Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
409 PM EST Tue Feb 09 2021

Valid 00Z Wed Feb 10 2021 - 00Z Sat Feb 13 2021

...Central Appalachians and Mid-Atlantic...
Days 2-3...

Two waves of wintry precipitation are expected to produce a
multitude of precip types through the forecast period.

The first is associated with a modest shortwave ejecting through
confluent mid-level flow and moving atop the Mid-Atlantic
Wednesday night into Thursday. This feature will be accompanied by
modest PVA and upper diffluence within the RRQ of a departing jet
streak over New England. More impressively, WAA and isentropic
ascent will cause precipitation to expand northeastward while the
weak wave moves east through Thursday morning. Within the cold
sector of this system, a period of moderate snow is likely, with
the heaviest accumulations expected in the upslope W/SW terrain of
WV and western MD into the Laurel Highlands of PA. WPC
probabilities for 4 inches are as high as 50% in these areas, with
lesser probabilities extending to near I-95 in MD and VA. South of
there, some light freezing rain is likely as the WAA drives a
modest warm nose atop the cold surface wedge, with some light
sleet in between the two zones. WPC probabilities for 0.1" of
freezing rain on D2 are generally 10-30%, and highest in the VA
foothills.

After a respite much of Thursday, a secondary wave will lift
northeast, this one with an associated surface low moving off the
NC coast and away to the northeast. This will produce another
round of WAA precipitation, with snow north and sleet/freezing
rain to the south. The guidance today has trended south with this
feature, likely due to a stronger response of the high pressure
wedged down from the north. While there still exists a lot of
spread in the model solutions, it appears the heaviest snowfall
with this second wave will occur in a west to east oriented band
from the Panhandle of WV eastward towards the DE coast where WPC
probabilities range from 30-60% for 4 inches, highest in the
terrain of WV. This second wave is likely to have more significant
precipitation with it as the WAA and isentropic ascent are
accompanied by stronger synoptic lift as a divergence maxima moves
overhead. There could locally be snowfall amounts much higher than
4 inches, but confidence in placement of these is too low at this
timescale to mention, especially with uncertainty in how the
system will track over the next few days. Like the first system,
there is also likely to be a narrow corridor of moderate freezing
rain where the warm nose tops the cold surface wedge and sub
freezing low-level temperatures. WPC probabilities for 0.1" of
freezing rain on D3 have increased to 30-50%, highest in far
northern NC into central VA.


...Southern Plains, Mississippi Valley, Ohio Valley...
Days 1-3...

...Damaging freezing rain event likely Wednesday and Thursday...

A shortwave moving slowly south of the Four Corners will shed
pieces of energy northeastward and into confluent moist flow
through the Southern Plains and into the Mississippi and Ohio
Valleys Wednesday through Friday. At the same time, an expansive
jet streak centered over the Great Lakes will leave a persistent
RRQ atop the region, and this favorable diffluence will interact
with a low-level baroclinic zone and the mid-level impulses to
spawn waves of low pressure moving northeast along the stalled
surface front. Ahead of the main trough axis and downwind of the
low pressure waves, low-level WAA will intensify, with isentropic
lift on the 290K-300K surfaces increasing atop an arctic high
pressure. The combination of deep synoptic ascent atop this
increasing isentropic upglide will lead to expanding
precipitation, with all p-types expected. For this area, the most
significant impactful wintry precip is likely to be freezing rain
as a warm nose lifts northward atop still very cold surface
temperatures. While the guidance has continued to trend southward
with its axis of freezing rain, due to a stronger ridge to the
north counteracting the warm nose, there is still likely to be a
corridor of heavy freezing rain. Forecast soundings for parts of
AR, TN, and KY indicate warm nose intensity and depth nearing the
90th percentile for long duration freezing rain events, and the
WSE means have shown an increase in point max freezing rain
accretion. While some runoff is expected in the heavier rates,
there is a high probability for 0.25" of accretion from near
Little Rock, AR northeastward to Jackson, KY. It is likely that
some areas will receive more than 0.5", creating a damaging icing
event for parts of the area. Surrounding this axis of heavy
freezing rain, light accretions are likely, including far western
WV, northern TN, and back into OK/TX where locally backed flow
will lead to better isentropic ascent around a secondary wave of
low pressure and prolonged light freezing rain or freezing drizzle
is likely.

North of this axis, some moderate to heavy sleet and light to
moderate snow is expected where the warm nose is either weaker or
non-existent. Where the most sleet accumulates, snowfall amounts
will be tempered, but there are some modest WPC probabilities for
2" of snow/sleet, highest in northern KY. By Thursday night the
best forcing expands eastward and moisture gets shunted to the
south, bringing an end to this prolonged and damaging freezing
rain event.


...Western United States...
Days 1-3...

Lingering snowfall across the central CO Rockies is expected on D1
as residual moist mid-level flow and a weak shortwave ejecting to
the east combine to drive ascent. WPC probabilities on D1 for 6
inches are moderate in some of the higher peaks.

Much more significant snowfall is likely to spread across the
Pacific Northwest and then down through much of the West beginning
late D2 and especially on D3. As broad cyclonic flow envelops much
of the CONUS, confluent mid-level NW flow will drive moisture
southeastward from the Pacific, while an amplified shortwave drops
from British Columbia to the OR/CA coast Friday morning. This
feature will be accompanied by an upper level divergence maxima in
the LFQ of a sinking Pacific jet streak to drive ascent, while a
surface reflection wave of low pressure advects towards the OR
coast on D3. The guidance has become slightly more aggressive with
moisture, and subtly further north with the low track today, and
while it is a near certainty that heavy snow will spread into the
terrain of the Sierra, as well as the ranges from the Olympics of
WA southeast as far as the CO Rockies, there is also potential for
lowland snow and ice with this event as snow levels crash. WPC
probabilities in the mountains are high for 8 inches, with more
than 12" likely in the more favorable terrain of the Sierra,
Cascades, and down into the CO Rockies.

With the northern push of the low today, moisture is more likely
to spread into the Portland and Seattle areas, with snowfall
likely even into the cities. The precipitation may start as some
freezing rain/sleet as warm advection tops cold surface
temperatures as a wedge of high pressure builds east of the
Cascades and funnels through the passes, but should change to all
snow outside of the OR Coast where WPC probabilities for freezing
rain are as high as 30% for 0.1". For the lowlands south of
Seattle, WPC probabilities have increased for 4 inches and now
show a moderate risk for this accumulation, with higher amounts
likely in the Columbia Gorge. The biggest uncertainty involves the
cities themselves. While Seattle should be cold enough for all
snow, they may be on the edge of the heaviest precip, although WSE
plumes continue to show an increase in snowfall and it is likely
that more than 2" accumulates in Seattle. For Portland, the
snowfall is less certain as some mixing is likely early and the
city may try to warm above freezing with the further north low
track, but probabilities are high for 1" there as well.


...Great Lakes...
Day 1...

A strengthening area of low pressure moving away from New England
this evening will leave robust CAA in its wake, embedded within
broad troughing across the eastern CONUS. The air mass is
exceedingly cold such that the favorable DGZ is at or below the
surface, leading to inefficient ice crystal growth. However,
enough latent heat off the Lakes and modest lapse rates should
provide enough instability for small regions of LES in the
Keweenaw Peninsula and downwind of Lake Ontario. WPC probabilities
are high for 4 inches in these narrow corridors.


Weiss