Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
353 PM EST Wed Dec 16 2020

Valid 00Z Thu Dec 17 2020 - 00Z Sun Dec 20 2020

...Major Winter Storm Ramping Up Now; Will Affect the Mid Atlantic
and Northeast through Thursday...

...Mid-Atlantic and Northeast...
(Day 1; 00Z Thursday to 00Z Friday)

A major winter storm is ramping up this afternoon across the
central Appalachians and Mid Atlantic, and it will quickly push
through much of the rest of the Northeast region over the next 24
hours. Some context is important. This storm is expected to
deliver more snow than was observed all of last season (Winter
2019-2020) at some locations, especially in Pennsylvania. And some
locations are currently forecast to approach or exceed December
record two-day snowfalls. Exact records will depend on where the
heaviest snow bands set up, but the potential for at least some
monthly records is realistic.

Overall, models have continued to converge with both temperature
profiles and precipitation, which leads to growing confidence in
most areas. The greatest differences continued to exist along the
peripheries in two key areas: (1) a mixed precipitation zone with
an advancing warm nose in the DCA-NYC I-95 corridor and along the
southern New England coast; and (2) northward extent of a band of
heavy snowfall associated with mid-level frontogenesis.

Along the southern edge, a multi-model consensus with hi-res model
contribution was favored for the thermal profiles and placement of
precipitation type transitions. Area radars, such as KLWX and
KDIX, and surface observations such as mPING, are already showing
the snow transition being pushed northwest roughly in line with
the 12Z HREF dominant precipitation type. This lends credence to
the forecast blending approach. Furthermore, snow ratios could be
affected by a larger region of significant riming to the northwest
of the snow transition. Recent runs of the NAM Nest have been
consistent in showing at least moderate rime factors penetrating
as far as 150 miles inland. This may be too aggressive, and the
NAM has been on the warmer and north side of guidance for several
days. Nevertheless, the general concept of reduced snow ratios in
an area closer to the precip type transition seems sound, and this
was applied to the forecast. It's further supported by the 18Z IAD
sounding which was taken while snow was still falling, and showed
a relatively deep (950-800mb) nearly isothermal layer around -1C.

Meanwhile, confidence is growing in an enhanced band of heavy snow
from central to northeast Pennsylvania, through south-central New
York near the southern Catskills, and perhaps into the Berkshires
and southern Greens in western Massachusetts and southern Vermont.
There is a strong model signal for a pivoting ribbon of
frontogenesis and strong vertical motion in the dendritic growth
zone in that area, and it is also likely to be situated just north
of the most significant riming shown by the hi-res models. WPC
PWPF probabilities for 12 inches of snow are 80-95 percent in
those areas. The highest probabilities are situated in the Poconos
and Catskills regions of NE PA and SC NY with a 60-75 percent
chance of 18 inches in that more concentrated area. Applying the
underlying ensemble data to snow amount thresholds for the Winter
Storm Severity Index suggests a very high likelihood (70+ percent)
of reaching the major category in the aforementioned swath from C
PA into W MA. It should be noted that slight shifts in the
position of the band could redistribute the impacts, but
confidence in the presence of such a band of heavy snow is
relatively high.

Although some sleet and freezing rain will be possible, much of
the accumulation potential with those precipitation types was
concentrated prior to 00Z. Therefore, the probability of receiving
at least 0.1 inches of ice accumulation is less than 10 percent.


...Western States...
(Days 1-3; 00Z Thursday to 00Z Sunday)

An active pattern remains with the northwest CONUS with the first
wave arriving on the coast late tonight into Thursday morning, and
another wave reaching the Pacific Northwest on Friday. Ample
Pacific moisture will produce heavy snow for higher elevations of
much of the central and northern portions of the Western U.S. over
the next few days. Taking the next three days in total,
probabilities of 1+ foot of snow is over 70 percent in much of the
Washington Cascades, with many other ranges likely to see at least
6-8 inches of snow in the same time frame.


Lamers