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

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000 FXUS64 KMRX 201854 AFDMRX Area Forecast Discussion National Weather Service Morristown TN 254 PM EDT Thu Apr 20 2017 .Short term (Tonight and Friday)...
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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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