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

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000 FXUS64 KMRX 070818 AFDMRX Area Forecast Discussion National Weather Service Morristown TN 318 AM EST Wed Dec 7 2016 .SHORT TERM (Today and Tonight)...
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A long wave trough migrates slowly east across the North Central states today through tonight with a couple weak shorter wavelength features ripping across the central and eastern portion of the nation. The first is building east into toward the Ohio Valley with a broken ac/ci deck with the second shifting east across the the Lower Mississippi Valley into the Southeastern U.S. tonight. A typical swath of low level moisture trails in the cold air behind a front advancing across the Appalachian rise early this morning. The low level layer is being lifted on the Cumberland Plateau and along the Smoky Mountains with some downsloping generating breaks in the Great Valley. Always difficult forecasting these scenarios in the valley in terms of whether to go mostly cloudy or mostly sunny in the valley. Will play it relatively cloudy early in the day with improvement to more sunshine toward mid day and during the afternoon as the moist layer gradually dissipates. Cold advection will be occurring during the day today with a northerly surface flow but the leading edge of stronger, arctic push won`t make it across the Cumberland Plateau until Thursday and Thursday night. The models depict a rather modest increase in moisture depth with the weak wave tonight, but not enough to get too excited about. There could be just enough weak lifting in the southwest flow aloft to generate a couple sprinkles or snow flurries across the forecast area tonight. The temperature profile would trend toward some flurries; especially later tonight into the early morning hours Thursday when low temperatures drop into the middle and lower 30s. But again, we`re talking about little more than trace precipitation amounts. .LONG TERM (Thursday through Tuesday)...The NAM has shown a drier trend for Thursday, with little if any QPF across our area, and mainly in the mountains and SW NC. Will keep POPs just in the slight chance range. What little precip there may be will transition to snow through the day as cold advection builds in behind the cold front. Temperatures may peak in the morning and hold steady or fall through the afternoon. On Thursday night/Friday morning the NAM is showing a 20 kt NW flow and an increase of moisture between 925-850 mb below -10C, which could be enough to produce some flurries in SW VA and the TN mountains. Highs on Friday continue to be the coldest that we have seen this season, in the 30s for most of the Valley. A building ridge and a dry W-NW flow will be across the region for Saturday, and temperatures will rebound into the 50s on Sunday as the ridge shifts to our east. Models show a low pressure system tracking across the Northern Plains and Great Lakes on Sunday, with its associated cold front crossing our area Sunday night/Monday, but with the GFS and ECMWF showing some timing differences. The Superblend solution of likely POPs Sunday night will be followed. For now, temperatures favor an all-rain event. A generally zonal flow aloft sets up late in the forecast period, which should give near normal temperatures on Monday and Tuesday. Beyond that, the models are hinting at another arctic blast in the Wednesday/Thursday time frame.
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Chattanooga Airport, TN 53 36 44 23 / 0 20 20 10 Knoxville McGhee Tyson Airport, TN 50 34 43 21 / 0 20 20 10 Oak Ridge, TN 51 33 43 21 / 0 10 20 0 Tri Cities Airport, TN 49 33 41 19 / 10 20 20 10
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