720
FXUS64 KMRX 260209
AFDMRX

Area Forecast Discussion
National Weather Service Morristown TN
1009 PM EDT Sat May 25 2024

...New UPDATE...

.UPDATE...
Issued at 959 PM EDT Sat May 25 2024

Temperatures this evening were running a bit cooler than
expected, so hourly values were reduced to encompass this trend.
Also, there has been virtually no precipitation/convection in or
near the area, so PoPs were also reduced. Otherwise, the remaining
elements were updated based on the latest high-res guidance. The
main question for tonight will be how much fog develops by the
morning, which is likely due to the recent rainfall. With respect
to the severe weather event in the Sunday to Sunday night period,
some of the 00Z high-res guidance has come in through the event.
The sources paint a picture of a significant severe weather event
across much of the Ohio and Tennessee River Valleys with multiple
rounds of discrete and linear convection. The overnight/early
morning forecast update will examine the event closer, but the
initial indications suggest the potential for a significant and
long-duration event in our area.

&&

.SHORT TERM...
(This evening through Sunday)
Issued at 322 PM EDT Sat May 25 2024

Key Messages:

1. Fog and low clouds will likely form overnight in a moist and
stagnant air mass.

2. Showers and thunderstorms are possible on Sunday as upstream
convection may move into the area, although confidence is low on
timing and intensity of storms.

Discussion:

Mesoanalysis shows a marginally unstable environment that has been
worked over by morning convection and cloud cover. We are starting
to see some clearing in southern sections, and with heating and
mid/upper 60s dewpoints, some isolated showers may pop up through
the rest of the afternoon. These diurnally-driven showers should
dissipate around sunset. The overnight period will likely remain
dry, but with fog and low clouds developing in the moist air mass.

Confidence is low regarding the potential for showers and
thunderstorms tomorrow, and their timing, if they occur at all.
The NAM is quite bullish on bringing a MCV from today`s expected
convection in TX/OK across northern MS/AL and southern TN tonight,
reaching our area early Sunday morning. The CAMS show nothing at
this time, and they keep storms from a MCS well to our northwest
until the mid to late afternoon. The NAM solution will be
discounted but some slight to low chance PoPs will be put in for
the Plateau in the morning. As mentioned, the CAMS bring the
remains of a MCS across KY Sunday morning and early afternoon, and
into the northern Plateau around 19-22Z. The environment at this
time may be capable of supporting some strong to severe storms
with damaging winds and large hail, with 0-6 km shear of 35-45 kt,
MLCAPE near 2000 J/kg, and DCAPE of 1000-1200 J/kg, but that
assumes that there was no morning convection to stabilize the
environment. The areas that would have the potential for severe
storms Sunday afternoon and evening would be north of I-40. The
midnight shift will likely have a better feel for the severe
threat on Sunday as the evolution of upstream convection will be
more evident then. For now, PoPs on Sunday afternoon will be kept
in the chance range.

&&

.LONG TERM...
(Sunday night through next Saturday)
Issued at 322 PM EDT Sat May 25 2024

Key Messages:

1. Round of strong to severe storms should move through Sunday night
into Monday morning.

2. Drier and cooler weather to begin Tuesday night through the end of
the week.

3. A slight chance of showers and thunderstorms again Saturday with a
warming trend.

Discussion:

Sunday night the best chance for severe storms still looks like will
be after midnight and into the mid to late morning hours Monday as a
slow moving low moves towards the Great Lakes Region, ahead of a
front through the Ohio and Tennessee Valley Sunday night and Monday.
The convection will be ongoing over the upper Midwest and western
Great Lakes early Sunday evening. Some guidance indicates storm
development ahead of the line across TN which would have a threat
for all severe hazards given the steep mid-level lapse rates and
strong low-level and deep-layer shear. Any storms that form during
the evening could be supercells and could occur during the 00Z to
around 04z before the line moves in or develops to the west.

This convection should become more widespread during the late
evening and move southeast ahead of the cold front. The primary
threat for the convection will be damaging winds although there will
be some tornado threat given the strengthening low-level jet during
the evening hours. The orientation of the line will be the primary
limiting factor to a greater tornado threat with this overnight
squall line. The NAM model shows rapid development southward from
the Ohio Valley across central KY southwest to north MS about 06Z
Monday and moving quickly east across the forecast area between 09
and 15Z. The squall line will have a very moist/unstable boundary
layer and strong deep layer shear to support a damaging wind threat
through the overnight hours into Monday morning. Forecast soundings
indicate the environment could be conducive to producing all types
of severe weather with damaging straight-line winds as the most
widespread threat, however large hail, flooding, and a few tornadoes
cannot be ruled out at this time. Rainfall amounts Sunday night and
Monday morning could be as high as around 2 inches across the
plateau, southeast TN and southwest NC.

Late Monday morning or afternoon the front will move through the
region bringing in drier air from the northwest, but additional
storm development is possible if the front slows down and the
atmosphere destabilizes in the afternoon hours behind the morning
convection along the front. Do not think any later storms will be
severe but there could still be gusty winds and small hail possible.

Late Monday or Monday evening the precipitation should end as the
front moves east of the Appalchians. There will be clearing skies
Tuesday and much lower dew points in the 50s. Temperatures will be
in the mid 70s northeast to the lower 80s southwest. The deep upper
trough moving in Tuesday behind the front and surface high from the
northwest will keep the more pleasant conditions around through
Friday. Across the NE US the trough elongates some late Wednesday so
some clouds may move into the northeast sections and produce a
sprinkle or two. Temperatures will moderate to near seasonal normals
by Saturday as the upper trough/elongated upper low moves off the
east coast. Weak ridging moves into the TN valley and some moisture
to the west may be enough to produce a few showers and storms late
Saturday.

&&

.AVIATION...
(00Z TAFS)
Issued at 701 PM EDT Sat May 25 2024

Aviation conditions for most of the night will be quiet with a few
clouds around 5,000 feet and fairly light winds. Towards the early
morning hours, development of fog/low clouds is expected,
especially at TYS and TRI. At these sites, reduction to IFR or
less is likely with the worst conditions being indicated for TRI.
Morning fog/low clouds will gradually dissipate through the
morning with VFR prevailing into the afternoon hours. The main
question for the late afternoon to the evening will be if and
when storms (strong to severe) move through the area before 00Z.
Currently, TYS and TRI look to have higher chances for impact at
the terminals from 20Z onward. Right now, this still remains
uncertain, but any storms during this time could produce damaging
winds and large hail with a lower but still present tornado
threat.

&&

.PRELIMINARY POINT TEMPS/POPS...
Chattanooga Airport, TN             68  88  70  86 /  20  30  60  60
Knoxville McGhee Tyson Airport, TN  66  86  68  81 /  20  30  70  80
Oak Ridge, TN                       65  86  67  82 /  20  40  70  80
Tri Cities Airport, TN              62  84  64  79 /  20  50  50  90

&&

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

$$

SHORT TERM...DGS
LONG TERM....TD
AVIATION...BW