Johnson City, Tennessee 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

368
FXUS64 KMRX 180930
AFDMRX

Area Forecast Discussion
National Weather Service Morristown TN
430 AM EST Sat Jan 18 2025

...New SHORT TERM, LONG TERM...

.SHORT TERM...
(Today and tonight)
Issued at 357 AM EST Sat Jan 18 2025

Key Messages:

1. Widespread showers this morning become increasingly isolated
this afternoon. Still a dreary day, despite rainfall mostly ending
after midday.

2. Strong cold front moves through tonight. Cold air strong enough
to change rain to at least a wintry mix in the plateau and parts
of our Virginia counties before daybreak tomorrow.

Discussion:

Overnight, winds were unexpectedly strong in the valley along the
leading edge of the precipitation shield moving in. Gusts to
around 40 mph were seen at the Knoxville airport, and some of our
typical mountain wave sites like Camp Creek and Cove Mountain
gusted to near 50 and 60 mph respectively. These winds are on
their way down so no headlines were issued.

For today, we`ll remain situated between a southern stream jet
streak across Alabama and Georgia, and an approaching northern
stream system to our northwest. Widespread rainfall ongoing across
the CWA is largely due to a wave passing by in the southern stream
jet, with upper support expected to wane from midday through the
afternoon and evening hours. As such, expect shower coverage to
begin to dwindle by the late morning hours, with rainfall becoming
increasingly isolated in nature this afternoon. Given we`re not
completely void of upper support, and the low levels will remain
saturated, wouldn`t be surprised to see areas of drizzle be
commonplace this afternoon and evening. Temperatures, even in the
higher elevations, seem less than supportive of snowfall this
morning and low/mid level temperatures are forecast to increase
through the day so I don`t think snow accumulations are anything
to worry with during the day today.

Tonight into Sunday, a strong cold front driven by the northern
stream jet will move through the forecast area, clearing the
Appalachians after midnight tonight. Strong cold air advection
likely doesn`t occur until after daybreak tomorrow, which will
largely be a concern for the long term period. However,
temperatures at elevation will begin to cool off fairly rapidly
tonight. With increasing jet support from the interaction of the
northern and southern branch jets across the southeast CONUS,
precipitation will develop in the wake of the front, leading to
at least a rain/snow mix after midnight tonight if not outright
snow. To that end, have some light snow accumulations, generally
less than half an inch, in the plateau and up into Lee and Wise
counties in Virginia tonight. Any appreciable snowfall holds off
until after daybreak tomorrow.

&&

.LONG TERM...
(Sunday through Friday)
Issued at 357 AM EST Sat Jan 18 2025

Key Messages:

1. A system moving through Saturday night through Monday morning,
will bring northwest flow snowfall to the area. Light accumulation
possible for the middle to upper Tennessee Valley. Highest amounts
will be confined to the Southern Appalachians and southwest Virginia
where either a Winter Weather Advisory or Winter Storm Watch has
been issued.

2. VERY COLD Sunday through Thursday morning. Coldest temperatures
of the season for many. Monday morning through Wednesday morning
will be the coldest with lows ranging in the teens for most of the
TN Valley to single digits elsewhere. Wind chill could make it feel
as cold as 15 below zero across the highest terrain.

3. Trend warmer end of the week with great uncertainty over
Tues/Wed system and the weekend.

Discussion:

The long term period begins just about when Arctic-like air will
fill in behind a cold frontal passage. A low pressure center will be
over the Carolinas Sunday morning with wrap around flow from the
Virginias into the TN valley region. Jet dynamics as well as
northwest low-level flow, will increase over the region later in the
day Sunday that will keep precipitation going over the favored
northwest flow terrain. Increasing NW winds will create a downslope
effect for the valley, so precipitation amounts will lighten or
become flurries. The low is expected to exit off of the Mid-Atlantic
and deepen as it heads for the Canadian Maritimes Sunday into
Monday. Wrap-around and thus, northwest flow, will continue across
the higher terrain early Monday morning. With that effect, heavy
snowfall amounts can be expected for the highest terrain in
southwest Virginia and the Appalachians bordering North Carolina. A
Winter Weather Advisory is now in effect for southwest Virginia
except Scott County, and a Winter Storm Watch from Johnson down to
Monroe County in Tennessee. LREF probs give Knoxville a 55% chance
of exceeding a half inch of snowfall through Monday morning.
Probabilities increase as you head north and obviously over the
mountains. Tri-Cities now shows a 80-90% chance of exceeding one
half inch of accumulation. Valley locations will be highly dependent
on how much moisture can make it into the valley. A forecast
sounding from the NAM near the Smokies Sunday does show low-level
moisture up to about 800 mb or so, below zero C through the column,
and NW winds in the 25 to 30 KT range.

To keep the subject on precipitation, guidance becomes all over the
place with the next two possible systems Tuesday into Wednesday and
next weekend. The Arctic high 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. Brief zonal
flow and the next digging trough will set the scene for next weekend
when we`re supposed to recover from the very cold temperatures.
Models just aren`t sure if it will be the southern stream system
that will impact the area or one dropping in from the north.

Now the cold we`ll finish with. This is expected to be the most
impactful CWA wide of the long term, as not everywhere will see
snow. Longwave troughing will set up over the area as we head into
Sunday. Heights will drop near 20 dam when comparing the current
time with later Sunday. 850 temps will range from the teens below to
low 20`s below C. Expect very cold morning temperatures Monday
through Wednesday with values in the low teens, single digits, or
even some readings below 0. Any sort of wind will make it feel much
colder, which Monday morning appears will have strongest winds
overnight as strong high pressure races in behind a deepening low
off of New England. Expect wind chills to be single digits or colder
throughout the CWA, with possibly reaching dangerous values of teens
below zero. 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 on the 20th 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. Though we will still remain below normal towards the
end of the week, temperatures do trend warmer Thursday onward, with
highs approaching 40s again to round out the long term period.

&&

.AVIATION...
(06Z TAFS)
Issued at 1248 AM EST Sat Jan 18 2025

First wave of rainfall has already arrived at KCHA, but is having
difficulties moving further north due to entrenched dry air so
have delayed onset of SHRA at KTYS and KTRI a tad. Guidance is
universally insistent on bringing flight categories down to MVFR
and even IFR at all terminals later today. Hard to go against that
kind of consistency, but I`m also not seeing those CIGS anywhere
upstream where guidance says they should exist now. As such, kept
the trend but delayed the onset of those categories by a bit.
Lastly, showers should persist for a 6-9hr window. Afterwards the
low levels will remain saturated so some -DZ could occur, but will
leave that out of the TAFs for now.

&&

.PRELIMINARY POINT TEMPS/POPS...
Chattanooga Airport, TN             54  34  39  13 /  80  40  10   0
Knoxville McGhee Tyson Airport, TN  53  32  37  11 /  80  60  80  10
Oak Ridge, TN                       50  31  36  10 /  80  60  60  10
Tri Cities Airport, TN              47  32  37  10 /  80  60  80  20

&&

.MRX WATCHES/WARNINGS/ADVISORIES...
NC...NONE.
TN...Winter Storm Watch from Sunday morning through Monday morning
     for Blount Smoky Mountains-Cocke Smoky Mountains-Johnson-
     Sevier Smoky Mountains-Southeast Carter-Southeast Greene-
     Southeast Monroe-Unicoi.

VA...Winter Weather Advisory from 7 AM Sunday to 7 AM EST Monday for
     Lee-Russell-Washington VA-Wise.

&&

$$

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