Tornado and Flash Flooding in Tennessee, August 7, 2023

Area Forecast Discussion
National Weather Service Morristown TN
949 AM EDT Mon Aug 7 2023

...New UPDATE...

.UPDATE...
Issued at 948 AM EDT Mon Aug 7 2023

Cumulus development is ongoing across central Kentucky which is
increasing confidence that the ingredients needed to create a severe
weather event are actually materializing this morning. Initiating is
ongoing near the strengthening LLJ as the aggressive shortwave moves
through. Mesoanalysis shows that all the severe parameters are
impressive this morning and with clouds clearing out this morning
the debris CU doesn't look like it's going to get in the way of
increasing instability in the low levels of the atmosphere. All
weather threats are possible today, including rotating supercells.
Based on current radar trends and refreshed CAMs the timing of
incoming storms has slowed down just a tad compared to previous
forecast with storms looking to move onto the plateau around noon
and quickly moving eastward, pushing east of the mountains before
sunset.

Main weather message for today is to keep an eye radar and the
latest warnings, especially for the commute home from school or work
as this is the peak time frame for severe storms.

&&

.SHORT TERM...
(Today and tonight)
Issued at 252 AM EDT Mon Aug 7 2023

Key Messages:

1. Severe thunderstorms expected today.

2. Isolated to scattered weaker convection tonight ahead of a cold
front that pushes through.

Discussion:

An unusual pattern for August is setting up for today.  An
unseasonably strong upper-level 300mb jet will set up across our
area, and an anomalously intense 500 mb shortwave will move eastward
across the Tennessee and Ohio River Valleys during the day. In
response to the divergence associated with the strong upper level
jet, the 850mb LLJ will intensify across the Tennessee Valley with
speeds of around 40 kt expected. NAEFS forecast anomalies show the
850/700/500 mb winds all around the max compared to climatology for
this time of year. Models continue to depict around 40 to 50 kts of
effective bulk shear across the area, and there is little question
that the wind profile will support supercell and multicell
convective modes.

How much instability will be available is still less certain, given
that there is still potential of morning convection and convective
debris cloudiness that may hinder destabilization. Most model
solutions keep any morning activity to a minimum and focus on a
convective line that surges through our area this afternoon. HREF
mean SBCAPE values generally show 2000+ J/kg as the line moves in,
but some other individual models such as the RAP and NAM show values
approaching or exceeding 3000 J/kg.  While there is still some
uncertainty about the magnitude of the CAPE, current thinking is
that there will be sufficient convective energy combined with the
shear for a significant severe weather threat for today.  CAMS have
come into reasonable agreement, and generally show a line of strong
to severe storms moving through our area between 15Z and 21Z.
Damaging winds are expected to be the most widespread impact, but
large hail will also be possible.  There is also a low chance for
tornadoes, and the HREF does show significant updraft helicity
tracks across our are this afternoon.  Localized flooding is also
possible, although the storms will likely be moving fast enough to
limit flooding potential outside of any areas that experience
training.

For tonight, a trailing cold front will be pushing through and we
may see some additional scattered to isolated weak convection, but
this should end from west to east as drier air begins to push in
behind the front.

&&

.LONG TERM...
(Tuesday through Sunday)
Issued at 252 AM EDT Mon Aug 7 2023

Key Messages:

1. Additional rounds of numerous to widespread showers and storms
expected Wednesday into Thursday. Daily chances of showers and
storms may persist into the weekend.

2. Temperatures will generally be near to slightly below normal
through the extended.

Discussion:

Cooler air will advect into the region Tuesday in a post-frontal air
mass as temperatures become slightly below normal across the region.
A brief quasi-zonal synoptic setup aloft will also keep most
locations dry, with the exception to this being maybe a few light
upslope showers associated with any lingering wrap around moisture.
As we head into Wednesday a shortwave trough is expected to begin
progressing across central portions of the CONUS. H85 flow will
shift to a southwesterly pattern in response. Increasing isentropic
lift will bring a return of PoPs Wednesday into Thursday as the
shortwave shifts eastward.

Overall, long range guidance such as the Euro and GFS want to agree
on the disturbance moving through the forecast area late Wednesday
into mid-day Thursday. Most recent NAM run is a little bit slower
with better shortwave forcing not becoming more apparent until
Thursday afternoon. Timing will play an important role in available
instability and associated severe weather chances. Additionally,
coverage of showers ahead of the shortwave forcing also has the
potential to limit instability Thursday if great enough. Ensemble
guidance paints PWAT between 1.7 and 1.9 inches with 24hr
accumulation generally between 1-2 inches for portions of the area.
Some flooding concerns may also develop but will depend on the
presence of a surface boundary and potential for training convection
along with aforementioned instability. Given the uncertainty
combined with the severe weather threat this afternoon will utilize
low probability in the HWO for now.

Daily chances of showers and storms look to continue into the
weekend, though details remain uncertain at this time. General
trends suggest a more diurnal pattern is possible but will also
depend on where a frontal boundary sets up and potential for any
small impulses through the mean flow. Temperatures will remain
slightly below to near normal through the extended.

&&

.AVIATION...
(12Z TAFS)
Issued at 650 AM EDT Mon Aug 7 2023

Some MVFR level cigs around at times early, and TRI may see MVFR
cigs again late. Otherwise, mainly VFR conditions expected all
sites outside of convection. Expect some strong to severe storms
to move through the area this afternoon, and will carry VCTS with
tempo +TSRA all sites. Winds will breezy out ahead of the storms,
gusting to around 25 kts or more from the southwest.

&&

.PRELIMINARY POINT TEMPS/POPS...
Chattanooga Airport, TN             91  69  87  69 /  80  30  20  20
Knoxville McGhee Tyson Airport, TN  88  68  84  66 /  80  30  10  10
Oak Ridge, TN                       86  67  83  65 /  80  20  10  10
Tri Cities Airport, TN              86  65  79  61 /  80  50  10  10

&&

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

SHORT TERM...
AVIATION...

Hazardous Weather Outlook
National Weather Service Morristown TN
1146 AM EDT Mon Aug 7 2023

NCZ060-061-TNZ012>018-035>047-067>074-081>087-098>102-VAZ001-002-
005-006-008-081600-
Cherokee-Clay-Scott TN-Campbell-Claiborne-Hancock-Hawkins-
Sullivan-Johnson-Morgan-Anderson-Union-Grainger-Hamblen-
Northwest Cocke-Cocke Smoky Mountains-Northwest Greene-
Southeast Greene-Washington TN-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-Scott VA-Russell-Washington VA-
1146 AM EDT Mon Aug 7 2023 /1046 AM CDT Mon Aug 7 2023/

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

.DAY ONE...This Afternoon and Tonight

Thunderstorms are expected today, and some of these storms will
likely become strong to severe. Primary risks will be damaging
winds and large hail. There is also a threat of torrential
downpours capable of producing localized flooding, and an
tornadoes will be possible as well.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday

The probability for widespread hazardous weather is low.

.SPOTTER INFORMATION STATEMENT...

Spotter reports may be needed today.