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

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000 FXUS64 KMRX 161954 AFDMRX Area Forecast Discussion National Weather Service Morristown TN 254 PM EST Sat Dec 16 2017 .SHORT TERM...
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(Tonight and Sunday)... The surface high pressure is shifting to the south and east of the area with weak southerly to southwesterly winds at the surface. Current water vapor imagery and RAP upper air analysis indicates a closed low spinning to the west near the Texas and New Mexico line. A shortwave ridge ahead of this trough can be noted across the Northern Gulf of Mexico into the Central Plains. These higher heights aloft in combination with southerly flow and mostly sunny skies have allowed temperatures to rise up into the 40s this afternoon. A few locations will top out in the 50s across the Central and Southern Valley. For tonight, the trough to the west will lift northeastward across the Southern Plains. At the same time, a warm front will begin to lift slowly north across the Northern Gulf of Mexico. Isentropic lift ahead of this boundary will saturate the atmosphere from the top down. The atmosphere is rather dry currently with PW values in the 0.2-0.3 inch range, therefore, it will take some time before precipitation begins to fall. Overnight lows will be a few degrees warmer tonight and range from the upper 20 to low 30s. Precipitation will begin near daybreak tomorrow across the Southern Valley and Plateau. With temperatures near freezing during this time there could be a brief period at the onset of light sleet or freezing rain but do not expect any impacts due to the light and brief nature. Just above the surface at the 850 mb level temperatures will be well above freezing with values in the 6-8 degree Celsius range. A cool and rainy day is generally forecast for Sunday with the boundary remaining to the south. It looks like the best isentropic lift will be over the area late morning to mid afternoon. PW values during this timeframe will increase to around an inch. Model soundings indicate that this saturations will be below 500-600 mb and due to this expect rainfall amounts will be light and generally under a quarter of an inch. With the persistent clouds and light rain, reduced highs on Sunday with temperatures only making it up into the low to mid 40s. .LONG TERM...(Sunday night through Friday)... Models continue to come into better agreement through midweek with upper ridging resulting in above normal temperatures across the area. Minor shortwave traverses through westerly flow to the south of our area on Monday with slight chance PoPs across our southern sections. Stronger shortwave trough moves eastward across the Southern Plains on Tuesday with southwest flow and isentropic lift increasing across the Southern Appalachians. This will result in chance PoPs for most on Tuesday with likely PoPs possible across southern portions of the forecast area by late Tuesday and Tuesday night. The trough axis will move through the area on Wednesday with the strongest lift and moisture combination across southeast TN/southwest NC. In the wake of this trough, models are in good agreement with ridging developing across the southeast CONUS leading to a moist southwest flow across the area ahead of a large scale trough and cold front late next week and into next weekend. It must be emphasized that there continues to be poor agreement on the exact track and strength of the system next weekend as well as the exact position of the surface cold front. There is impactful differences between model solutions with poor run to run model consistency. Highlighting the uncertainty, even the GEFS ensembles have fairly large spread with a standard deviation for surface temperatures next Saturday around 10 deg F. In short, there is still a lot of uncertainty with how things will setup next weekend, but there is growing certainty and agreement for a pattern favorable for precipitation. However, it would still be irresponsible to speculate with any certainty on specific precip type or coverage next weekend through Christmas based on current modeling and ensemble solutions.
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Chattanooga Airport, TN 32 47 43 60 / 10 60 50 20 Knoxville McGhee Tyson Airport, TN 31 45 40 59 / 0 60 40 10 Oak Ridge, TN 31 44 39 59 / 0 60 30 10 Tri Cities Airport, TN 28 46 37 57 / 0 40 30 10
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