Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
325 PM EST Sat Dec 23 2023

Valid 00Z Sun Dec 24 2023 - 00Z Wed Dec 27 2023

...Central Rockies through the Northern Plains and Upper Midwest...
Days 1-3...

Interacting northern and southern stream shortwaves will emerge
from the Intermountain West on Sunday and then phase into an
extremely large and anomalous closed low over the Plains by
Monday. This feature will continue to deepen on Monday, with
700-500mb heights reaching -1 to -2 sigma according to NAEFS
across the Plains, forcing an extremely amplified mid-level wave
to result in only very slow eastward translation of the closed low
through the end of the period. This closed low combined with an
intensifying subtropical jet streak to its south reaching 150kts,
with some weak coupling likely with a jet streak over Canada, will
result in multiple rounds of snow, with a significant winter storm
becoming likely for parts of the Plains on Christmas.

D1 will feature waves of snow moving across the Northern and
Central Rockies as the individual shortwaves begins their approach
towards each other. Both of these features will move rapidly, the
southern stream to the northeast over the Four Corners, the
northern stream to the southeast across the Rockies, providing
widespread but overall modest ascent. However, waves of upper
diffluence combined with the mid-level PVA/height falls, will move
into increasingly saturated regions as downstream flow from the
S/SW at 700mb advects substantial moisture northward, while also
serving to produce additional topographic upslope flow into the
terrain. This will result in periods of moderate to heavy snow
from as far north as the Black Hills to as far south as the Sangre
de Cristos, with significant accumulations likely into the Front
Range and Palmer Divide of CO. WPC probabilities for more than 4
inches of snow are 50-80% in the higher terrain, with locally more
than 8 inches likely. Although snowfall accumulations into the
High Plains of CO are not expected to be as significant as in the
terrain, as a surface low develops in the lee of the Rockies, an
enhanced band of fgen above a strong cold front will likely result
in some heavier snowfall rates from the I-25 corridor and points
northeast into NE. The WPC prototype snow band tool suggests
snowfall rates here reaching 1-2"/hr at times, which could cause
significant travel impacts Sunday despite WPC probabilities for
more than 4 inches of snow that are below 30%.

The more significant snowfall into the Plains begins D2 /Christmas
Eve Night/ but becomes most substantial D3 /Christmas Night/.
During this time, the phasing of the mid-level impulses occurs
driving the anomalous closed low described above. While a lead
surface wave is likely to eject quickly northeast along the
eastward advancing cold front D2 producing a swath of moderate
snow from central NE to southern ND where WPC probabilities for
more than 4 inches reach 20-30%, the main event will develop from
the Corn Belt and into the eastern Missouri Valley D3.

During this secondary evolution, a strong surface low will develop
near Missouri and then lift north/northwest as it gets captured by
the strong upper low. This will occur in tandem with intense moist
advection characterized by intense 285-290K isentropic upglide
with mixing ratios of 3-4 g/kg, surging cyclonically around the
low into an impressive TROWAL late Monday into Tuesday, with this
WCB originating from an axis of PWs of more than +3 sigma
according to NAEFS. With high pressure anchored to the northwest,
this strengthening low, and models have trended even deeper today,
will likely have a robust isallobaric response to cool the column
despite the impressive WAA surging around it, resulting in an area
of heavy rain to freezing rain to snow. While there is still some
uncertainty in the placement of the heaviest snow axis, the
GFS/ECMWF/CMC are all relatively aligned in the 500mb development,
with the DESI clusters 1 and 2, which comprise nearly 80% ECE, 50%
GEPS, and 30% GEFS matching the CIPS analog guidance quite well.
This suggests an axis of heavy snow developing late Monday and
persisting through Tuesday for portions of the eastern Missouri
Valley across NE and SD, which is also where the best forcing to
drive a pivoting deformation band exists. Although the SREF DGZ
depth probabilities appear modest at this time, and strong winds
beneath it could cause dendrite fracturing, there is a good signal
for fgen lifting ascent into the TROWAL and DGZ supporting heavy
snow rates, and WPC probabilities for more than 4 inches of snow
on D2 peak above 50% in NE, and continue on D3 reaching 80% in
parts of SD/NE. Locally, this could result in more than 1 foot of
snow, most likely where this band pivots, and when combined with
strong winds could result in near-blizzard conditions.

Although some uncertainty in the exact placement of the low still
exists, there will almost certainly be a mixed-precip transition
zone as the WAA runs into the colder air, which could result in a
stripe of significant freezing rain accretions as well. The
greatest risk for significant accretions are from northwest IA
through the Buffalo Ridge/Coteau des Prairies, and into northern
MN where WPC probabilities both D2 and D3 reach above 40% for more
than 0.1". Locally more than 0.25" of ice is possible, and the NBM
90th% suggest isolated amounts approaching 0.5", however, a lack
of robust dry advection to maintain low wet-bulb temperatures
combined with heavy precipitation rates should limit freezing rain
somewhat, reducing the potential for these excessive freezing rain
amounts. Impactful and significant freezing rain is still likely,
though.


...Pacific Northwest...
Days 2-3...

A deepening shortwave trough over the Pacific will advect eastward
Monday into Tuesday before lifting onshore near the British
Columbia/Washington state border Tuesday morning. Downstream of
this feature, mid-level confluent flow will intensify as the
gradient becomes pinched south of the deepening impulse, which
will then spread into a divergent region across the Pacific
Northwest D2-D3. Pieces of the vorticity maxima will shed east
into this divergent area, producing additional lift, and working
in tandem with at least subtle upper diffluence within the LFQ of
an approaching jet streak to drive robust synoptic lift into the
region. IVT surging eastward has a high probability of exceeding
250 kg/ms late Monday into Tuesday, with even a 50-60% probability
of exceeding 500 kg/ms, suggesting ample moisture to be wrung out
as heavy precipitation. With PWs progged to reach +2 sigma
according to NAEFS, and snow levels rising ahead of the
accompanying warm front from around 3000 ft early to 5000 ft late,
this will result as areas of heavy snow, but primarily in the
northern Cascades and above pass level as reflected by WPC
probabilities for more than 4 inches of snow climbing from 5-10%
D2 to 50-80% on D3, with some light snowfall possible at
Washington and Stevens Passes on D3.

An additional concern with this evolution is the threat for
significant freezing rain along and east of the Cascades, through
the Columbia Gorge, and into the Columbia Basin. Sub-freezing
temperatures embedded within Canadian high pressure centered over
the Northern Rockies will maintain an axis of cold surface
temperatures on light easterly winds banking into the Cascades and
settling through the lower elevations. As the WAA associated with
the IVT moves into the region and crests the Cascades, this will
likely result in precipitation falling into the sub-freezing
temperatures as freezing rain. Available moisture across the
region has increased with recent model runs, so the threat for
significant icing has increased, and current WPC probabilities for
more than 0.1" of ice reach as high as 30-40% near the Columbia
Gorge, with 10-30% probabilities extending along the downwind
foothills of the Cascades and into the western portions of the
Columbia Basin.


...Key Messages for the Christmas Day Winter Storm...

--Significant winter storm develops Sunday night
A significant winter storm will impact parts of the
northern/central Plains and Upper Midwest with heavy snow,
freezing rain, and strong winds, during the Christmas holiday
(Sunday through Tuesday). Widespread travel impacts are likely.

--Heavy snow for parts of South Dakota & Nebraska
Heavy snow developing early Monday will spread across SD and NE
into Tuesday with snowfall rates likely reaching 1"/hr at times.
This will result in heavy snow accumulations which have a high
chance (70-90%) of exceeding 4 inches. In the axis of heaviest
snow, there is a 30-50% chance of more than 1 foot of snow.

--Dangerous travel due to near-blizzard conditions
The combination of heavy snow and strong winds which may reach
30-40 mph across the Plains, could produce near-blizzard
conditions. This will result in dangerous travel due to severely
restricted visibility and snow-covered roadways.

--Freezing rain to cause hazardous conditions
A wintry mix of sleet and freezing rain will likely overspread
parts of the Upper Midwest beginning Monday afternoon. There is a
moderate chance (50-70%) for at least 0.1" of ice in parts of the
area. This will likely lead to slippery roads and sidewalks, with
isolated power outages also possible.


Weiss