Area Forecast Discussion
Issued by NWS Knoxville/Tri Cities, TN

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FXUS64 KMRX 221720 AAA

Area Forecast Discussion...UPDATED
National Weather Service Morristown TN
120 PM EDT Wed May 22 2024

...New AVIATION...

Issued at 1048 AM EDT Wed May 22 2024

A few showers and thunderstorms are located in northern portions
of Middle TN, moving NE. This activity could reach our northern
Plateau counties this afternoon, and the forecast of low/slight
chance PoPs there looks good. Models show development of showers
in NW NC near our border with RNK, and the forecast of a slight
chance PoP in the NE TN mountains looks good. The update will make
a slight upward adjustment in temperatures based on yesterday`s
highs and the latest guidance, plus a few ob sites are running
warmer than forecast.


(Today and tonight)
Issued at 313 AM EDT Wed May 22 2024

Key Messages:

1. Most locations will be dry and warm again today.

2. Tonight chances for scattered showers or storms will increase
along the Cumberland Plateau first then eastward late.


Our widespread pause in the rain and storms has nearly come to an
end. We get one last nice today before the gloom comes back.
HREF, NBM, and soundings indicate increased cloud coverage today,
with the bulk of it existing higher up in the atmosphere.
Nighttime Microphysics currently shows cloud debris overhead with
thicker clouds west of us where scattered convection has been
taking place. Expecting a mix of sun and high clouds today, and
the Sun should shine through this afternoon and help boost us back
into the mid 80s once again.

On a wider level, a shortwave pivoting within the mean upper
level flow will be orbiting around a Great Lakes cyclone. In the
Ohio River valley a subtropical jet will waver in strength and a
weak surface front will be slowing down out in the vicinity of the
Ohio River. High resolution guidance has a few viewpoints of
evolution and timing of potential clusters of thunderstorms, so
will side with the HREF`s mean precipitation depiction for this
discussion. HREF has some available CAPE, but generally less than
1,000 J to work with along with less than supportive thermal
profiles given a nighttime arrival for better rain chances.
Chances of rain should generally increase after dark tonight,
primarily after midnight, first along the Cumberland Plateau then
slowly eastward as we approach dawn on Thursday. There doesn`t
appear to be any severe concerns, but weak elevated thunderstorms
to rain showers is the most likely outcome. Temperatures tonight
will be mild and muggy, bolstered by dewpoints in the low 60s once
again and much thicker cloud cover.


(Thursday through Tuesday)
Issued at 313 AM EDT Wed May 22 2024

Key Messages:

1. An unsettled period with multiple rounds of showers and
thunderstorms for the period.

2. Some of the storms may be strong to severe at times, with the
higher chances for severe storms looking like the weekend and
possibly into Monday. Localized flooding will also be possible.


We start the period with a surface cold front to our north, and
broad west-southwest flow aloft. The surface cold front will weaken
and sag south before stalling over the region.  Some weak short wave
energy will slide across the area Thursday, and showers and
thunderstorms can be expected. Models soundings and ensemble data
generally suggest modest instability with MLCAPES likely in the 500
to 1000 J/kg range. However, there will be some shear to aid storm
organization with effective shear mostly around 25-35 kts indicated,
so a few stronger to severe storms will be possible although
Thursday does not currently look like a significant severe
thunderstorm day. Damaging winds and hail will be the primary

Thursday`s system will just be the first of a series of short waves
that will move across the area during the period, bringing multiple
rounds of showers and thunderstorms. The details of each individual
wave are still unclear given model differences and the inherent
uncertainties of handling these types of features in the longer
ranges. However, ensemble data does suggest a trend of increasing
convective energy, with mean SBCAPES peaking in the 1500-2000 range
each day for much of the area by the weekend. An uptick in shear is
also indicated, thus the threat of strong to severe storms is
expected to increase by the weekend and may continue Monday as well
although there are timing differences with a front that moves
through late in the period.  Damaging wind gusts and large hail will
likely be the primary threats. In addition,the multiple rounds of
precipitation will bring a threat of localized flooding in areas
that see repeated or prolonged heavy rainfall.

Models do show a more significant upper trough moving in late in the
period and the aforementioned cold front sweeping through.  However,
they are struggling with the timing of this, and thus the end of the
period sees heightened uncertainty.  There may still be quite a few
showers/storms around on Tuesday, but there is also a possibility
that drier air will be moving in behind the front providing a
significantly drier day. The NBM has chance PoPs for Tuesday which
looks reasonable given the current level of uncertainty.


(18Z TAFS)
Issued at 118 PM EDT Wed May 22 2024

Scattered clouds around 4-5 kft are expected at the TAF sites
through the afternoon and evening, with winds from the SW at 8-12
kt. Overnight, some showers may pass just north of TRI, so will
mention VCSH there for now. A greater chance of showers and
thunderstorm arrives late in the TAF period at TYS and TRI, and a
PROB30 for MVFR vis/cigs with thunderstorms will be mentioned.


Chattanooga Airport, TN             88  68  86  67 /   0  20  40  30
Knoxville McGhee Tyson Airport, TN  87  67  81  65 /  10  20  70  40
Oak Ridge, TN                       86  66  80  64 /  10  30  70  40
Tri Cities Airport, TN              87  63  78  61 /  10  20  80  40