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

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FXUS64 KMRX 160635

National Weather Service Morristown TN
235 AM EDT Sun May 16 2021

.SHORT TERM...(Today and Tonight)...

Key Messages:

1. Light rain possible across mainly the northern areas today.
Dry, cloudy, and warm conditions expected in the south.

A weak warm front was situated roughly along the I-40 corridor
across most of TN early this morning. Regional radar imagery shows
some light returns across middle and western TN as well as western
KY, that are shifting ESE with time, associated with weak isentropic
lift along the 295k and 300K surfaces. Guidance lifts the warm front
north of the Knoxville metro area during the morning hours as low
level southerly flow strengthens slightly in response to a shortwave
ejecting east across the southern plains. Accordingly, the best low
level ascent and rain chances will also shirt north to areas
nearer the TN/VA border. Suspect rainfall will be light for the
most part today as forecast soundings show only meager
instability. Freezing levels are fairly low so there could be a
few stray lightning strikes, but will leave thunder out of the
forecast due to weak capping aloft and aforementioned low
instability. Kept PoPs limited to chance levels and areas mostly
north of I-40 as coverage will likely be spotty.

Temperature wise, didn`t veer too much from the previous forecast
and the latest NBM guidance. Upper 70s to near 80 seem plausible in
the southern valley, with upper 60s to low 70s in the north. That
said, if rainfall is more widespread than forecast, afternoon highs
will be several degrees cooler than forecast. Guidance seems split
about 50/50 on what scenario will unfold, with the cooler guidance
not lifting the warm front north as quickly as others and yielding
rainfall well south of the Knoxville metro area. It`s too early to
tell exactly how this will go and will likely be further nailed down
with a mid-morning update from the day shift. Slight to low-end
chance PoPs will continue in SW VA and far northeast TN into the
overnight hours as weak impulses continue to shift east across the
warm front boundary. Overnight lows will be fairly mild, with mid
50s on tap for the valley and perhaps upper 50s for mid-slope
locales that settle into the inversion.


.LONG TERM...(Monday through Saturday)...

Key Messages:

1. Isolated to scattered showers and thunderstorms are expected
Monday and Tuesday with an isolated thunderstorm possible in the
mountains on Wednesday. Dry weather is expected Thursday through

2. Above to well-above normal temperatures are expected late in the
week and into the weekend with much of the area reaching well into
the 80s by Friday and Saturday.

Monday through Wednesday

To start the period, a closed low will be in place over the southern
Rockies with shortwave ridging across the Mississippi Valley. At the
surface, a frontal boundary will also be noted, extending from the
central Great Plains and into Kentucky. With this initial setup, the
region will be south of the front with southerly flow lending WAA
and increasing Gulf moisture. The front, along with embedded
shortwave energy and subtle upper jet divergence, will make the case
for convection on Monday, especially in northern areas near the
boundary. Vertical profiles reveal instability sufficient for
mention of thunderstorms, but shear/dynamics are both too to support
strong/severe convection. During the day on Tuesday, the frontal
boundary will remain nearly stationary, making for continued
isolated to scattered showers and thunderstorms. The NAM is fairly
aggressive with destabilization, which may be sufficient for an
isolated stronger storm. But, the pattern suggests that this
potential is very limited. By Wednesday, the Bermuda High
strengthens to ~1,030 mb, making for increasing subsidence across
the eastern CONUS. This also comes as ridging strengthens across the
eastern U.S., following increasing troughing in the west, driven by
an increasingly negative Pacific/North American pattern (PNA).
Moisture and orographic considerations may be sufficient for an
isolated afternoon thunderstorm in the TN/NC mountains but is
unlikely elsewhere.

Thursday through Saturday

Later in the week, high pressure builds further to the west, also
shifting the Gulf stream of moisture westward. This also comes as
the increasingly negative PNA continues to enhance eastern ridging,
especially by Friday. By the end of the period, ensemble and
deterministic model guidance suggests 500mb heights to exceed 590
dam over the southeast, which is over 2 standard deviations above
normal. This is also confirmed by the NAEFS table. As such, above to
well-above normal temperatures are looking more and more likely to
end the period and just beyond as well. It can also be noted that
the area will likely remain dry during this timeframe with the lack
of moisture. With all of these considerations, surface moisture
looks to remain fairly low with any resulting heat indices likely
being much below heat advisory criteria. Nevertheless, this will
prove to be the warmest period of the season, by far.




Scattered showers will spread east through the TN valley through
the day in the vicinity of a warm front. Expect mostly VFR
conditions through the period, with perhaps some MVFR ceilings at
KTRI late as rainfall coverage increases. Most precip should be
north of KTYS, but did add in a VCSH for a few hours after
daybreak due to light returns moving eastward across middle TN
early this morning. Winds are expected to be pretty light, but
should pick up an increasingly southerly direction at KCHA and
KTYS as warm front lifts north.



Chattanooga Airport, TN             81  57  83  62  82 /  10   0   0  10  20
Knoxville McGhee Tyson Airport, TN  76  56  80  59  80 /  10  10  10  10  20
Oak Ridge, TN                       76  55  80  58  80 /  20  10  10  10  30
Tri Cities Airport, TN              70  55  74  54  77 /  30  30  50  20  30



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