central Rockies and surrounding terrain, while
remaining much lower than climo elsewhere. Across the Northern
High Plains, the overlap of some fgen and deformation could result
in higher SLR, but the DGZ appears quite high which should limit
snow growth production somewhat.

All of this together will result in impressive snowfall totals
from the OR Cascades southward through the Wasatch and San Juans
and north into the Northern Rockies where WPC probabilities are
moderate to high for 8 inches, with locally more than 12 inches
likely above 5000 ft. On D2, the best ascent shifts east such that
the greatest WPC probabilities for 8 inches become more restricted
from the San Juans northward through WY and into the Little Belts
of MT, reaching 30-50%, with a secondary maxima of 20-40%
probabilities occurring in eastern MT. By D3 a subsequent but
weaker shortwave will spread additional heavy snowfall exceeding 8
inches back into the WA Cascades and Northern Rockies. Event total
snowfall could eclipse 20 inches in a few of the highest terrain
locations in the CO Rockies, the Tetons, and parts of the
Absarokas. Moderate snow totals nearing 6 inches are possible as
far south as the White Mountains of AZ and the Sangre de Cristos,
with impactful accumulating snowfall also likely at many of the
mountain passes above 5000 ft.

The probability of significant icing is less than 10 percent.

Weiss