279
FXUS64 KMRX 171912
AFDMRX

Area Forecast Discussion
National Weather Service Morristown TN
312 PM EDT Wed Jul 17 2024

...New SHORT TERM, LONG TERM...

.SHORT TERM...
(This evening through Thursday)
Issued at 237 PM EDT Wed Jul 17 2024

Key Messages:

1. Increasing coverage of showers and thunderstorms over the next
24 hours.

2. Temperatures likely below normal on Thursday.

Discussion:

Upper heights will continue to lower through the period as a large
scale trough over the Great Lakes and into the mid and lower
Mississippi river valley shifts slowly southeastward. The result
will be increasingly high coverage of showers and thunderstorms
across the forecast area. These will be largely diurnally driven,
especially coverage wise as we`re seeing this afternoon, but do
expect rain chances overnight to some degree as almost all
guidance indicates. Forecast soundings and SPC Mesoanalysis page
show mixed layer CAPE in the 1,500-2,000 J/kg this afternoon, with
downdraft CAPE values in the 800-1,000 J/kg range. SPC expanded
their marginal risk area, mainly for an isolated damaging wind
threat. This seems reasonable to me. Effective deep layer shear is
weak, less than 20kt, across the board but with this amount of
instability coincident with PWAT values in excess of 1.8", tall
upright storms will experience precip loading and pose a risk of
some strong winds.

For tonight into Thursday, we should get another round or two of
convection as the upper trough and associated surface front
approach from the northwest. Increasing influence from the right
entrance region of a jet on the eastern periphery of that upper
trough should promote some nocturnal convection tonight, then
another round of diurnally driven afternoon storms are expected
tomorrow. Coverage may be more limited to the eastern half of the
CWA (areas along/east of the I-75 and I-81 corridors) tomorrow
depending on how quickly the front arrives/moves through. PoPs
reflect this of course. Temperatures should actually fall to below
seasonal values tomorrow.

&&

.LONG TERM...
(Thursday night through next Wednesday)
Issued at 237 PM EDT Wed Jul 17 2024

Key Messages:

1. A frontal boundary will move through the region Thursday night,
followed by a brief decrease in rain chances.

2. A break from the heat is expected behind the front, with slightly
below normal temperatures throughout the long term.

3. POPs remain elevated throughout the long term due to higher than
normal moisture content across the area.

Discussion:

In the upper levels, a trough will be over the Mississippi Valley.
The ridge over the Southeast will strengthen by Saturday and
especially Sunday extending over the region as a trough remains to
the west. At the surface, a cold front will move through the region
Thursday evening bringing a lull to rain chances. This boundary
doesn`t make it far and lifts back into the region by Saturday with
the blocking pattern in the Southeast. POPs on Friday may be a tad
high but keeps best rain chances in the East Tennessee Mountains and
NC counties. Saturday and beyond, there will be fairly high chances
for showers and thunderstorms mainly in the afternoon and evening
hours. Deep layer southerly flow will supply ample moisture during
this forecast period. Precipitable water values will be near 2
inches by the end of the weekend. This might be the drought busting
pattern that we desperately need. Speaking of good news, with
increased rain chances and cloud cover, highs will be lower this
period, maybe even a few degrees below normal. Fingers crossed.

&&

.AVIATION...
(18Z TAFS)
Issued at 141 PM EDT Wed Jul 17 2024

The next 24 hours will see two, or possibly three, rounds of
SHRA/TSRA across eastern Tennessee as a cold front slowly
approaches and moves through the region. Expect the first round to
die down around 01z-02z, with another a few hours before 12z, and
then another tomorrow during the day. Coverage this afternoon
should be greatest so have prevailing TSRA in at all sites to
account for this. Will carry VCSH for the late night round and
leave tomorrow`s convection to later TAF issuances as confidence
in timing is less. May see some MVFR flight categories later
tonight due to the rain, but not confident enough in their
occurrence to go with anything greater than a SCT025 base at all
sites for the time being.

&&

.PRELIMINARY POINT TEMPS/POPS...
Chattanooga Airport, TN             73  88  69  86 /  70  70  20  50
Knoxville McGhee Tyson Airport, TN  71  85  67  86 /  80  60  20  50
Oak Ridge, TN                       71  84  66  86 /  80  50  10  30
Tri Cities Airport, TN              68  81  62  85 /  90  70  10  40

&&

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

$$

SHORT TERM...CD
LONG TERM....McD
AVIATION...CD