Flash Flood and Debris Flow in Tennessee, May 26 to 27, 2024

Area Forecast Discussion...UPDATED
National Weather Service Morristown TN
1030 AM EDT Sun May 26 2024

...New UPDATE...

.UPDATE...
Issued at 1017 AM EDT Sun May 26 2024

An update has been issued to raise the threat of severe storms
this afternoon. Models are indicating a very favorable environment
for severe storms is developing over Middle and East TN, and the
ongoing severe storms on the TN/KY border are expected to feed off
an unstable air mass and potentially intensify as they move east.
ACARS soundings from TYS this morning show an elevated dry layer
between 700-500 mb that will favor damaging wind gusts. NAM
soundings at TYS show steepening lapse rates around 8 C/km and
MLCAPE around 2500 J/kg. Effective shear values will be in the
40-50 kt range, supporting supercells. LCL heights and 0-1 km
shear will be on the marginal side but within the range of
potentially supporting tornadoes. The greatest threat area will be
the northern Plateau, possibly as far south as I-40 and the
Knoxville area. Timing of the storms appears to be between 1230-1
pm on the Plateau to around 6-7 pm in NE TN. Models appear to be a
bit too slow with the current storm motion.

A second round of severe storms is expected tonight as an MCS
spreads into the area, likely after midnight. All severe modes are
possible with these storms, even with the overnight timing.

&&

.SHORT TERM...
(Today and tonight)
Issued at 221 AM EDT Sun May 26 2024

Key Messages:

1. Fog is possible early this morning in a moist and stagnant air
mass.

2. Showers and thunderstorms are possible this afternoon as
upstream convection may move into the area, although confidence is
low on timing, coverage and intensity of storms.

3. High confidence that a line of strong to severe storms will
move through the region late Sunday night/early Monday morning.

4. Localized flooding will be possible in places where soils are
more saturated from recent rains.

Discussion:

In the upper levels, nearly zonal flow will be present today with
a shortwave moving into the Tennessee Valley late tonight. At the
surface, a low will move into Missouri/Illinois this afternoon
with the cold front still west of the Mississippi at sunset. The
low will move slowly into the Great Lakes Region by morning with
the cold front just west of our region by sunrise.

With clear skies maximizing radiational cooling, calm winds and
abundant ground moisture, fog development is possible in the
early morning hours. Currently (3 AM EDT) fog is very patchy.

Uncertainty is high regarding shower/storm activity this
afternoon. The CAMs are not in good agreement on coverage and
timing of storms this afternoon/evening. The CAMs that do have
activity this evening have isolated strong to severe storms and
even some helicity tracks. The environment will be favorable for
severe this afternoon/evening with CAPE in the 2K to 3K range,
Effective shear 50 to 60 knots and good mid level lapse rates. The
limiting factor this afternoon/evening is lack of trigger with the
main front and trough well to the west but if a minor wave moves
through the pattern we could get some strong activity this
afternoon/evening and possibly a couple tornadoes.

There is higher confidence in storms moving through late tonight
as the cold front and trough approach the region. CAMs are in good
agreement on a squall line moving through around 2 AM to 7 AM.
CAPE will be lower at this time less than 1K in most areas but
shear will be favorable around 50 to 60 knots. The main threat
will be damaging straight-line winds but a few isolated tornadoes
cannot be ruled out with favorable 0-1 km shear (30 knots) and 0-1
km helicity (200 to 300).

Rain amounts will generally be 1 to 2 inches for most locations.
The line of storms will be moving fairly quickly but will likely
have high rain rates. Flooding potential will be higher in places
where storms do materialize this afternoon/evening and places that
have had heavy rain recently. Overall, flooding is expected to be
fairly isolated.

&&

.LONG TERM...
(Monday through Saturday)
Issued at 221 AM EDT Sun May 26 2024

Key Messages:

1. Another round of afternoon/evening possible on Monday

2. Drier and cooler weather to begin Tuesday through the end of the
week. with a gradual warming trend into the weekend.

Discussion:

By late Monday morning the front will be moving through the region
as the line of storms from overnight should be well to the east of
the Appalachians. Moving into the afternoon/evening hours we will
likely see another round of thunderstorms along shortwave energy
traversing the south side of the low pressure system over the Great
Lakes. Storms on Monday are expected to be much weaker and more
isolated in coverage, with the greatest chances to see storms being
north of Interstate 40. If the sky is able to clear out earlier in
the day on Monday the atmosphere could destabilize enough that some
of the stronger storms are capable of producing strong winds gusts
and hail during the afternoon/evening. Once the sun begins to set
Monday night the better synoptic forcing and diurnal convection
should quickly come to an end.

For the rest of the work week we'll generally see a trend of cooler
weather, a few degrees below seasonal normals as we sit at the base
of the trough which will help keep light westerly to northwesterly
winds over the region. We also could see some light showers along
the spine of the Appalachians with the northwest flow, but generally
most of the eastern Tennessee Valley should remain dry Tuesday
through Friday.

Rain chances return over the weekend as a low tries to move through
the eastern United States. PoP chances will remain lower at this
time due to uncertainty with how far east the system is able to
progress before taking a more northward course towards the Great
Lakes Region. The ridging that will build in to fight against this
system will also likely increase temperatures to above seasonal
normals by Saturday/Sunday.

&&

.AVIATION...
(12Z TAFS)
Issued at 724 AM EDT Sun May 26 2024

Fog is reducing visibilty near TRI but will dissipate shortly
after 12Z. Model guidance is not in good agreement on shower and
storm potential this afternoon and evening. Confidence is low on
timing and coverage but the best chances appear to be near TYS and
TRI but it is possible storms may be more isolated. The best
chance for widespread thunder will be late tonight/early tomorrow
morning when a line of storms moves through. Some storms may be
strong to severe.

&&

.PRELIMINARY POINT TEMPS/POPS...
Chattanooga Airport, TN             90  70  87  64 /  20  80  50  10
Knoxville McGhee Tyson Airport, TN  85  68  83  62 /  50  80  70  10
Oak Ridge, TN                       85  67  84  60 /  70  80  60  10
Tri Cities Airport, TN              84  64  80  60 /  80  80  70  20

&&

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

SHORT TERM...DGS
AVIATION...McD

Hazardous Weather Outlook...UPDATED
National Weather Service Morristown TN
1014 AM EDT Sun May 26 2024

NCZ060-061-TNZ012>018-035>047-067>074-081>087-098>102-VAZ001-002-
005-006-008-271415-
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-
1014 AM EDT Sun May 26 2024 /914 AM CDT Sun May 26 2024/

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

.DAY ONE...Today and Tonight

Two rounds of severe thunderstorms are expected today and
tonight.

The first round is expected between 1 PM and 7 PM this afternoon
and evening, mainly for areas along and north of I-40. These
storms will be capable of producing damaging wind gusts to 70 mph,
large hail to ping-pong ball size, and tornadoes.

A second round of severe storms is expected tonight, likely
between midnight and 5 AM. These storms will be capable of
producing damaging wind gusts, large hail, and isolated tornadoes.

In addition, repeated or prolonged periods of heavy rainfall may
produce localized flooding in a few locations.

.DAYS TWO THROUGH SEVEN...Monday through Saturday

The probability for widespread hazardous weather is low.

.SPOTTER INFORMATION STATEMENT...

Spotter activation will be needed this afternoon and tonight.