006
FXUS64 KMRX 191536
AFDMRX

Area Forecast Discussion
National Weather Service Morristown TN
1036 AM EST Sun Jan 19 2025

...New UPDATE...

.UPDATE...
Issued at 958 AM EST Sun Jan 19 2025

Rain has changed to snow or rain/snow mix in most places where
precip is falling. Hi-res guidance has a light rain or rain/snow
mix continuing for the next few hours. By early afternoon, precip
should be mostly confined to a band of precip from Blount County
northward to the Tri-Cities area. This rain/snow mix will
transition to snow by early afternoon but temps are expected to
remain above freezing until mid or late afternoon in most of the
Valley. Until temps get below freezing accumulation will be
difficult in the Valley and most of the good lift and moisture
will be moved out by mid to late afternoon as a trough moves
through. However, snow showers are likely on the backside this
afternoon and there will be potential for accumulation from those
mainly in places where the Winter Weather Advisory is. Those
showers will be more hit and miss but one to two inches in spots
still looks reasonable. Forecast for mountain snow is still
on-track, with heaviest snow this afternoon and evening in the
higher elevations. Temps will drop below freezing mid to late
afternoon in most of the Valley. Outside of snow showers, flurries
will continue this afternoon possibly into the evening hours.

&&

.SHORT TERM...
(Today and tonight)
Issued at 420 AM EST Sun Jan 19 2025

Key Messages:

1. Widespread rain/snow showers start around daybreak then change to
all snow by mid/late morning.

2. Totals will be very light in the low elevation areas, but a
respectable event appears likely in the TN mountains.

3. Very cold temperatures follow the snow tonight, with wind chill
values dropping into the 15-20 below zero range for higher
elevations, and even 5-10 below elsewhere.

Discussion:

Upper shortwave that will drive snowfall across our area later this
morning can be seen moving through the Arklatex region on water
vapor imagery at the moment. Downstream, some banded areas of light
precip are already developing over western TN/KY in response.
Guidance continues to advertise that the 12z-15z timeframe will see
precip begin in earnest across the CWA, with late morning to early
afternoon being the most intense period of synoptically driven
precip before we transition to a northwest flow this afternoon
through the overnight hours.

Precip types: The surface cold front is well to our southeast this
morning, with mid northerly flow already in place in our CWA and
some obs sites already falling into the 30s in VA and our northern
plateau counties. Those are the exceptions currently though, with
most places in the low 40s. Still think that these general temp
trends hold through daybreak and the onset of precip. Model
soundings show freezing levels falling rapidly today though, so a
rain/snow mix appears likely at the beginning, but believe we
transition to mostly snow by late morning as strong cold air
advection begins.

Snowfall totals: This is the lowest confidence aspect of the
forecast. For the valley and elevations below roughly 1500 feet, I
believe snow accumulations will be limited, with the best chances
restricted to areas well north of Knoxville most likely. Recent
warmth and rain, the snowfall occurring during the daytime hours,
and a general lack of heavy precipitation rates should all work
against the possibility of any significant accumulations. At higher
elevations however, especially say 2,500 to 3,000 ft or more, the
transition to a northwest flow regime will coincide with
temperatures aloft plummeting. Ordinarily we lose saturation into
the DGZ during these types of events, but the H85 temps fall
into/beyond the DGZ range while that general part of the atmosphere
remains saturated and resulting snow ratios increase. As such,
totals in the mountains, especially the Smokies, have been adjusted
upwards a notable amount. While not as stark, amounts were increased
in the higher terrain of our southwest VA counties too. Believe
there`s support for several places in the east TN mountains seeing 8-
10" out of this event, with the potential for over a foot in the
Smokies. After these adjustments, upgraded portions of our Virginia
counties to a winter storm warning to account for the upward
adjustments in high elevation snowfall totals there.
Otherwise no changes to the headlines were made.

Extreme cold: Temperatures tonight are expected to drop into the low
teens to single digits, possibly negative single digits at
elevation. This will coincide with elevated or gusty winds during
the overnight hours and result in very low wind chill values.
There`s some uncertainty in how low these values drop. But
confidence was high enough to convert portions of the extreme cold
watch, to an extreme cold warning. Did that for all but Scott county
in our Virginia counties, and then ran the warning down our TN
mountain zones to Monroe county. Elsewhere, I converted to a cold
advisory.
There`s an argument to be made that parts of the Cumberland plateau
need to be converted to a warning, but I wasn`t confident enough in
how low wind chills will get there to go with a warning over an
advisory. Will let the day shift take another look at things and
convert if necessary.

&&

.LONG TERM...
(Monday through Saturday)
Issued at 420 AM EST Sun Jan 19 2025

Key Messages:

1. VERY COLD through Thursday morning. Coldest temperatures of the
season. The Extreme Cold Watch has either been upgraded to a warning
or Cold Weather Advisory.

2. Slight chance of a quick light snow for far eastern parts of the
area Tuesday into Wednesday, but still uncertain.

3. Additional storm systems Friday and possibly near Sunday may
bring the next chance of precipitation to the area, but models
aren`t too in line at this time.

4. Temperatures expected to rebound and trend warmer Thursday and
into the weekend.

Discussion:

The period begins Monday morning where a strong low will be centered
over the Canadian Maritimes and NW flow induced precipitation should
be ended. Longwave troughing will continue sinking south, as well as
surface high pressure strengthening as it moves in from the west.
With these features in mind, the tightened MSLP gradient across the
higher terrain will bring some gusty winds, which is why we are
expecting the coldest temperatures AND wind chills Monday morning.
Many higher elevation spots will have wind chill values in the teens
to 20s below zero Fahrenheit. Valley locations can expect wind
chills in the single digits to a couple of degrees below 0. Extreme
Cold Watches are expected to be upgraded to Warnings in these areas.
The remainder of the forecast area will go from the aforementioned
watch to a Cold Weather Advisory. One caveat to keep in mind if
places will reach criteria is, cloud cover and how quickly it can
exit during the coldest hours of the morning Monday.

850 temps will range from the teens below to near 20 below C. Expect
very cold morning temperatures through Thursday with values in the
low teens, single digits, or even some readings below 0. Tuesday
morning will be a touch warmer than Monday and Wednesday due to
increasing cloud cover. It will still be cold, however! Wednesday
may be similar to Monday, except lighter winds. Skies will be much
clearer, allowing radiational cooling to be at it`s max capability.
Highs will surely be coldest on Monday with the entire CWA possibly
not seeing any readings above freezing. Highs in the mountains may
not even reach 20 or greater. Single digit highs very probable as
well over the highest terrain. Just to compare with what we usually
experience around this time in Knoxville for example, is 48 for a
high and 30 for a low. Nearly 20+ deg below normal. Make sure you`re
taking the extra precautions necessary to keep your home, pets,
plants, the vulnerable population, and more safe during this short
stretch. We do recover from this around Thursday during the day when
we see much of the forecast area above freezing again. A brief
respite of deep troughing will allow some warming along with
southerly return flow. By the weekend we will be in the 40s to near
50s area-wide.

The next systems to monitor will be around the Tuesday to Wednesday
timeframe and the weekend. The Arctic high and its presence will
determine how much precipitation will come of the mid-week system.
The farther west and north in the CWA, the lesser your chances of
seeing anything at all. Anything that falls though will be snow
given the temperatures we will be at. So far, it only looks to be a
couple tenths of an inch of snow along far eastern parts of the CWA.
Brief zonal flow and the next digging trough will set the scene for
next weekend when we`re expected to recover from the very cold
temperatures. A frontal system diving in from middle Canada may
bring some mountain snow showers around Friday, but moisture looks
pretty limited. Saturday looks to dry out and then attention turns
again at the every end of the forecast period on Sunday, where a
system with possibly more moisture might impact the region. Of
course given how far out in time this is, cannot say for certain
when and what to expect just yet.

&&

.AVIATION...
(12Z TAFS)
Issued at 645 AM EST Sun Jan 19 2025

SHRA are increasing in coverage across middle and eastern TN
early this morning. Expect light SN to mix in by 13-14z, including
at KTYS and KTRI, with a change to all SN by 16z or so. Showers
wrap up around 20z for dry conditions the remainder of the TAF
period. Believe KCHA stays south of the precip today. Otherwise,
CIGS and VSBY will remain low, likely around the MVFR/IFR
transition through this afternoon then gradually come up to
VFR/MVFR levels. Expect gusty winds to around 25kt at all sites
this afternoon and evening.

&&

.PRELIMINARY POINT TEMPS/POPS...
Chattanooga Airport, TN             38  12  28  18 /  20   0   0   0
Knoxville McGhee Tyson Airport, TN  36  11  24  14 /  80   0   0   0
Oak Ridge, TN                       35  10  25  14 /  70   0   0   0
Tri Cities Airport, TN              37   9  21  11 /  70  10   0   0

&&

.MRX WATCHES/WARNINGS/ADVISORIES...
NC...Extreme Cold Warning from midnight tonight to 2 PM EST Monday
     for Cherokee-Clay.

TN...Extreme Cold Warning from midnight tonight to 2 PM EST Monday
     for Blount Smoky Mountains-Cocke Smoky Mountains-East Polk-
     Johnson-Sevier Smoky Mountains-Southeast Carter-Southeast
     Greene-Southeast Monroe-Unicoi.

     Winter Weather Advisory until 10 PM EST this evening for
     Campbell-Claiborne-Grainger-Hamblen-Hancock-Hawkins-
     Jefferson-Morgan-Northwest Carter-Northwest Cocke-Northwest
     Greene-Scott TN-Sullivan-Union-Washington TN.

     Winter Storm Warning until 7 AM EST Monday for Blount Smoky
     Mountains-Cocke Smoky Mountains-Johnson-Sevier Smoky
     Mountains-Southeast Carter-Southeast Greene-Southeast
     Monroe-Unicoi.

VA...Winter Weather Advisory until 10 PM EST this evening for Scott
     VA.

     Winter Storm Warning until 7 AM EST Monday for Lee-Russell-
     Washington VA-Wise.

     Extreme Cold Warning from 1 AM to 1 PM EST Monday for Lee-
     Russell-Scott VA-Washington VA-Wise.

&&

$$

SHORT TERM...CD
LONG TERM....KS
AVIATION...CD