Heavy Snow in Virginia, December 24, 2020

AREA FORECAST DISCUSSION
National Weather Service Morristown TN
258 PM EST Thu Dec 24 2020

.SHORT TERM (Tonight and Friday)...

Obs and webcams around the Cumberland Plateau and eastern KY are
showing that the change to snow is beginning. Radar shows precip is
blossoming, likely in response to increasing midlevel frontogenetic
forcing. Over the next few hours, cold advection will continue to
strengthen across the area and the changeover line will push east,
with all precip in our area expected to be snow by 00Z. Snow banding
will be possible as negative EPV values will coincide with the
strong frontogenesis zone over NE TN and SW VA from 23-02Z. A rumble
of thunder cannot be ruled out in that area, and snowfall rates near
or above 1 inch per hour will be possible. The dynamic forcing will
exit our area by midnight, but we will continue to have very steep
lapse rates under the mid/upper level closed low that will keep
scattered to numerous snow showers going through the night. As this
low slows progresses over the area, the chance of snow showers and
flurries will persist into Christmas Day for locations north of I-
40, although additional accumulations will be very light and mainly
confined to the mountains.

The trend of the most recent model runs has been to reduce snow
amounts slightly. This forecast package will follow that trend, with
just a slight reduction in snow totals for the TN Valley from the
previous forecast. Expecting 1-3 inch amounts in the Advisory area,
with 3-5 inches in the lower elevations of the warning area, and 5-
10 inches in the higher elevations. The Warning and Advisory areas
will not be changed. Travel conditions tonight continue to look very
poor as heavy snowfall rates this evening will reduce visibility,
and the very cold temperatures coming in tonight will create an icy
layer underneath snow-covered roads.

Wind chill values tonight will be dangerously low, in the range of -
5 to -15 F, and the Wind Chill Advisory will be continued.

DGS

.LONG TERM (Friday night through Thursday)...

Not a whole lot of excitement during the long term. Friday night will
be the coldest of the period with lows ranging from near single
digits to the mid-to-upper teens. With breezy winds still in place
due to a tight pressure gradient, wind chill values from the single
digits to lower teens can be expected. A few flurries may linger
Friday night across extreme northeast TN and portions of southwest
VA as W/NW upslope flow tries to squeeze out any remaining moisture.
Flurries will end overnight as the trough pulls out of the area to
our northeast. Higher pressure follows on Saturday with the exiting
trough. We should see highs rebound into the mid 30s to low 40s by
Saturday afternoon with mostly clear skies. Temperatures not as cold
Saturday night with lows in the lower to mid 20s.

Ridging further builds into the area on Sunday as heights amplify in
response to a shortwave swinging in from the northwest. Highs on
Sunday will climb into the upper 40s to low 50s with again plenty of
sun but clouds start to increase late. Late Sunday night our
shortwave moves in and slight chance POPs will be in place during
the overnight hours. QPF will be very light as the bulk of the
moisture will be well to our north. Areas that are cold enough will
see some light snows showers but not enough to make an impact. A few
light sprinkles possible on Monday as the shortwave moves through
the area. Moisture will be very limited so any precip that reaches
the ground will be trace amounts. Monday night into Wednesday
conditions improve. Tuesday and Wednesday feature gradual warming
temperatures as heights build again in response to a long wave
trough moving through the Great Plains. Wednesday will be the
warmest of the period with highs in the upper 40s to mid 50s. As of
now, it looks like most of the day on Wednesday will be dry but POPs
quickly ramp up during the evening hours with increasing chances
through Thursday. This event is still a ways out but another round
of moderate rain along with another mountain wave event look
probable. It remains to be seen whether we see any snow on the
backside as cold air rushes in. Stay tuned!

SR

&&

.AVIATION...
12Z TAF DISCUSSION.

Poor flight conditions expected at all sites today as a cold
front moves through the area with widespread rain changing to snow
during the afternoon hours. Conditions will continue to
deteriorate to IFR this morning at CHA and TYS with downslope flow
at TRI keeping VFR conditions through midday. By this afternoon,
IFR conditions are expected with moderate to heavy snow possible
at times for TYS and TRI. S/SSW winds remain strong and gusty this
morning before shifting to the WSW/W this afternoon behind the
cold front. Conditions gradually improve to MVFR at TYS and TRI,
and to VFR at CHA, late in the period.

JB

&&

.PRELIMINARY POINT TEMPS/POPS...
Chattanooga Airport, TN             22  31  19  46  27 /  20   0   0   0   0
Knoxville McGhee Tyson Airport, TN  21  26  17  42  26 /  50  20  10   0   0
Oak Ridge, TN                       21  27  17  42  25 /  50  20  10   0   0
Tri Cities Airport, TN              19  22  12  37  21 /  90  40  10   0   0

&&

.MRX WATCHES/WARNINGS/ADVISORIES...
NC...Winter Weather Advisory until 4 AM EST Friday for Cherokee-Clay.

TN...Winter Storm Warning until 4 AM EST Friday for Blount Smoky
     Mountains-Cocke Smoky Mountains-Hancock-Hawkins-Johnson-
     Northwest Carter-Northwest Greene-Sevier Smoky Mountains-
     Southeast Carter-Southeast Greene-Southeast Monroe-Sullivan-
     Unicoi-Washington.

     Wind Chill Advisory from 10 PM this evening to 4 AM EST Saturday
     for Blount Smoky Mountains-Cocke Smoky Mountains-Johnson-
     Sevier Smoky Mountains-Southeast Carter-Southeast Greene-
     Southeast Monroe-Unicoi.

     Winter Weather Advisory until 4 AM EST Friday for Claiborne-East
     Polk-Grainger-Hamblen-Jefferson-Knox-Loudon-North Sevier-
     Northwest Blount-Northwest Cocke-Northwest Monroe-Union-
     West Polk.

     Winter Weather Advisory until 4 AM EST /3 AM CST/ Friday for
     Anderson-Bledsoe-Campbell-Morgan-Roane-Scott.

VA...Winter Storm Warning until 4 AM EST Friday for Lee-Russell-Scott-
     Washington-Wise.

&&

Hazardous Weather Outlook...UPDATED
National Weather Service Morristown TN
314 PM EST Thu Dec 24 2020

NCZ060-061-TNZ012>018-035>047-067>074-081>087-098>102-VAZ001-002-
005-006-008-252015-
Cherokee-Clay-Scott-Campbell-Claiborne-Hancock-Hawkins-Sullivan-
Johnson-Morgan-Anderson-Union-Grainger-Hamblen-Northwest Cocke-
Cocke Smoky Mountains-Northwest Greene-Southeast Greene-
Washington-Unicoi-Northwest Carter-Southeast Carter-Roane-Loudon-
Knox-Jefferson-Northwest Blount-Blount Smoky Mountains-
North Sevier-Sevier Smoky Mountains-Sequatchie-Bledsoe-Rhea-Meigs-
McMinn-Northwest Monroe-Southeast Monroe-Marion-Hamilton-Bradley-
West Polk-East Polk-Lee-Wise-Russell-
314 PM EST Thu Dec 24 2020 /214 PM CST Thu Dec 24 2020/

This Hazardous Weather Outlook is for portions of southwest North
Carolina...east Tennessee and southwest Virginia.

.DAY ONE...This Afternoon and Tonight

Heavy snow is forecast this evening across northeast Tennessee,
southwest Virginia, and the mountains of East Tennessee. A Winter
Storm Warning is in effect for this area. In lower elevations of
northeast Tennessee and southwest Virginia, 3 to 5 inches of snow
are expected. In the moutains, 5 to 10 inches of snow will be
possible.

In the Cumberland Plateau and central Tennessee Valley, including
Knoxville, a Winter Weather Advisory is in effect, and 1-3 inches
of snow are foreast through tonight.

Travel conditions tonight will be very poor as heavy snowfall
rates this evening will reduce visibility, and the very cold
temperatures coming in tonight will create an icy layer underneath
snow-covered roads. Black ice will also be possible tonight across
the entire area.

The very cold airmass along with breezy west to northwest winds
Thursday night will produce dangerous wind chills from 5 below
zero to 15 below zero across the higher terrain where a Wind Chill
Advisory is in effect.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday

Scattered snow showers and flurries will continue into Friday,
with additional light accumulations only expected in the higher
elevations. Roads will continue to be icy into Friday.

Otherwise, the probability for widespread hazardous weather is
low.

.SPOTTER INFORMATION STATEMENT...

Spotter reports of snowfall amounts will be needed this afternoon
and tonight.

Special Weather Statement...UPDATED
National Weather Service Morristown TN
305 PM EST Thu Dec 24 2020

TNZ081-083>085-098>100-250300-
Sequatchie-Rhea-Meigs-McMinn-Marion-Hamilton-Bradley-
Including the cities of Cagle, Dunlap, Cartwright, Lone Oak,
Dayton, Evensville, Old Washington, Grandview, Spring City,
Big Spring, Athens, Clear Water, Dentville, Etowah,
South Pittsburg, Haletown (Guild), Jasper, Martin Springs,
Whitwell, Powells Crossroads, Monteagle, Chattanooga, Cleveland,
and Tasso
305 PM EST Thu Dec 24 2020 /205 PM CST Thu Dec 24 2020/

...Wintry and Turning Colder This Evening and Christmas Day...

Light snow accumulations are possible late through this evening
across the southern Cumberland Plateau and southern Tennessee
Valley. Most lower elevations will see a dusting to one half inch
of snow, with up to an inch across the highest elevations of the
plateau.

Colder air rushing in could allow for some black ice to develop.
Patches of black ice will be possible tonight and Christmas
morning so use caution if traveling on area roads.

Please listen to NOAA Weather Radio or your local media for the
latest updates on this situation.