-- Discussion containing changed information from previous version are highlighted. --
506
FXUS64 KMRX 230120
AFDMRX

Area Forecast Discussion
National Weather Service Morristown TN
920 PM EDT Sun Sep 22 2024

...New UPDATE...

.UPDATE...
-- Changed Discussion --
Issued at 915 PM EDT Sun Sep 22 2024

Showers and a few storms have ended except for an area along the
TN and VA border this evening. Expect more isolated activity to
continue along and near the KY/TN border and into southwest
Virginia as weak shortwaves traverse from west to east across
northern TN and southern KY. Patchy fog is likely as well mainly
northern two thirds of the forecast area. Cloud cover was
decreasing in the south and west but continues in the east. Expect
clouds to increase from the northwest to the northeast as showers
and storms develop ahead of a cold front from the south central MO
border with AR eastward into south central KY. There will be
increasing rain chances along the TN/KY border into southwest
Virginia late tonight and early Monday morning because of this
feature. Updated forecast has been sent.
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&&

.SHORT TERM...
(This evening through Monday)
Issued at 303 PM EDT Sun Sep 22 2024

Key Messages:

1. Isolated showers and storms continue overnight. Fog expected
for any area that receives rain.

2. Isolated to scattered showers and storms tomorrow.

3. Hot again tomorrow, temps around 5 to 10 deg above normal.

Discussion:

A few isolated showers and storms will continue through the night
as weak short wave energy passes through the region. Any area
that receives rainfall will likely see fog thereafter.

Tomorrow, the flow becomes more zonal as high pressure shifts to our
southeast. This zonal flow, combined with additional shortwave
energy moving through, will increase chances of showers and storms
across the area. Though there will be increased coverage, not all
areas will see showers or storms. There is a marginal risk for
severe storms tomorrow but I am not that excited about it. The
reason for the marginal is increased shear in the mid to upper
levels with 0-6km shear around 30kts. Shear in the lower levels is
weak to non-existent. With the increased mid to upper shear, storms
will be able to be longer lived than summer-time pulse variety.
However, mid level lapse rates of 4 to 5 C/km are unimpressive. The
main threat with any stronger storm will be gusty winds due to some
drier air at the surface. Again though, not that excited about the
severe potential.

&&

.LONG TERM...
(Monday night through next Sunday)
Issued at 303 PM EDT Sun Sep 22 2024

Key Messages:

1. A significant pattern change is in the offing during the extended
period with ridging aloft being replaced by a trough over the mid-
section of the nation. End result, increasing chances of much needed
rainfall across the region.

2. Ensemble Prediction System along with the latest deterministic
models are beginning to align with a significant tropical cyclone
developing in the Caribbean and Gulf of Mexico then moving north.

3. Depending on how this tropical cyclone interacts with the upper
trough over the mid-section of the nation and upper ridge over
Florida, this low may move into the Tennessee valley and southern
Appalachians late week.

Discussion:

For Monday night and Tuesday, first in series of short-waves lift
out of the developing mid-nation upper trough. Best dynamics will be
over the northern half of the region. MLCAPES will be in the 750-
1500 range with very limited mid-level lapse rates and shear. Main
concern will be lightning and gusty winds up to 30-40 mph Tuesday
afternoon.

For Tuesday night and Wednesday, the upper trough continues to
deepen over the mid-section of the nation with a frontal boundary
becoming quasi-stationary over the Tennessee valley. MLCAPES of 500-
100 with PWS around 1 standard deviation above normal per NAEFS.
Main concern will be locally heavy rainfall. Storm motion will be
nearly parallel with the boundary so can not rule out localized
runoff issues.

For Thursday into next weekend, a significant tropical system will
likely (40-60 percent probabilities of development in the Caribbean
and Gulf of Mexico) be affecting the Gulf coast states. Latest
Ensemble Forecast System (EPS) and deterministic models showing this
low moving north into the Tennessee valley and southern
Appalachians. Here are the potential concerns:

1. Heavy rainfall and flash flooding, especially the Plateau,
southeast Tennessee, and southwest North Carolina.
2. Windy conditions, especially for the favored mountain wave high
wind areas of far east Tennessee Mountains, and the Plateau.

As this system eventually develops, models will likely form more of
a consensus of track and timing, and potential impacts.

&&

.AVIATION...
(00Z TAFS)
Issued at 740 PM EDT Sun Sep 22 2024

VFR conditions forecast for most of tonight as showers are ending.
However, added MVFR fog at TRI for a few hours with model guidance
forecasting it and brief period of fog this morning. Ceilings
generally lower Monday with chance of showers and storms in the
afternoon. Have prob30 groups for thunderstorms after 00Z at CHA
and TYS. Better rain chances at TRI Monday for showers and storms
with earlier start and will have showers prevailing late in the
afternoon.

&&

.PRELIMINARY POINT TEMPS/POPS...
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Chattanooga Airport, TN             70  92  69  91 /   0  30  10  40
Knoxville McGhee Tyson Airport, TN  69  90  67  89 /  30  50  20  60
Oak Ridge, TN                       68  89  66  86 /  20  50  20  70
Tri Cities Airport, TN              65  85  64  83 /  30  70  40  70
-- End Changed Discussion --

&&

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

$$

SHORT TERM...TD
LONG TERM....DH
AVIATION...TD