606
FXUS64 KMRX 160232
AFDMRX

Area Forecast Discussion
National Weather Service Morristown TN
1032 PM EDT Thu Aug 15 2024

...New UPDATE...

.UPDATE...
Issued at 1023 PM EDT Thu Aug 15 2024

Forecast is on track tonight with mostly clear sky conditions. The
main impacts will be on Friday when a line of showers and
thunderstorms is expected to move across the area. Exact timing is
uncertain, but recent CAMs are showing additional convective
initiation associated with the convective cluster currently
across Missouri. This cluster would develop into an MCS and move
towards our area mid-day to early afternoon on Friday.

While this will need to be monitored, the overall severe risk
continues to be marginal. The upper shortwave and PVA will be
focused north of our area across the Ohio Valley with limited to
no height falls for the East Tennessee area. This will result in
poor mid- level lapse rates and limited instability with HRRR
forecast soundings showing MLCAPE around 1000 to 1500 J/Kg with
highest values across our western counties near the Cumberland
Plateau. Effective bulk shear is also weak at around 15 to 20 kt.
While the severe potential is limited, there is still a low risk.
Even with sub- severe winds, some downed trees will be expected.
In addition, RAP forecast PW values around 2.0 to 2.2 inches will
result in heavy rain rates and localized flash flooding potential
on Friday.


&&

.SHORT TERM...
(This evening through Friday)
Issued at 300 PM EDT Thu Aug 15 2024

Key Messages:

1. Isolated showers through this evening over parts of higher
terrain.

2. Scattered to numerous showers and storms Friday, with some
strong to severe storms and localized flooding possible. Damaging
winds would be the primary threat with any severe storms.

Discussion:

The rest of this evening and tonight should be mostly quiet, with
some isolated shower development over the plateau and mountains.
At least a couple hundredths has been recorded as of this time in
some areas. Activity should wane with the loss of day time
heating. Patches of fog may also be possible due to the influx of
moisture funneling our way under calm conditions overnight.

Friday: our mostly dry spell will come to an end this weekend with
an approaching system and attendant cold front. A mid to upper
level trough is forecast to slide east across the northern tier of
the country. The low center will pretty much be stacked underneath
it. Pre-frontal showers and thunderstorms will impact the area as
early as just before or around sunrise from the west and
northwest. SPC currently has us in a MRGL risk for severe weather
and they mention the possibility of the morning convection
diurnally strengthening as we approach Friday afternoon. 20 to
30KT effective shear along with CAPE in the 1000 J/Kg range is
enough to support multicellular storms. Exact timing, evolution,
movement, and the possibility or not of multiple rounds is still
pretty unclear even with CAMs less than 24 hours out. All in all
we can expect the possibility of strong to severe storms with
damaging winds the primary threat. Heavy rainfall could also lead
to localized flash flooding. Hopefully the latter will hold off
with how dry we have lately been.

&&

.LONG TERM...
(Friday night through next Thursday)
Issued at 300 PM EDT Thu Aug 15 2024

Key Messages:

1. Scattered to numerous showers and storms Friday evening and
Saturday, with some strong to severe storms possible.

2. More scattered showers and storms possible Sunday and Monday,
with better coverage across the northern areas.

3. Higher pressure building south with a more northwest flow and
drier Tuesday and Wednesday with cooler high temperatures and low
temperatures dropping into the 50s.

Discussion:

Models in general agreement with upper low over Wisconsin and
Michigan to start the extended forecast period Friday night. The
upper low will drift, move southeast through Saturday and Sunday
opening up by Sunday night into the northeast states and digging an
upper trough across the eastern states into the TN Valley and
southeast through Sunday night. Multiple shortwaves will progress
through the NW flow and across the Tennessee Valley Friday night and
continuing through Saturday and Sunday. A cold front will be
approaching the area as well on Saturday. The trough looks to shift
eastward Monday into Monday night with improving conditions Monday
night and continuing through Wednesday.

Models are still different in the details with the timing of the
shortwaves coming through but are showing Friday evening and late
Saturday afternoon and evening as the best times for significant
rainfall and thunderstorms. Sunday the trough and cold front will
have shifted farther south with more convection south of the
Tennessee state line in MS, AL, GA and the Carolinas with the GFS
and NAM12 showing that will be where more significant convection
develops. Model runs have been showing differences so stay tuned.
With the threat for heavy rainfall some isolated flooding is
possible Friday night but a better chance will be Saturday night if
Friday night is wet too. Sunday and Sunday night into Monday expect
storms to more directly effect the northern half of the area with
another significant shortwave moving through the Ohio Valley Sunday.

Overall the atmosphere looks supportive of more organized convection
with some strong to severe storms possible Friday evening. The slow-
moving upper trough/low continues east across the Great Lakes/OH
Valley Saturday. The weak surface low over southern MI is forecast
to gradually develop into southern Canada/Ontario. The weak front
extending southward from the low should slowly move east-southeast
across the OH Valley and Mid-Atlantic into the Southeast through
Saturday night. The boundary, although not well defined and having
only modest low-level convergence, should serve as a focus for
strong to severe convection. Diurnal heating of a moist airmass
along/ahead of the front should bring weak to moderate
destabilization Saturday afternoon, even though mid-level lapse
rates should remain modest. Enhanced mid-level flow associated with
the upper trough/low should focus storms over parts of the lower OH
Valley into the TN Valley and Mid-South. But, sufficient deep-layer
shear to support some updraft organization should be present over
much of the broad warm sector. Multiple clusters may develop along
the length of the front Saturday afternoon and pose primarily an
isolated strong to damaging wind threat as they spread east-
southeast through Saturday evening. So Marginal risk is across the
forecast area. Damaging winds will be the primary severe threat with
these storms. In addition, any areas that see repeated and/or
prolonged rounds of heavy rainfall may experience localized
flooding.

Cooler temperatures will be moving in behind the front Sunday and
Monday, with temperatures at or a little below normal Sunday and
below normal on Monday. Highs Monday only in the 60s to lower 70s
higher elevations and 70s to the lower 80s. On Tuesday and
continuing into Wednesday a cooler surface high pressure will build
into the eastern Tennessee Valley and southern Appalachians.
Dewpoints will lower into the more comfortable 50s at most sites
with actual lows in the 50s Tuesday into Thursday mornings. A weak
surface trough is expected to move through the upper flow Thursday
and may trigger a few showers or storms. Temperatures will be
warming back into the mid to upper 80s Thursday.

&&

.AVIATION...
(00Z TAFS)
Issued at 724 PM EDT Thu Aug 15 2024

VFR conditions continue tonight with a low probability of fog near
the TRI terminal. Rain chances increase on Friday with VCSH from
mid-morning throughout the afternoon. Have included prob30 TSRA at
all terminals as a thunderstorm complex is forecast to move across
the region, but timing is still uncertain. SW winds remain mostly
5-10 kt with higher gusts near any thunderstorms.

&&

.PRELIMINARY POINT TEMPS/POPS...
Chattanooga Airport, TN             72  94  73  93 /   0  50  50  70
Knoxville McGhee Tyson Airport, TN  72  91  71  90 /   0  50  70  70
Oak Ridge, TN                       71  89  70  89 /  10  60  70  70
Tri Cities Airport, TN              67  86  69  85 /  10  50  60  50

&&

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

$$

SHORT TERM...KS
LONG TERM....TD
AVIATION...JB