Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
351 AM EST Tue Dec 27 2022

Valid 12Z Tue Dec 27 2022 - 12Z Fri Dec 30 2022

...Great Lakes...
Day 1...

Cyclonic flow over Lakes Erie and Ontario continue into this
afternoon with single bands gradually diminishing. Day 1 snow
probabilities are moderate for 4"+ over the northern Tug Hill. A
shortwave trough currently over the Canadian Prairies  crosses
Lake Superior this evening, providing some modest PVA over the
northern Great Lakes bringing snow to the eastern U.P. where are
there low Day 1 probabilities for 4" of snow. There is potential
for a narrow stripe of freezing rain this afternoon and evening
over the Upper Midwest south of Lake Superior, but sleet is the
more likely mixed precip south of the snow bands given the very
cold surface temperatures. 


...The West...
Days 1-3...

A deep upper low currently off the WA coast weakens/opens as it
moves over WA this afternoon, but the surrounding trough digs
ahead of a potent NWly jet off the Pacific, becoming a
full-latitude trough by tonight. A strong atmospheric river south
of this wave continues to track south over CA today, pushing into
Mexico tonight. While the heights over the Pacific Northwest are
low, the temperatures in the 700-500mb layer are remarkably mild
for late December over California and the Southwest where snow
levels in the core of the plume are 10,000ft or higher. Snow
levels decrease to around 4000ft through this afternoon and
evening behind a cold front making its way through California.
Broad onshore flow will maintain a heavy, wet snow in the Sierra
Nevada, Cascade Range, and as far inland as the Salmon River and
Sawtooth of central Idaho. Day 1 WPC PWPF shows probabilities for
>12" of snowfall between 60-80% in portions of these ranges
tonight through tonight. WPC WSSI continues to depict "Extreme"
impact potential in the Sierra Nevada weighted toward the Snow
Load component of the WSSI algorithm. Travel is likely to be
dangerous, if not impossible, in the hardest hit areas. In
addition, there is a strong wind component and the combination of
high snow load and gusty winds could lead to downed tree limbs and
power lines in the northern and central Sierra Nevada today.

By tonight, the strong NWly jet streak will push in from the
Pacific and force the core of the best 700mb moisture flux into
the Intermountain West. With falling 700-500mb heights across the
region from the upper trough moving in aloft, this will cause snow
levels to drop and snow to fall heavily from the Tetons and
Wasatch on south to the San Juans and the Mogollon Rim. Snow
levels will initially be quite high in the Four Corners region
tonight, but they will fall to 6,000' by Wednesday afternoon. The
slug of Pacific moisture is impressive with NAEFS showing
climatological percentiles >99% in the 500-700mb layer across
north-central Arizona and southwest Colorado. Day 1.5 WPC PWPF
shows 50-80% probabilities for >8" of snow for the Wasatch and
most of the Colorado Rockies west of the Continental Divide with
high snow levels keeping heavy snow to just the highest parts of
AZ. The San Juans have the best odds of seeing >12" snowfall
totals with 60-80% probabilities. Similar to their neighbors in
the Sierra Nevada, it will be a combination of Snow Amount and
Snow Load that drives the WSSI to indicate "Moderate" to "Major"
impacts in parts of these mountain ranges. Treacherous travel
conditions are expected in these ranges Wednesday and into
Wednesday night.


...Central Plains & Midwest...
Days 2-3...

The moisture associated with the upper trough traversing the
Intermountain West will make its way across the Nation's Heartland
where a wave of low pressure is set to form in lee of the Rockies
by late Wednesday. Confidence for snow east of the Front Range is
currently highest north from the Palmer Divide where there are
20-40% probabilities for 4"+ snow on Day 2.5. While bands of snow
are anticipated northwest of the Colorado Low crossing the Plains
Wednesday night into Thursday, the thermal environment is rather
marginal. Where the band sets up there will be potentially locally
moderate to even heavy snow, but probabilities for even 2"
northeast from CO are currently quite low. Overrunning ahead of
the low could also allow some freezing rain from the middle
Missouri Valley into/through MN.

Jackson