-- Discussion containing changed information from previous version are highlighted. --
272
FXUS64 KMRX 220158
AFDMRX

Area Forecast Discussion
National Weather Service Morristown TN
958 PM EDT Sat Sep 21 2024

...New UPDATE...

.UPDATE...
Issued at 955 PM EDT Sat Sep 21 2024

New update for the rest of the night as showers have almost ended
with the last activity over the southern Tennessee Valley. Blow
off clouds from upstream convection in middle Tennessee and south
central Kentucky was keeping variable high clouds over the east
Tennessee Valley but this has been decreasing over the last hour.
Have added fog over much of the region for late tonight and early
Sunday as the air mass is not quite as dry as this morning and
yesterday with fog likely forming where winds are calm and skies
partly cloudy. A few areas may have dense fog. Lows still expected
to be in the mild 60s overnight tonight. Updated forecast already
sent.

&&

.UPDATE...
Issued at 728 PM EDT Sat Sep 21 2024

A few showers have developed across the southern part of east
Tennessee and another area of showers and storms was moving
southeast toward the western plateau just to the west of the
county warning area. Added some low rain chances for rest of the
evening in this area. Elsewhere could see a few showers in the
northeast and an isolated storm but that should end by mid
evening. Hourly temperatures and dew points also updated. Updated
forecast sent.

&&

.SHORT TERM...
(This evening through Sunday)
Issued at 240 PM EDT Sat Sep 21 2024

Key Messages:

1. Isolated showers and possibly a thunderstorm through early
evening. Greatest coverage over Plateau and southwest
Virginia/northeast Tennessee.

2. Unseasonably hot conditions today and Sunday. Temperatures
near 10 degrees above normal.

Discussion:

A weak wave is rotating around the upper ridge across the Ohio
valley and central Appalachians. Meso-analysis shows best
instability across the northern Plateau into southwest
VA/northeast Tennessee with MLCAPES of 500-1000. These areas have
the best chance of showers and possibly a thunderstorm through
early this evening.

Locations that see any rainfall will have a decent chance of fog
development overnight. Otherwise, partly cloudy sky with a stray
shower possible.

For Sunday, upper ridge remains over the region allowing for
another unseasonably hot day with temperatures from 5 to 10
degrees above normal. HREF and deterministic models agree with
another short-wave moving toward the area during the afternoon.
MLCAPES will be best across the Plateau and western sections of
the Tennessee valley. Widely scattered to scattered late day
showers and possibly a thunderstorm are expected.

&&

.LONG TERM...
(Sunday night through next Saturday)
Issued at 240 PM EDT Sat Sep 21 2024

Key Messages:

1. Increasing chance of convection during the extended but
confidence is low on probabilities and timing due to evolution of
the tropical system in the gulf.

2. Can not stress enough to not focus on deterministic models on
timing, strength, and location of tropical systems late this week.
Main message is that the CPC is depicting a greater than 60
percent probability of tropical development in the Gulf.

Discussion:

For Monday through Wednesday, ensemble cluster analysis shows a
slow change in the upper level pattern with a northern and
southern stream short-waves weakening the upper ridge over the
area. This will allow for increasing coverage of convection over
the region especially along and north of interstate 40. Some much
needed rainfall is possible.

For Thursday through Saturday, forecast confidence decreases as
CPC probabilities shows tropical development in the gulf combined
with continued upper trough over the mid-section of the nation.
The evolution of these systems will play a large role in the
timing, direction, and strength of the tropical system and
associated rainfall.

&&

.AVIATION...
(00Z TAFS)
Issued at 702 PM EDT Sat Sep 21 2024

A few showers developing across east central and SE TN early
tonight so expect a few hours of isolated showers/storms but not
enough coverage to put in evening TAF forecast. Main concerns
later tonight will be fog development late tonight/Sunday morning
at TRI between 08-12Z Sunday. Due to lack of fog last night and
overall dry soil, forecasting only A TEMPO MVFR. Otherwise, VFR
conditions expected with weak surface pressure gradient continuing.
Only light wind speeds. Isolated showers expected to develop across
the Plateau counties, SW Virginia and NE Tennessee Sunday but
confidence is not high enough to include in TAFs.

&&

.PRELIMINARY POINT TEMPS/POPS...
-- Changed Discussion --
Chattanooga Airport, TN             71  94  70  93 /  20  10   0  20
Knoxville McGhee Tyson Airport, TN  69  92  68  90 /  10  10  20  30
Oak Ridge, TN                       69  91  67  89 /   0  20  20  40
Tri Cities Airport, TN              63  88  63  84 /   0  10  20  60
-- End Changed Discussion --

&&

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

$$

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