Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
343 AM EST Tue Dec 26 2023

Valid 12Z Tue Dec 26 2023 - 12Z Fri Dec 29 2023

...Western High Plains/Northern Plains/Upper Midwest...
Day 1...

The major winter storm continues this morning but will start to
wind down through the day. The upper low over northeastern KS will
slowly move eastward today as the surface low completes its
cyclonic loop over the Corn Belt and the entire system becomes
vertically stacked this afternoon. At this point in the life
cycle, dynamical forcing has lessened considerably but the
rotating upper low will still maintain a modest, though weakening,
overly wrapped moisture connection to the Gulf/western Atlantic
through the day. The storm will devolve into three parts for at
least the first half of the day 1 period -- overrunning >0C air
atop a cold <0C surface over the High Plains (ND/SD into northern
MN) where freezing rain will persist for day 1. Additional ice
accumulations near 0.25" are likely in eastern ND into northern SD
where WPC probabilities for at least that much exceed 50%. A
larger area stretched from western SD ENE to the MN Arrowhead will
likely see at least some ice accretion on top of whatever has
fallen already. Hardest hit areas may see near 0.75" of icing from
the system.

To the west, over the western High Plains, combination of the nose
of the moisture plume (stretched from east of Florida up to the
Great Lakes and then all the way to western SD) and upslope
enhancement will aid in producing an additional several inches of
snow, especially this morning, over southwestern SD into eastern
WY and northeastern CO. Strong winds will maintain near-blizzard
conditions early in the day, with lowered visibility from blowing
snow through much of the day. WPC probabilities for at least
another 4 inches of snow are greater than 50% in many of these
areas, though mesoscale banding within favored convergent zones
will make for a patchier snowfall distribution. Finally, near the
upper low (and surface low) track, cold core mid-level
temperatures and near freezing surface temperatures will support
snow and snow showers today across northern KS eastward to MO. A
few inches of snow are possible beneath the upper low, which will
continue into day 2 but probabilities for at least 4 inches are
less than 10 percent.


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

A rather active eastern Pacific will trend toward in increasingly
split jet, sending waves of precipitation into the West Coast over
the next several days. Last of the freezing rain over parts of the
Cascades and Hood River Valley/Columbia Gorge will diminish today,
with snow levels roughly around 5000-6000ft. Next system will send
a weakening front into coastal areas late Wed with the parent low
well north. Best moisture plume will aim into NorCal, with
generally high snow levels of 6000-7000+ft for the
Shasta/Trinity/Siskiyou region and accumulations above 6 inches at
higher elevations.


...Northern Maine...
Days 2-3...

A northern stream front out of Quebec will dip down into northern
Maine, bringing in marginally cold air on Tuesday. The approaching
and slowly weakening upper low out of the Ohio Valley on Thursday
will favor a steady increase in moisture to the region, with
additional influence from coastal cyclogenesis near the
Mid-Atlantic. As this low lifts northeastward, combination of
height falls and broad lift via the LFQ of the upper jet will
promote light precipitation over Maine, with snow to the north and
a wintry mix over interior sections inland from the coast. A light
glaze of ice is likely with light snow to the north, where WPC
probabilities of at least 4 inches are low (generally around 10%).


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

--Winter storm starts to wind down today
The major winter storm will slowly start to ease over parts of the
northern/central Plains and Upper Midwest through the day. Areas
of blowing snow and freezing rain may impact travel through the
day.

--Additional snow for the Plains
Areas of moderate to heavy snow are expected this morning over the
western High Plains. Another 4-6" is likely with locally higher
amounts.

--Blowing snow conditions today
The combination of either falling snow or snow on the ground with
strong winds will lead to areas of blowing snow and near-blizzard
conditions this morning. Travel may 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 today across the
Dakotas and northern Minnesota. Additional ice accumulations
exceeding 0.10" are forecast in this area, with storm totals above
0.50†likely. Impacts have been scattered power outages and tree
damage, as well as dangerous travel due to icy roads and sidewalks.


Fracasso