303
FXUS64 KMRX 180208
AFDMRX

Area Forecast Discussion
National Weather Service Morristown TN
1008 PM EDT Sat Aug 17 2024

...New UPDATE...

.UPDATE...
Issued at 1002 PM EDT Sat Aug 17 2024

Convection is ongoing across the area with several marginally
severe storms having occurred this evening. The environment is
still supportive of some storms becoming strong to severe with
impressive 35 to 40 kts of effective bulk shear and over 1,000
J/kg of MLCAPE still remaining. Several cells have shown signs of
rotation in the mid-levels with low-level flow and shear being
very limited. This is expected to continue over the next several
hours but with decreasing intensity due to recent convection and
loss of daytime heating. PoPs were adjusted to focus more in
northern portions of the area with temperatures also reduced due
to the continued convection late in the day. Remaining weather
elements were updated based on the latest high-res model guidance.

&&

.SHORT TERM...
(This evening through Sunday)
Issued at 301 PM EDT Sat Aug 17 2024

Key Messages:

1. Scattered to numerous showers/storms through the afternoon and
tonight, with a few severe storms possible.

2. Scattered to numerous showers tomorrow in northern sections under
a broad trough.

Discussion:

Mesoanalysis shows ample instability across northern AL and southern
Middle TN and the southern TN Valley that will drive thunderstorms
for the next few hours. The presence of some weak shear will pose a
threat of severe storms, with damaging winds behind the main
concern, and a lesser concern for large hail. Meanwhile, convection
is blossoming across KY near and ahead of a cold front. This
activity will be moving E-SE through an unstable environment as it
approaches our area, and it will likely expand into a line of
convection. Expect that it will enter our KY border counties in the
23-03Z time frame, with a threat of damaging wind gusts. With some
upper dynamic support from a jet streak, it could maintain its
intensity past sunset as it crosses NE TN and SW VA through
midnight. These storms should exit the area between 06-09Z with the
passage of a shortwave trough axis and weak surface cold front.

We will continue to have a broad cyclonic mid/upper level flow
across the region on Sunday, with another embedded shortwave trough
moving around the broad trough during the day. Low level instability
will be weak behind the surface frontal passage, but lower temps
aloft under the trough and ample moisture depth will support
convective showers and a few isolated thunderstorms in the
afternoon, mainly north of I-40. Temperatures in northern sections
will be in the 70s to lower 80s with the expected precip and cloud
cover, while southern sections will be in the 85 to 90 range under
mostly sunny skies.

&&

.LONG TERM...
(Sunday night through next Saturday)
Issued at 301 PM EDT Sat Aug 17 2024

Key Messages:

1. Shower and thunderstorm chances continue Sunday evening through
Monday behind cold front, but will be mostly across the northeast
half of east TN and southwest VA. Showers and thunderstorms will not
be as strong.

2. Dry weather and near to below normal temperatures expected
Tuesday through about Thursday.

3. Upper ridging shifts east from the central states beginning later
Thursday and continues into the weekend. A more southerly flow will
warm temperatures back to near normal with very little chance of
rain through Saturday.

Discussion:

Sunday night into Monday will be the last chance to see showers and
or storms for several days as the forecast area will lie near the
base of a slow moving upper trough and low over the eastern United
States. The best chance to see convection for the early periods will
be north of Interstate 40, and especially near the TN/VA border and
into southwest Virginia and far northeast Tennessee. Much less
moisture will be present in the atmosphere behind a cold front
moving south of the region Sunday evening for thunderstorms to tap
into and the overall rainfall amounts should be lighter for
locations that see a quick round of rain Sunday evening and night
and into Monday. The main weather news in the extended this week
will be the cooler temperatures starting Monday and much lower dew
points through the week after Monday.

Sunday night and Monday the forecast area will still be on the edge
of upper trough over the eastern third of the US with shortwave
energy triggering some scattered showers and storms across the
northeast half of the area. There will be more clouds in the
northeast and temperatures Monday in the 70s to lower 80s with
mountain locations staying in the 60s. Only the southern valley
will stay warmer in the mid 80s and little chance of rain. A drier
airmass will be settling in from the northwest late Monday and
Monday night. The forecast area will then be under the eastern edge
of a strengthening upper ridge over the central United States. The
drier airmass out of the north and northwest will allow temperatures
to drop into the 50s to lower 60s Tuesday and mainly 50s Wednesday.
Highs Tuesday and Wednesday will be below average in the 70s to mid
80s as surface high pressure will drop southeast from the upper
midwest and mid Mississippi Valley towards the Ohio Valley and Great
Lakes. Some long range models are trying to increase moisture east
of the Appalachian crest Thursday but this looks overdone and will
keep the overall trend that the hot upper ridge shifts east for the
last 2 days of the extended and the surface high moves east of the
Appalachians as the upper trough lifts northeast. This should warm
temperatures back to near average and keep the region dry through
Saturday.

&&

.AVIATION...
(00Z TAFS)
Issued at 719 PM EDT Sat Aug 17 2024

The main aviation impact this evening will be storms approaching
from the north and west. Based on the latest trends, TRI is most
likely to see impacts with lesser confidence at TYS and minimal
confidence at CHA. Any storms could grow and produce damaging
winds, but the main question is if they will impact at the
terminals or not. Activity is likely to diminish around midnight
with the next impact being fog, which is also most likely to
happen at TRI over the others. If decent rainfall is seen at the
other terminals, fog will become more likely but was left out for
now. For Sunday, winds will shift to a more westerly direction
behind a frontal boundary. Additional rain chances are possible
with TRI likely to keep low clouds through the day.

&&

.PRELIMINARY POINT TEMPS/POPS...
Chattanooga Airport, TN             70  90  68  86 /  30  10  10   0
Knoxville McGhee Tyson Airport, TN  70  85  66  82 /  30  40  20  20
Oak Ridge, TN                       69  84  66  81 /  40  40  10  20
Tri Cities Airport, TN              67  77  64  76 /  50  60  40  60

&&

.MRX WATCHES/WARNINGS/ADVISORIES...
NC...NONE.
TN...NONE.
VA...NONE.
&&

$$

SHORT TERM...DGS
LONG TERM....TD
AVIATION...BW