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

Valid 00Z Wed Dec 27 2023 - 00Z Sat Dec 30 2023

...Northern and Central Plains/Upper Midwest/Mid Mississippi
Valley/Central Appalachians...
Days 1-3...
As the vertically-stacked low that is currently centered near the
eastern Kansas-Nebraska border gradually fills and drifts east,
the ongoing large-scale winter weather impacts will continue to
gradually ease.  As indicated by the WPC PWPF, the threat for
additional widespread heavy snow accumulations is ending, with
probabilities for accumulations of 4 inches or more less than 10
percent for most areas for all three days.  Ongoing strong winds
over the High Plains should begin to diminish early in the period
as well as the tight pressure gradient west of the low begins to
slacken.  However, freezing rain with accumulating ice will remain
a concern into this evening across portions of the Dakotas into
northern Minnesota, with WPC PWPF showing a narrow axis of
moderate to high probabilities for additional ice accumulations of
0.10 inch or more from north-central South Dakota to east-central
North Dakota.  

Although the large-scale threats are waning, there remains the
potential for localized impacts, especially near the low as it
drifts from the central Plains through the mid Mississippi Valley.
 Mid level energy and reinforcing cold air moving south of the
center may support periods of brief but intense precipitation,
with some of the deterministic guidance depicting strong ascent
within the DGZ.  Both the NAM and GFS snow squall parameters
indicate a modest threat for snow squalls, peaking during the
afternoon hours across southern Missouri.

Light snow accumulations are forecast to follow the low as it
moves slowly through the mid Mississippi and into the lower Ohio
Valley by late week.  Some models show mid-level energy wrapping
around the north side of the low from the Great Lakes, along with
low-to-mid level frontogenesis, supporting a swath of heavier
amounts forming on the northwest side of a redeveloping surface
low.  The NAM is one of the leading examples, but is also an
outlier as evidenced by the low probabilities for accumulating
snow.  The WPC PWPF shows modestly higher probabilities farther
east over the central Appalachians where precipitation is expected
to develop as snow or quickly changeover to snow over the high
elevations on Friday.  Slight probabilities for accumulations of 4
inches or more are shown by the PWPF across portions of the West
Virginia Allegheny Mountains.

...Pacific Northwest and Northern California...
Days 1-2...
Shortwave energy ejecting out ahead of an upper low is forecast to
push a weakening frontal boundary near the coast as the parent low
lifts to the north.  The heaviest precipitation is expected to
center across Northern California on Wednesday, where snow level
above 6000 ft will limit the threat for widespread heavy
accumulations.  Backing flow ahead of an upper trough amplifying
offshore will further limit the potential for widespread heavy
snow accumulations Thursday and Friday.   

...Northern New England...
Days 2-3...
A northern stream front out of Quebec will dip down into northern
Maine, bringing in marginally colder air later today. 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, the combination
of height falls and broad lift via upper jet dynamics will promote
light precipitation over Maine, with snow to the north and a
wintry mix over interior sections inland from the coast on
Thursday.  A second northern stream wave amplifying over Quebec
may help to push the colder air farther south behind a departing
coastal system, spreading frozen precipitation across more of
northern New England.  Overall, the WPC PWPF suggests light
accumulations for both snow and ice.

Pereira