745
FXUS64 KMRX 250317
AFDMRX

Area Forecast Discussion
National Weather Service Morristown TN
1117 PM EDT Tue Sep 24 2024

...New UPDATE...

.UPDATE...
Issued at 1101 PM EDT Tue Sep 24 2024

Scattered to numerous showers with a few embedded thunderstorms
continue across the region this evening. Given waning instability
under cloudy night skies, there is very minimal threat for any
strong to severe storms overnight. However, it cannot be said the
threat is zero with favorable bulk shear in excess of 40kts. The
greatest concern for the overnight will be isolated instances of
flash flooding. Tropical moisture continues to stream southwest to
northeast across the region. Upper level support and mean flow
nearly parallel to a sw/ne oriented surface boundary may promote
training convection, in which case flash flooding may occur. No
changes have been made to the flood watch however, as the greatest
threat remains mid-day onwards when frontal forcing further
approaches the region, increasing coverage of activity and precip
efficiency. Lastly, cannot rule out fog during the overnight due
to a very moist PBL. Lower confidence exists to how
dense/widespread fog will become with prominent cloud cover and
potential for showers / light breeze to keep the PBL mixed to
some extent.


&&

.SHORT TERM...
(This evening through Wednesday)
Issued at 330 PM EDT Tue Sep 24 2024

Key Messages:

1. A few strong to severe storms are possible through early evening,
with the best chances across the northern Cumberland Plateau. A
Severe Thunderstorm Watch is in effect for Morgan, Scott, and
Campbell through 8 PM EDT. The main threats are damaging straight
line-winds, locally heavy rainfall, and a very low probability of an
isolated tornado along with 1 inch hail.

2. Upper low taps into tropical moisture tomorrow, with a moderate
to heavy precip axis setting up across our area. This will allow for
the training of showers and storms which will lead to an
increasing flood threat.

Discussion:

A few strong to severe storms are possible through early evening.
The best chances will be across the northern Cumberland Plateau
where a Severe Thunderstorm Watch is in effect until 8 PM EDT. SPC`s
meso page currently shows roughly 40 to 50kts of 0-6km shear in
place with 30kts at 0-3km and 20kts at 0-1km. Instability is
moderate with 1500 J/kg MLCAPE. The main threats with these storms
will be damaging straight-line winds, locally heavy rainfall, and a
very low probability of an isolated tornado and 1" hail. The severe
threat should lessen later this evening but a few strong storms are
possible through the overnight hours due to elevated instability
remaining in place along with elevated shear values.

Tomorrow into tomorrow night brings another concern. A frontal
boundary associated with the upper trough will move into the region
and stall. The upper trough will then begin to tap into Gulf
moisture and pull it northward into the area. Strong upper
divergence, mean southerly flow aligned parallel to the boundary,
and ample moisture, will allow for the training of showers and
storms. This means a moderate to heavy rainfall axis will likely
setup somewhere across our area. As of now, the most likely area
will be along I-75 and east towards the east TN mountains. However,
out of caution, a Flood Watch will go into effect at 15Z tomorrow
for the majority of the area. Because of the heavy rainfall
potential, WPC has upgraded the ERO to moderate across portions of
our area. 12Z HREF means paint a good swath of 2 to 3 inches of
rainfall within the moderate ERO. Because of this, flooding will
begin to become a concern.

&&

.LONG TERM...
(Wednesday night through next Tuesday)
Issued at 330 PM EDT Tue Sep 24 2024

Key Messages:

1. Remnants from Tropical Storm Helene expected to impact the region
Thursday and Friday. Flooding possible as well as gusty winds. Flood
Watch in effect for southeastern Tennessee, including all southern
Appalachian counties, and southwest North Carolina.

2. Unsettled weather will continue throughout the long term period
with temperatures near seasonal norms.

Discussion:

The period starting Thursday morning will feature a closed upper low
to our west and what will eventually become Hurricane Helene
approximately due west of the Keys. Before impacts are felt by
Helene, this upper low to our west will help feed moisture into our
area well before rains from Helene arrive. Which may pose some
issues for SE Tennessee, SW North Carolina, and the southern
Appalachians with several inches of accumulating rainfall. Which
yes, is good for many areas that are dry, but this will be too much
in such a short period of time. Flood Watches are in effect for the
aforementioned areas that will begin Wednesday morning and go
through Friday. Not including amounts in the short term forecast
period, but accumulations beginning Thursday morning and ending on
Saturday, place many areas with 3 inches total rain. Right now, the
rainfall hot spots are SW North Carolina, Polk, Monroe, Blount,
Sevier counties in Tennessee. Unicoi and Carter Counties may feel
the impacts of orographic lift as well.

Other areas will receive a few inches of rainfall, as well, but
track of the storm and downslope drying similar to what mountain wave
events do, will make rainfall amounts more uncertain. Winds will
be the next impact element with those expected to increase as we
go through the day on Thursday. Peak winds and gusts will be late
Thursday and Friday morning when the center of Helene is just to
our south. Higher elevation spots will likely see gusts in the 40
mph range and into the 50 mph range for the highest peaks. Lower
elevation spots such as the valley will commonly see gusts in the
25 to 35 mph range. No wind hazards have been issued yet, but most
definitely will as we get closer. The main impact from Helene
without question will be the high rainfall amounts and what that
will do once the soils saturate.

Once Helene further weakens and deteriorates later Friday, through
the weekend and into next week, our area will remain under a closed
low aloft which will keep temperatures cooler and unsettled weather
in the forecast. Of course pinning this down beyond a tropical
system that hasn`t really gotten itself together yet, is not
possible, so slight to isolated chances of showers paint the last
few days of the long term.

&&

.AVIATION...
(00Z TAFS)
Issued at 741 PM EDT Tue Sep 24 2024

Unsettled weather continues over the next 24 hours. Expect
coverage of showers and storms to be more isolated and scattered
over the next few hours, but increasing in coverage overnight into
early Wednesday morning. With any strong showers or storms it will
be possible to see reduced visibilities. Additionally, with plenty
of moisture across the region low ceilings are possible as well,
though probabilities of MVFR cigs remains less than 40% so have
currently strayed away from any predominant mentions at this time.

&&

.PRELIMINARY POINT TEMPS/POPS...
Chattanooga Airport, TN             69  78  66  75 /  70  80  80  90
Knoxville McGhee Tyson Airport, TN  67  76  65  75 /  80  90  80  70
Oak Ridge, TN                       66  76  64  75 /  80  90  80  90
Tri Cities Airport, TN              65  76  64  75 /  60  80  70  60

&&

.MRX WATCHES/WARNINGS/ADVISORIES...
NC...Flood Watch from 11 AM EDT Wednesday through Friday afternoon
     for Cherokee-Clay.

TN...Flood Watch from 11 AM EDT /10 AM CDT/ Wednesday through Friday
     afternoon for Bledsoe-Blount Smoky Mountains-Bradley-Cocke
     Smoky Mountains-East Polk-Hamilton-Johnson-Marion-McMinn-
     Meigs-Northwest Monroe-Rhea-Sequatchie-Sevier Smoky
     Mountains-Southeast Carter-Southeast Greene-Southeast
     Monroe-Unicoi-West Polk.

     Flood Watch from 11 AM EDT Wednesday through Thursday morning
     for Anderson-Claiborne-Grainger-Hamblen-Hancock-Hawkins-
     Jefferson-Knox-Loudon-North Sevier-Northwest Blount-
     Northwest Carter-Northwest Cocke-Northwest Greene-Roane-
     Sullivan-Union-Washington TN.

VA...Flood Watch from 11 AM EDT Wednesday through Thursday morning
     for Lee-Russell-Scott VA-Washington VA-Wise.

&&

$$

SHORT TERM...
AVIATION...KRS