Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
251 PM EST Thu Dec 14 2023

Valid 00Z Fri Dec 15 2023 - 00Z Mon Dec 18 2023


...Southern Rockies into the High Plains...
Day 1...

The trough moving across the Southern Rockies today will eject
into the Southern Plains tonight into Friday, with some
re-amplification into a closed low likely over KS/OK. As this
advects eastward, most of the available moisture will shift east
as well noted by the greatest PW anomalies exceeding +2 sigma on
the NAEFS tables surging into the Central Plains. This combined
with the best forcing moving away will likely bring a slow wane to
precipitation from west to east, with snowfall reducing as well.
The exception tonight into Friday may still be a deformation axis
progged to develop across western KS and into the OK/TX Panhandles
as the upper low amplifies. This region will experience an overlap
of deformation, 850-700mb fgen, and subtle LFQ upper diffluence to
drive ascent which could manifest as a pivoting band of moderate
to heavy snowfall through late Friday morning. The column remains
thermally marginal for heavy snow, so p-type will be dependent on
precipitation rate, but the latest WPC prototype snow band tool
indicates a high likelihood for brief 1-2"/hr rates drifting
eastward across this region. This could result in additional
snowfall accumulations, but WPC probabilities for more than 2
inches are less than 5%, and will be highly dependent on whether
this secondary band can develop.


...Northern Plains and Upper Midwest...
Days 1-2...

Complex northern and southern stream interaction could result in a
band of heavy snowfall across portions of the Northern Plains and
into the Upper Midwest Friday evening through Saturday. The
northern stream shortwave digging out of Saskatchewan Friday will
amplify into a closed low over the Dakotas, and then interact with
a southern stream impulse lifting out of the southern Plains.
There remains considerable spread as to exactly how this will
manifest as these impulses phase D2, but what is more certain is
that impressive moisture (PW anomalies reaching +4 sigma according
to NAEFS) will merge into the northern stream westerlies resulting
in an axis of impressive moisture convergence draped west to east
across the region. This moisture will begin to surge cyclonically
around a developing low pressure along a warm front in ND/MN
Friday night on impressive 285K-290K isentropic upglide. At the
same time, a zonally oriented jet streak centered over northern
Quebec will leave at least its peripheral RRQ in the vicinity,
resulting in additional synoptic lift and possibly enhancing the
low-level fgen in response. Regional forecast soundings suggest a
marginal thermal structure for snowfall, but with increasing
ascent and temperatures below freezing almost to the surface, any
enhanced lift could drive precipitation rains intense enough to
dynamically cool the column to produce snow. The best ascent is
currently progged to remain below the DGZ, so snow growth may not
be ideal, but with anomalous moisture in place, where any banding
can occur, this could result in modest snowfall accumulations of
an inch or two, with locally higher snowfall possible along the
Arrowhead where additional moisture from Lake Superior combined
with some upslope into the Iron Ranges drives WPC probabilities
for 4+ inches to 5-10%. There may also be some light freezing rain
accretions exceeding 0.01", primarily across ND where WPC
probabilities are above 30% due to a lack of ice in the DGZ with
sub-freezing but saturated low-levels of the column.

Weiss