Area Forecast Discussion
Issued by NWS Knoxville/Tri Cities, TN-- Highlight Changed Discussion --
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FXUS64 KMRX 201854
Area Forecast Discussion
National Weather Service Morristown TN
254 PM EDT Thu Apr 20 2017
.Short term (Tonight and Friday)...
-- Changed Discussion --Scattered convection firing along the Plateau and Mountains will
continue to develop this afternoon in an increasingly moist and
less unstable environment. SBCAPE values are progged to be around
1000-1500 J/kg this afternoon, with MLCAPE values progged to be
as high as 700-800 J/kg by 21-22z timeframe. There is very little
shear to work with, so am not expecting very much organization
with this activity (which will favor the high terrain thanks to
some added orrographic lift). However, a localized strong storm is
possible - with gusty winds and small hail being the main
threats. PWATs are progged to be around 1.2", so a quick heavy
downpous will also accompany any activity that develops.
These storms should die quickly with the loss of heating, resulting
in a mostly/partly clear evening. However, clouds will quickly be
on the increase after midnight as a weak front drops south out of
KY late tonight into Friday morning. This will generate another
round of slower moving showers and storms progged to move into the
northern half of the region between 09-15z. The main threats with
this activty will be localized flooding due to the slow-moving
nature of these storms -- as well as gusty winds and some small
hail. A better threat for scattered severe storms may evolve south
of I-40 south (which will be precip free longer) as the
atmosphere destablizes in the afternoon thanks to some better
heating. The front will sag south providing a focus for more
organized convection there between 18-00z.
.Long term (Friday night through Thursday)...Friday night...the
boundary that remains over the area through Friday moves north...
with a low pressure system and associated front approaching the
southern Appalachians from the west. Scattered rain showers will
continue through Friday night in to early Saturday morning with
just enough instability to include a slight chance of thunder
in the forecast.
Saturday...rain chances begin increasing through the morning
with enough lift...instability...and moisture for scattered
showers and thunderstorms to become more widespread through
the early afternoon. With minimal wind shear...high PW values
and weak lift...strong storms will be possible with a low chance
for isolated severe. By Saturday evening...the low level winds
shift from SW to W...with low level warm air advection and moisture
decreasing. As cooler drier air moves in to the area through
Saturday night...thunder becomes limited...with only scattered
light showers remaining through the night.
Sunday...cooler and drier air will continue to advect in to the
area behind the low pressure with enough wrap around moisture to
keep light scattered showers in the forecast through the afternoon.
Temperatures will be several degrees below normal as opposed to
several degrees above normal that has been occurring for several
days. By Sunday late afternoon...the low pressure will be well
east of the Appalachians...with rain chances quickly diminishing.
Sunday night through Thursday...a weak ridge moves over the area.
The NW flow slowly shifts to the W and then SW through the
beginning of the work week...with a gradual warming trend to
temperatures through the rest of the extended. Mostly clear
skies and dry weather will be present.
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