-- Discussion containing changed information from previous version are highlighted. --
307
FXUS64 KMRX 231121
AFDMRX

Area Forecast Discussion
National Weather Service Morristown TN
721 AM EDT Thu May 23 2024

...New AVIATION...

.SHORT TERM...
(Today and tonight)
Issued at 303 AM EDT Thu May 23 2024

Key Messages:

1. Scattered showers and thunderstorms expected primarily in the
afternoon to early evening hours today. Storms could be strong to
low end severe, with gusts up to 60 mph and quarter size hail the
top end threat.

2. Additional shower development possible late overnight and
early Friday morning, no severe expected.

Discussion:

Looking at the second day of showers and thunderstorms later
today. Want to make a brief mention of the potential for showers
this morning to about noon for northeastern Tennessee into
southwest Virginia. Some of the hourly HRRR runs have showers
developing and moving to the northeast, along with the HREF and
GFS, this seems to match up well with isentropic ascent across the
same area that the GFS is depicting. These showers aren`t
expected to have a severe component to them. Moving onto the main
show in the afternoon, if CAMs are to be believed, then
thunderstorms should begin to blossom across Middle Tennessee and
into Kentucky around noon before moving across East Tennessee and
southwest Virginia through the remainder of the daylight hours.
Convection initiation looks to be bolstered by divergence aloft
thanks to the weak jet streak structure present over the region.
As far as the environment goes, it`s supportive of three hazards:
wind, hail, and heavy downpours. We`ll start with the overall
environment and work backwards. PWATs for tomorrow are running
between 1.4 and 1.5 inches, which is around the 90th percentile
for late May per Nashville climatology. Additionally, freezing
level on soundings looks to be 11,000 feet and we`ll have 800 CAPE
north of Knoxville, and 1000 to 1300J Knoxville and south. Storms
should be semi-efficient at producing heavy rainfall, but
ultimately storms are expected to be confined to a few hours and
be moving along. The other contextual information is 3 hour flash
flood guidance looks to be 2 to 3 inches, so it would be a low
risk overall for flash flooding.

Moving onto severe potential, SPC has the area under a marginal risk
for a low chance of severe hail and wind. This seems reasonable
here. GFS and HRRR soundings depict fairly straight hodographs with
35 to 40 knots of effective shear, enough to sustain and organize an
updraft but the straight hodograph is not indicative of a tornado
risk. Those high freezing levels will act to work against hail
potential, but would not be surprised to see some hailstones in
the stronger cells approaching an inch. Overall given the expected
environment for this afternoon general thunderstorms are most
likely, with potential for a few being strong to low end severe.

Thunderstorms should with time tomorrow afternoon progress to the
southeast, and then exit in the evening hours. After that guidance
has another lull for the area, though scattered showers or
thunderstorms may develop again over the Plateau late tonight
towards early Friday morning. If the early morning Friday round
occurs, look for those to just be general variety showers, maybe a
rumble of thunder, given much thinner CAPE profiles and the
nighttime low level inversion.

&&

.LONG TERM...
(Friday through Wednesday)
Issued at 303 AM EDT Thu May 23 2024

Key Messages:

1. An unsettled period with multiple rounds of showers and
thunderstorms for much of the period.

2. Some of the storms may be strong to severe at times, with the
higher chances for severe storms looking like Sunday and/or Monday.
Localized flooding will also be possible.

3. Drier and cooler air moving in toward the end of the period, but
a few additional showers and/or storms still cannot be ruled out.

Discussion:

We start the period with quasi-zonal flow aloft and a short wave
approaching from the west. This short wave will swing across the
area Friday, bringing another round of showers and thunderstorms. A
look at model soundings shows disagreement on how much convective
energy will be available, but overall model and ensemble data
suggests MLCAPES will likely be in the 1000-1500 J/kg range, and
while shear looks weak for much of the area there is some increase
across the south during the day with effective shear around the 25-
30kt range indicated. A few of the storms may become strong to
severe Friday with damaging winds and hail the primary threats.

Models have been consistent in showing the unsettled pattern
continuing through the weekend into early next week, as a series of
short waves of varying strength affect the area followed by a deeper
trough and cold front. However, the details of timing/strength of
each wave as well as the timing of the trough/cold front have shown
worse agreement and poor run to run consistency.  Thus there is high
confidence in multiple rounds of showers/storms, but low confidence
in the details. Saturday`s short wave may have more convective
energy to work with than Friday`s but shear does not look much more
significant. However, by Sunday into Monday ensemble data suggests
deep layer shear may reach 40-50kts across much of the area, along
with significant instability. Additional rounds of showers and
thunderstorms can be expected through the weekend into Monday, and
some storms may be strong to severe Saturday but especially Sunday
and possibly into Monday (depending on how quickly the front moves
through). With the increased shear by the Sunday/Monday time frame,
supercellular structures look more likely so while the primary
threats will continue to be damaging winds and hail, there looks to
be an uptick in the tornado threat as well. The details of all of
this are still unclear as mentioned above, but it will bear close
watching as we get closer.

While there is no specific period of significantly heightened flood
risk indicated during the Friday through Monday time frame, given
the multiple rounds of showers and storms there will be a risk of
localized flooding in areas that see repeated and/or prolonged
periods of heavy rainfall.

While models have been waffling on the timing of the cold front,
most now indicate it will be through by late Monday or Monday night
as a deeper upper trough takes up residence over the eastern US.
Drier and cooler air will move in for Tuesday and Wednesday, but
there may be additional short waves moving through the trough so the
forecast continues to have chances, albeit significantly lower than
the preceding days, of showers and thunderstorms for these latter
periods.

&&

.AVIATION...
-- Changed Discussion --
(12Z TAFS)
Issued at 710 AM EDT Thu May 23 2024

All TAF sites are expected to be VFR during the day today outside
of any VIS restrictions during afternoon TSRA. Any direct TS at
the terminals today will contain strong, gusty winds. Confidence
is medium on timing of storms, but expectation is for the bulk of
the storms to be out of the area after 00z. A low chance for rain
at CHA late in the period, but otherwise CIGs will lift this
evening and allow for VIS to drop to MVFR at both TYS and TRI by
06z, with further reductions at TRI possible.
-- End Changed Discussion --

&&

.PRELIMINARY POINT TEMPS/POPS...
Chattanooga Airport, TN             85  67  83  67 /  50  50  70  50
Knoxville McGhee Tyson Airport, TN  81  65  81  65 /  80  40  70  60
Oak Ridge, TN                       80  64  81  64 /  80  40  70  60
Tri Cities Airport, TN              78  61  79  62 /  80  50  70  50

&&

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

$$

SHORT TERM...Wellington
AVIATION...Wellington