Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
349 PM EST Thu Dec 20 2018

Valid 00Z Fri Dec 21 2018 - 00Z Mon Dec 24 2018

...Southern and Central Appalachians...

An anomalously deep cyclone will move up the eastern periphery of
the Appalachians on Friday and Saturday. The low track further
inland on the East Coast should lead to widespread rain in most
areas east of the mountains, with snow potential largely confined
to the post-cyclone cold air advection regime in the Appalachians.
Therefore, the expectation is that heavy snow potential will be
tied closely to favored upslope regions in the Appalachians for
northwesterly flow. Areas outside the immediate terrain gradients
and ridges should see less potential for accumulation due to a
combination of initially warm temperatures, NVA aloft following
the passage of the occluding cyclone, and some drying in the
mid-levels which may impair ice crystal growth in areas without
focused ascent (as with upslope regions).

Given the strength of the cyclone, the concern would be that
global models with a greater horizontal grid spacing may
underestimate the focused orographic ascent and thus snowfall
potential. Model forecast MSLP values along the low track are more
than 4 standard deviations below normal in the central and
southern Appalachians, and December record low MSLP values could
be challenged in the vicinity of western NC and VA. The large and
strong cyclone will have a strong wind field, and the 850mb wind
speeds in the northwesterly flow behind the cyclone should be
above the 90th percentile for December as well. Therefore,
orographic ascent may be much stronger than is typical for
northwesterly flow snow events in the Appalachians. The 3km NAM
Nest shows considerably more snowfall than model consensus and
global models generally, and this suggests that snowfall amounts
above the 75th-90th percentile may be more achievable in the
highest elevations of the Appalachians. Forecast snow amounts were
trended up slightly to account for this concern, and there is now
a High Risk (greater than 70 percent chance) of 4+ inch snow
amounts in the Allegheny Mountains of WV and southern PA, as well
as the highest peaks of the Blue Ridge Mountains along the NC-TN
border.

...Utah, Colorado and Southern Wyoming...

A progressive +PV anomaly will quickly move across the UT-CO
region in a period of approximately 12 hours (Saturday 03-15Z).
Nevertheless, a relatively strong frontogenetic circulation is
expected just ahead of the advancing wave, and the circulation
should extend into the dendritic growth zone. Therefore,
individual locations within the region should experience a 3-6
hour period where strong vertical motion coincides with a
saturated DGZ, and thus snow rates could be rather heavy.
Furthermore, models show a broad plume of -EPV extending into the
mountains of UT-CO, and vertical cross sections show the potential
for some layers of CSI/CI intersecting the same ice crystal growth
layers. The presence of instability aloft could lead to some
banded snow and focused areas of very heavy snow rates.
Accordingly, snowfall amounts were bumped up across the region --
both due to the potential for heavy snow rates and an increase in
model QPF as well. There is a Moderate to High Risk (at least 40
percent chance) of 4+ inch snow amounts in most of the higher
elevation areas from the Wasatch and Uinta Mountains in UT, to a
variety of ranges in northern Colorado. Although amounts are
expected to be less in the lower elevation areas, the WPC winter
storm ensemble distribution has shifted decidedly toward higher
amounts in these areas as well, including the Wasatch Front.

...Northwest, including the Cascades...

An ongoing wave of precipitation across the Northwest will be
waning over the next 12 hours, with more significant mid-upper
level height rises spreading across the region on Friday. This
will lead to a continuation of some heavy snow in the higher
terrain across the region early in the Day 1 period. Snow levels
should fall below 4000 feet in the Cascades in this time period,
and below 5000 feet in the interior Northwest. A lull in the
precipitation is expected for the Day 2 period, with another
significant shortwave approaching by Saturday Night. Lingering
cold air over the region should keep snow levels quite low east of
the Cascades on Saturday Night, generally below 3000 feet, before
warm air advection gradually raises snow levels on Sunday. This
should lead to significant snow in the WA-OR Cascades, with
accumulating snow also affecting a broader area of eastern WA-OR
and into ID.


The probability for significant icing (0.25 inches or more) is
less than 10 percent.

Lamers