Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
329 PM EST Mon Dec 25 2023

Valid 00Z Tue Dec 26 2023 - 00Z Fri Dec 29 2023

...Central Rockies through the Northern Plains and Upper Midwest...
Days 1-2...

A major winter storm will bring heavy snow with blizzard
conditions, and significant ice accretions to portions of the
Plains and Upper Midwest over the next two days.

Surface low pressure consolidating near the Mid-Mississippi Valley
today will deepen further as it tracks W/NW before occluding over
the Upper Midwest tonight. This low will then retrograde to the
west and then southwest through the Central Plains into Tuesday as
it gets captured by an extremely anomalous closed low over the
middle part of the country (NAEFS height anomalies reaching -3
sigma), before finally filling and ejecting out to the east by
Wednesday. Tremendous moisture advection characterized by PWs of
around +4 sigma according to NAEFS will rotate cyclonically around
this low on impressive isentropic upglide into a robust TROWAL,
fueling a large area of heavy precipitation from Kansas through
Minnesota, much of which will be snow and freezing rain, before
things dry out on Wednesday.

For the snow areas, this is likely to be a significant blizzard,
especially for portions of NE and SD, and potentially into
northern KS. Intense isentropic ascent at 290K will surge the
moisture westward, which beneath the intensifying TROWAL will
result in an expanding area of heavy snow. At the same time, there
is likely to be a robust deformation axis that sets up to the west
of the surface low, and collocated with some low-level fgen to
drive an axis of even more intense ascent. The setup seems to
support a pivoting band of heavy snow, with rates of 1-2"/hr
predicted by the WPC prototype snow band tool. As the entire
system retrogrades westward, this band will drift west while
pivoting N to S around the low, resulting in a swath of very heavy
snow, and WPC probabilities for 6+ inches of snow reach above 70%
from northern NE through much of west/central SD, and locally
12-18" of snow is likely. This heavy snow combined with winds that
may gust above 50mph at times will produce blizzard conditions and
dangerous travel. As the low fills and ejects eastward late D2
into D3, some axis of heavy snow may persist, but with gradually
waning intensity and coverage. For D2, multiple areas of moderate
probabilities for 2-4" of snow continue near the SD/NE border and
parts of northeast KS, with a secondary maxima over MO beneath the
core of the upper low and associated deformation/steep lapse rates.

Farther to the north, the intense WAA within the WCB and
responsible for the intense theta-e advection into the TROWAL,
will also surge a warm nose of 850mb temps > 0C over still cold
surface temperatures and sub-freezing wet bulb temperatures. This
will result in axis of heavy freezing rain, especially north of a
large dry slot that will lift northward through D1. Where moisture
is most pronounced and sub-freezing surface wet bulb temperatures
can persist, freezing rain accretions are likely to be
significant, and WPC probabilities for 0.25" or more are now above
80% in parts of eastern ND, with a large swath of moderate
probabilities for 0.1" encompassing much of eastern SD, ND, and
northern MN. There remains some uncertainty into where this
heaviest icing will occur, especially since it appears morning
observations are a bit warmer than model progs, but higher
elevations of the Coteau des Prairies and into eastern ND, may be
the focus for the greatest impacts due to freezing rain, with
damaging ice above 0.5" possible in some areas.


...Pacific Northwest and Northern California...
Days 1-3...

A series of shortwaves moving into the Pacific Northwest will
maintain generally confluent onshore flow through the period,
driving waves of precipitation into the area through mid-week. The
first of these will be ongoing tonight as a warm front lifts
onshore coincident with the shortwave trough axis advecting
inland, and accompanied by increasing upper diffluence within the
LFQ of a Pacific jet streak. The overlap of this jet streak and
confluent onshore flow will drive IVT as high as 750kg/ms,
although the probabilities of IVT that high reaching the coast are
low. Regardless, this will result in an expanding area of heavy
precipitation ahead of the cold front which will move onshore
Tuesday aftn and rapidly dry out the column. Snow levels will
start around 7000 ft but fall steadily to around 3000-4000 ft by
the time the precip winds down. This should keep significant
snowfall accumulations above pass levels, and on D1 WPC
probabilities for more than 4 inches of snow are 50-80% in the
highest terrain of the Northern Cascades.

More concerning for D1 will likely be the freezing rain threat
along the Cascades and into the eastern foothills, as well as at
lower elevations along the Columbia Basin and Columbia Gorge.
Canadian high pressure anchored in place will maintain easterly
flow across WA/OR, funneling cold air at the surface. As the
precipitation expands eastward, this will result in warm air
overrunning the cold surface layer to produce freezing rain. There
is still some uncertainty into the depth of the cold air, but it
is likely at least modest ice accretions will occur in the
Cascades, including Stevens and Snoqualmie Passes, with more
significant icing likely along the Columbia Gorge. WPC
probabilities for 0.1" or more of ice are 10-30% along the
Cascades and into the Foothills, with a 10-20% chance of 0.25"
along the Hood River Valley and the Columbia Gorge.

After a respite much of D2, another shortwave will approach the
coast, driving additional ascent while maintaining an axis of IVT
through confluent flow and jet level energy into the region. The
maximum IVT is progged to shift a little farther south with this
second wave than the first one, which will allow precipitation to
expand as far south as the northern CA ranges of the
Shasta/Trinity/Siskiyou region and then continue up through the
Cascades. The axis of precipitation is progged to be rather
narrow, and snow levels should hover around 5000 ft, limiting
overall impacts. However, where some upslope ascent can
contribute, moderate snowfall accumulations are likely as
reflected by WPC probabilities peaking around 60% for 4+ inches of
snowfall.


...Northern Maine...
Day 3...

A spoke of vorticity rotating around a large mid-level gyre
drifting into the OH VLY will combine with the favorable diffluent
LFQ of a subtropical jet streak arcing northeast along the
Atlantic Seaboard to produce modest cyclogenesis off the
Mid-Atlantic Coast Wednesday night into Thursday. Moisture
wrapping northward around this surface low will surge into New
England during D3, with associated isentropic ascent driving an
expanding area of precipitation into the region. At the same time,
a cold front will drop slowly southward into northern New England,
resulting in a transition area from rain to snow towards Canada.
This should lead to at least an axis of moderate snowfall across
northern Maine on Thursday, where WPC probabilities for more than
4 inches of snow are 20-40%.


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

--Major winter storm continues through Tuesday
A major winter storm will expand across parts of the
northern/central Plains and Upper Midwest with heavy snow,
freezing rain, and strong winds through Tuesday. Widespread travel
impacts are likely.

--Heavy snow for the Central/Northern High Plains
Snow will increase in coverage and intensity through tonight with
snowfall rates at times reaching 1-2"/hr. This will lead to
significant snow accumulations, and there is a high chance (70+%)
of 12" or more of snow from south-central SD into northern NE.
More than 4" of snow is expected for most of NE, SD, and into the
High Plains of KS/CO/WY.

--Dangerous travel due to blizzard conditions
The combination of heavy snow and strong winds (gusts up to 55
mph) will produce blizzard conditions. Travel will be difficult to
near impossible due to snow-covered roads and whiteout conditions.
Gusty winds could bring down tree branches and power lines.

--Damaging icing for parts of the Northern Plains
A mix of sleet and freezing rain will continue across the eastern
Dakotas and then expand westward tonight. Significant ice
accumulations exceeding 0.25" are forecast in this area, with
isolated totals above 0.5" likely. This will result in scattered
power outages and tree damage, as well as lead to dangerous travel
due to icy roads and sidewalks.


Weiss