Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
252 PM EST Thu Nov 24 2022

Valid 00Z Fri Nov 25 2022 - 00Z Mon Nov 28 2022

...New Mexico to West Texas...
Day 1..

A potent closed low over New Mexico will drift slowly southward
tonight before ejecting to the northeast on Saturday, finally
filling as it gets absorbed into the westerlies Sunday morning.
This will be accompanied by paired jet streaks upstream and
downstream of the closed low, and deep layer ascent will become
widespread and intense across Texas/New Mexico through Saturday
morning. To the east of the main upper low, moisture will be drawn
northward and then rotate cyclonically into TX and NM, with PW
anomalies progged to reach as high as +2.5 sigma. The associated
theta-e ridge will lift within this WCB into a TROWAL around the
low, with moist isentropic upglide aiding to drive locally
enhanced mesoscale ascent across West Texas, the southern Texas
Panhandle, and much of southeast New Mexico. While low-level
thermals will be marginal noted by a near 0C isothermal layer at
the bottom of local soundings, the overlap of this moist ascent
and favorable instability aloft for some CSI suggests ascent and
precip loading will dynamically cool the temperatures to support
snow. It is likely some of this snowfall will be quite heavy,
noted by WPC snow band tool probabilities suggesting a long
duration of 1-2"/hr rates, highest in SE NM and along the terrain
of the Davis, Guadalupe, and Sacramento mountains. The guidance
has again shifted a bit SW today, and this has resulted in an
increase in snowfall despite SLRs that should be generally around
8:1, supporting a heavy wet snow which will create considerable
impacts to travel. WPC probabilities for 6+ inches are 60-90% in
the terrain of west Texas and much of southeast NM, with local
amounts of 12+ inches likely, especially in the Sacramento range.


...Pacific Northwest...
Days 1-3...

Dual shortwaves will lift onshore the Pacific Northwest late this
week through the weekend, bringing rounds of moderate to heavy
precipitation across the region. The mid-level flow in which these
shortwaves will be embedded remains quick driving transient
forcing as the cyclonic flow remains broad. While features are
expected to be modest in amplitude and fast moving, confluent
500mb flow will combine with Pacific jet streaks to produce at
least subtly enhanced moisture to wring out snowfall in the
terrain. Brief SW flow and WAA ahead of each shortwave trough will
enhance ascent and moisture as well, with the heaviest snow likely
in two rounds: Friday night into Saturday, and then again Sunday
aftn and evening. The WAA associated with the leading shortwave
Friday night will push snow levels up to 4000-5000 ft before
crashing, but most of the moisture will also erode as the snow
levels fall thanks to cold dry air advecting in from Canada. For
the second, and more significant event, snow levels will climb
briefly to 3000-4000 ft, but then will likely fall to 1500-2000
ft, suggesting snow will begin to impact the passes late in the
forecast period, with significant accumulations possible. For D1
and D2, WPC probabilities for more than 4 inches of snow are
confined to the WA Cascades, the Northern Rockies/Bitterroots, and
northern Big Horns, with a 20-40% chance each day. The more
significant event begins D3 as the somewhat stronger shortwave
moves onshore. More impressive ascent, better moisture, and lower
snow levels will result in WPC probabilities reaching 70-90% in
the WA Cascades and east into the Northern Rockies, with more than
2 feet of snow possible in the highest terrain. Lesser
accumulations of a few inches are likely into the OR Cascades and
Blue Mountains as well. Additionally, with snow levels falling
below the passes, significant snowfall exceeding 4" is likely at
Snoqualmie, Stevens, and Lookout passes.


The probability of significant icing exceeding 0.1" is less than
10%.

Weiss