394
FXUS64 KMRX 121100
AFDMRX

Area Forecast Discussion
National Weather Service Morristown TN
700 AM EDT Wed Mar 12 2025

...New AVIATION...

.SHORT TERM...
(Today and tonight)
Issued at 252 AM EDT Wed Mar 12 2025

Key Messages:

1. Low relative humidity values and enough wind for elevated fire
danger this afternoon/early evening.

Discussion:

We start the period with broad ridging at the surface and aloft with
the atmosphere quite dry.  We will see plenty of sunshine today and a
warm afternoon, with highs in the 70s in most valley locations. Good
mixing this afternoon will result in relative humidity values
dropping into the 20s across nearly all of the area, and a few spots
may dip into the 15 to 20 percent range. These low RH values, along
with southwest winds of 5 to around 10 mph with some higher gusts,
will lead to increased fire danger. A fire danger SPS will be
issued.

A short wave will be approaching from the west tonight, but we will
just see an increase in mainly high clouds during the latter half of
the night. Temperatures will be above normal tonight as well.

&&

.LONG TERM...
(Thursday through Tuesday)
Issued at 252 AM EDT Wed Mar 12 2025

Key Messages:

1. A weak system will move through the region Thursday night into
Friday, bringing chances for rain, mainly in southern portions of
the area.

2. A very strong and dynamic system will impact the region Friday
through Sunday with mountain wave winds being the first impact
Friday night through Saturday.

3. Uncertainty still remains for Saturday PM severe weather chances,
but the latest trends suggest increasing possibility of a
significant severe weather event, especially along the Cumberland
Plateau and into southeast Tennessee.

4. Multiple rounds of heavy rainfall will lead to chances for flash
flooding, especially in the western half of the area.

Discussion:

At the start of the period, a weak system will be over the
Mississippi River Valley. Southerly flow ahead of it will advect
moisture into southern portions of the area with this system
bringing a return of rain chances Thursday PM into Friday morning.
Overall, moisture will be limited in our area, especially further
north and east of Knoxville, with rain chances being focused mainly
in the south. With the weak forcing and limited moisture, rainfall
totals are expected to be light overall. Low-end instability does
support a mention of thunder chances, but anything more organized
will be much further south.

On Friday, focus will turn towards the development of the weekend
system. Its development begins as a deep, negatively tilted trough
ejects out of the Rockies with the rapidly deepening surface low
progressing over Nebraska/Kansas and then into the Upper Mississippi
River Valley. This update is now able to include the NAM, which
suggests a track further south than many global solutions. The
central pressure will likely be below 980mb, values that are record-
breaking for portions of this region. The following sections outline
the potential hazards, ranked in order of occurrence:

1. Mountain Waves/Fire Weather

Ahead of this system, height rises and a broad southerly 850mb jet
will make for unseasonably warm temperatures. Additionally, a very
strong MSLP gradient will be in place across the region, especially
along the mountains. During the day on Friday, values of 40 to 50
kts are indicated to our west. By Friday night into Saturday, the
latest guidance (now including the 00Z NAM) suggests 65 kts or
greater. Similar conditions linger through Saturday or Saturday
night, suggesting a long-duration and higher-end mountain wave event
is increasingly likely. With rain chances likely to hold off until
at least Saturday morning, this also raises concerns for fire
weather, given the limited rain potential on Thursday.

2. Severe Weather

As the system tracks north, a very broad warm sector is expected to
develop from the Ohio River Valley southward. The initial surface
low will be far to north, but the consensus is for a mesoscale type
of low developing well to our west. Some discrepancies still exist
in its depiction, but model guidance shows impressive upper-level
dynamics and broad divergence over our area and northward. Soundings
show an impressively curved hodograph, deep-layer shear of 60 kts or
more, and low-level SRH of 300 to 500 m2/s2. The main question will
be the timing and extent of instability. Saturday evening, the
global models show what would likely be a robust QLCS over Middle
Tennessee and possible discrete cells ahead of it. For southeast
Tennessee and much of the Cumberland Plateau, SBCAPE of over 500
J/kg seems likely, even with the limited model resolution, with
lessened confidence further north and east. The outcome will
definitely depend on how much low-level moisture and instability is
realized, which could be greatly affected by more mesoscale factors
that are not clearly depicted. However, the current data shows
increasing confidence of ingredients that would support a
significant, all-hazards severe risk, especially in the
aforementioned areas. Ahead of the main line, 850mb winds in excess
of 65 kts raise concerns for potential damaging winds at a minimum.
A lot can still change as model data at this range is still largely
global-scale. But the recent trends are increasing confidence in a
possibly significant severe weather event, especially along the
Cumberland Plateau and southeast Tennessee.

3. Flooding

Ahead of the approaching cold front, very strong 850mb moisture
transport moves just west or into our area with PWATs reaching to
near record high values of 1.4 to 1.5 inches. There may also be some
decaying convection earlier on Saturday, followed by multiple rounds
later on Saturday. With the mean flow nearly parallel to the
boundary, training is increasingly likely. This risk would be
greatest in western portions of the area, but activity could persist
well into the night further east as downsloping diminishes.

Sunday through Tuesday

During the day on Sunday, the frontal boundary is expected to
progress east of the area with moisture also quickly exiting. This
will shift rain chances further east throughout the day. Expansion
of high pressure from the south will promote subsidence and cooler
temperatures Sunday night into Monday. By Tuesday, high pressure
shifts further east with a return of southerly flow. Another system
is likely to develop and track to our north mid-week, but warmer
temperatures are the main expectation for Tuesday.

&&

.AVIATION...
(12Z TAFS)
Issued at 516 AM EDT Wed Mar 12 2025

VFR conditions expected for the period all sites. Light winds
will become southwest around 10kts with some higher gusts this
afternoon, then back to light overnight.


&&

.PRELIMINARY POINT TEMPS/POPS...
Chattanooga Airport, TN             78  51  71  53 /   0   0  40  20
Knoxville McGhee Tyson Airport, TN  75  49  71  46 /   0   0  20  10
Oak Ridge, TN                       76  48  71  48 /   0   0  20  10
Tri Cities Airport, TN              72  44  71  43 /   0   0  10   0

&&

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

$$

SHORT TERM...
LONG TERM....BW
AVIATION...