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

Valid 00Z Fri Dec 22 2023 - 00Z Mon Dec 25 2023

...Southern Rockies...
Days 1-2...

The feature responsible for periods of heavy snow in the southern
Rockies is an upper low currently off the southern California
coast. By Friday morning, the upper low speeds up and heads for
the Lower Colorado River Valley. The upper low will eventually
find itself tracking through southern Arizona Friday evening with
healthy 250-500mb diffluence over the San Francisco Peaks north of
Flagstaff, the Gila and White Mountains of eastern Arizona, and
eventually the San Juans of southwest Colorado. In addition, there
is no shortage of moisture present as NAEFS shows IVT values
(200-300 kg/m/s) and PWs (up to 0.50") that this time of year
exceed the 97.5 climatological percentile. While there is a lack
of sufficient cold air in the valleys, the strong diffluence aloft
combined with ample moisture and upslope flow enhancing
precipitation rates will lead to heavy snowfall in these mountains
ranges starting Friday afternoon and lingering into Saturday
morning. Latest WPC PWPF continue to depict high probabilities
(>70%) for snowfall totals >8" near Apache National Forest and
into the San Juans above 9,000ft. The WSSI indicates Moderate
Impacts are anticipated in these mountains ranges with Minor
Impacts at the lower elevations.

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

A shortwave trough approaching from the northeast Pacific looks to
deliver an influx of Pacific moisture and sufficient vertical
ascent aloft to generate periods of heavy snow in the higher
terrain of the Olympics and Cascades Friday morning. As winds
shift out of the west and prolong the fetch of 850mb moisture flux
in wake of the cold frontal passage, snow levels will drop to as
low as 2,000ft and SLRs will rise. The upslope component will
continue to support topographically induced heavy snowfall in the
Cascades Friday evening. WPC PWPF sports moderate-to-high chances
(50-80%) for snowfall totals >6" in the higher peaks of the
Cascades, but lower chances (30-50%) for the highest peaks of the
Olympics. The heaviest snowfall totals will likely reside above
most major passes in western Washington for this event. Meanwhile,
the cold front will work its way into the northern Rockies Friday
night, bringing periods of snow to ranges such as the Blue, Boise,
Bitterroots, Absaroka, and Tetons. WPC PWPF tops out with moderate
chances (40-60%) for snowfall totals >4" in the highest peaks of
these ranges Friday night and into Saturday.

...Central Rockies to the Upper Midwest...
Day 3...

By late Saturday, the two disturbances responsible for heavy snow
in the aforementioned regions above will work in tandem to produce
a new winter storm over the central Rockies and High Plains by
Christmas Eve. As the upper trough in the southern Rockies
approaches, lee cyclogenesis in eastern Colorado and 850-700mb
layer averaged frontogenesis will take shape over the Central
Plains and stretch into the Upper Midwest by Christmas Eve. Out
ahead of the deepening upper trough, vertical motion atop the
atmosphere will increase thanks to strengthening diffluence over
the Northern and Central Plains, while a cold front races in from
the Northern Rockies to provide a source of fresh, sub-freezing
air within the boundary layer. Deterministic guidance are, more or
less, in two particular camps. The ECMWF/CMC/UKMET show more
phasing of the Northwest disturbance and the upper trough becoming
cut off from the polar jet, leading to a slower and more organized
storm system. The GFS is not as phased with more influence from
the polar jet to the north, resulting in a faster and more open
upper trough. The GEFS mean shows a little interaction that the
GFS, so have chosen to not include the GFS in this forecast.

As for the setup itself, the ECMWF ENS situational awareness tools
shows a strong and southerly IVT in the Great Plains Saturday
night (reaching at least the 97.5 climatological percentile) that
will supply the necessary moisture to wrap around the
strengthening 700mb low in the Central High Plains. On the
northern and western flank of the mean 500-700mb low Sunday
morning, the expectation is for a warm conveyor belt to ensue and
lead to a deformation axis of snow that stretches from northern
Colorado and southern Wyoming to as far north as southwest South
Dakota. How much snow accumulates will ultimately come down to
both the track of the storm and the residency time of the
deformation axis. The WPC PWPF highlights the Front Range of the
Rockies and the Laramie Mountains of Wyoming to points east over
western Nebraska and the Black Hills. The higher peaks of the
Front Range and Laramie Range sport the highest odds (>70%) for
snowfall totals >4". Probabilities are more in the moderate range
(40-60%) in the High Plains of western Nebraska, eastern Wyoming,
and along both the Palmer Divide and the Black Hills. It is worth
noting this is a complex evolution involving an intricate timing
of phasing and forward speed, so residents should continue to
closely monitor the forecast into the upcoming weekend.

In terms of impacts, this storm arrives Christmas Eve weekend,
potentially causing travel headaches for residents in this storm's
path. While confidence in where the heaviest totals east of the
Rockies take shape, the combination of snowfall and gusty winds is
prompting the Probabilistic WSSI to show moderate chances (40-60%)
for Minor Impacts Snow Amounts and Blowing Snow to cause from
southern Wyoming and east-central Colorado to western Nebraska and
southwest South Dakota. late Saturday and into the day on
Christmas Eve. It is also worth noting, along the 850mb front over
the Upper Midwest, that enough sub-freezing temps will be present
for the threat of a wintry mix in the eastern Dakotas. Latest WPC
PWPF shows low chances (10-30%) for ice accumulations >0.1" in
southeast North Dakota, while the WSSI-P shows 20-30%
probabilities for Minor Impacts due to Ice Accumulation in eastern
South Dakota on Christmas Eve.


Mullinax