Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
238 AM EST Thu Dec 21 2023

Valid 12Z Thu Dec 21 2023 - 12Z Sun Dec 24 2023

...Sierra Nevada to the Southern Rockies...
Days 1-3...

The upper low west of SoCal will finally turn eastward today and
come ashore on Friday as it weakens into NW Mexico. High snow
levels will confine appreciable snow to above 8000ft in the
southern Sierra today where a few inches are likely. The next
phase of the system will be into the Four Corners region day 2
(Friday) as the moisture plumes translates eastward, still tapped
to the subtropical/tropical eastern North Pacific (PW anomalies +1
to +3 sigma). Again, mild air mass in place will keep snow
generally confined to the higher/highest elevations (above 8000ft)
which will capitalize on upslope enhancement (eastern AZ -- White
Mountains) and parts of the Mogollon Rim where WPC probabilities
for at least 4 inches are moderate (40-70%) or higher. By day 3
(Saturday), moisture flux on southwest flow will intersect the San
Juans and promote much heavier snow totals (probabilities for at
least 8 inches are high, greater than 70%, above about 10,000ft)
with modest totals across the Sangre de Cristos and Jemez
Mountains in NM. By the end of day 3, northern stream system
exiting the northern Great Basin will begin to dig through Utah
and slow the lead shortwave onto the High Plains, allowing a Gulf
surge of moisture to flow northward and start to wrap into the
developing surface low exiting southeastern CO by the end of the
forecast period. Snow will overspread much of the Front Range as
rain changes to snow east of I-25 by Sunday morning. Through then,
probabilities of 4 inches of snow or more are moderate (40-70%) in
the CO Rockies above about 8000ft. Snow is expected to expand and
increase eastward thereafter.


...Pacific Northwest to the Northern Rockies...
Days 2-3...

A sharp mid-level shortwave will push into the Pac NW on Friday
(day 2) with an attendant surface cold front. Ahead of it, a
modest moisture surge (PW anomalies +1 to +1.5 sigma) will be
sufficient for mountains snows over the Olympics, Cascades, Blue
Mountains, and into the Idaho ranges as snow levels initially
around 5000-6000ft drop to below 2000ft as snow tapers off. The
vort max will turn southeastward on Saturday through ID/WY and
likely close off into an upper low by the end of the period as it
nears NW CO, combining with the southern stream system just to its
southeast. Snow will overspread WY into the CO Rockies as snow
levels lower and rain changes to snow down to the valley floors in
WY. On day 3, WPC probabilities for at least 4 inches of snow are
moderate (40-70%) over parts of the Absarokas, Wind River Range,
Laramie Mountains, and Medicine Bow Mountains into CO.


The probability of significant ice across the CONUS is less than
10 percent.


Fracasso