348
FXUS64 KMRX 250743
AFDMRX

Area Forecast Discussion
National Weather Service Morristown TN
343 AM EDT Wed Sep 25 2024

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

.SHORT TERM...
(Today through Thursday)
Issued at 335 AM EDT Wed Sep 25 2024

Key Messages:

1. Scattered to numerous rain showers and thunderstorms to continue.
Torrential rainfall is possible, with flash flooding an increasing
risk with time.

2. A low end risk for severe weather this morning to around noon,
before more widespread shower development ends severe risk. While
a strong to severe wind gust is the more probable threat, cannot
rule out a potential tornado.

Discussion:

The upper trough that is dominating the southeast`s weather pattern
will pinch off as an upper level low today and begin its pivoting in
Missouri and Arkansas through Thursday. This is the main timeframe
of what the research literature calls a predecessor rainfall event.
Rich moisture and broad but strong ascent will allow for rain
showers and some thunderstorms to develop and move northwards
through the southern Appalachian region. Additionally, the cold
front associated with the upper low will stall out just to our west,
adding a focal point ahead of the front for showers to move parallel
to.

Hard to say what, if any, severe threat we have today. HRRR and NAM
indicate the best timeframe for a possible severe storm is in the
morning hours after sunrise to around noon, before rain showers
become too numerous and cut down on available convective
instability. But during that few hour period, effective shear is
of course still strong, and we might be just able to squeak a
surface based storm out paired with strong 3CAPE. The SPC has our
eastern half in a marginal risk for a low end tornado and wind
threat, which aligns with the potential seen in model soundings.
High res CAMs indicate by around noon we should see a blossoming
of showers and elevated thunderstorms begin moving south to north
across the region. This will eventually evolve into two periods of
widespread rain, one Wednesday afternoon into evening, and the
second more notable period likely much of Thursday. The Weather
Prediction Center has given us a moderate risk of excessive rain
along the mountains both today and Thursday, with a high risk in
North Carolina on Thursday. It is rare to have such elevated risk
for successive days and points to the high end potential for
numerous flash flooding events as the event progresses this week.

&&

.LONG TERM...
(Thursday Night through Tuesday)
Issued at 335 AM EDT Wed Sep 25 2024

Key Messages:

1. Remnants from Tropical Storm Helene expected to impact the region
Thursday night and Friday. Flooding possible as well as strong gusty
winds.

2. Unsettled weather will continue throughout the long term period
with temperatures near seasonal norms.

Discussion:

The latest official NHC forecast has the center of the remnants of
Helene approaching Thursday night and moving across southeast TN and
off to our NW during the day Friday. With this track, we will see
strong wind gusts and some additional very heavy rain. The strongest
winds are expected to be over the higher elevations of the
mountains, with gusts in excess of 60 mph likely. Some valley areas
will likely see gusts in excess of 40 mph as well. Still enough
uncertainty of the exact track of the system to hold off a bit
longer on any high wind watch for the mountains, but one may be
needed soon.  Heavy rainfall can be expected as well Thursday night
into Friday. The highest amounts are expected to be across the
eastern mountains as well as southeast Tennessee, with current QPF
values in excess of 3 inches for the 24 hour period. Downslope flow
should help to hold precip values a bit lower for the central and
northern areas west of the mountains, but even there amounts are
expected to exceed an additional inch. Depending on how much rain
and flooding occurs with the earlier rainfall, the flood watch may
end up needing to be extended in these northern and central areas.
And flooding continues to be a major concern for southeast TN and
the eastern mountain areas including SW NC, and the flood watch
there will be continued as is.  Flash flooding, and significant
river flooding are both a concern.

Once the tropical system exits, the upper low that will be hanging
over the region will continue to keep us with chances for showers
and a few thunderstorms over the remainder of the period.
Temperatures will generally not be too far from seasonal normals.

&&

.AVIATION...
(06Z TAFS)
Issued at 1258 AM EDT Wed Sep 25 2024

Scattered showers and thunderstorms to continue overnight,
primarily north of KCHA. VFR conditions will deteriorate to MVFR
VIS for much of the area after 18z tomorrow as a more widespread
round of showers and storms spreads across the area. Low
confidence on when CIGs deteriorate to MVFR, but will be more
likely during moderate to heavier rainfall. Outside of
thunderstorms, winds should remain light and somewhat variable.

&&

.PRELIMINARY POINT TEMPS/POPS...
Chattanooga Airport, TN             77  65  73  67 /  80  60  80  90
Knoxville McGhee Tyson Airport, TN  75  65  73  67 /  90  90  80  90
Oak Ridge, TN                       74  63  72  65 /  80  80  80  90
Tri Cities Airport, TN              76  64  73  66 /  90  90  80  80

&&

.MRX WATCHES/WARNINGS/ADVISORIES...
NC...Flood Watch from 11 AM EDT this morning through Friday afternoon
     for Cherokee-Clay.

TN...Flood Watch from 11 AM EDT /10 AM CDT/ this morning through
     Friday afternoon for Bledsoe-Blount Smoky Mountains-Bradley-
     Cocke Smoky Mountains-East Polk-Hamilton-Johnson-Marion-
     McMinn-Meigs-Northwest Monroe-Rhea-Sequatchie-Sevier Smoky
     Mountains-Southeast Carter-Southeast Greene-Southeast
     Monroe-Unicoi-West Polk.

     Flood Watch from 11 AM EDT this morning through Thursday morning
     for Anderson-Claiborne-Grainger-Hamblen-Hancock-Hawkins-
     Jefferson-Knox-Loudon-North Sevier-Northwest Blount-
     Northwest Carter-Northwest Cocke-Northwest Greene-Roane-
     Sullivan-Union-Washington TN.

VA...Flood Watch from 11 AM EDT this morning through Thursday morning
     for Lee-Russell-Scott VA-Washington VA-Wise.

&&

$$

SHORT TERM...Wellington
AVIATION...Wellington