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

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000 FXUS64 KMRX 171915 AFDMRX Area Forecast Discussion National Weather Service Morristown TN 315 PM EDT Wed May 17 2017 .SHORT TERM (This Afternoon through Thursday)...
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A hot afternoon across the Southern Appalachians with temperatures at 3 pm in the mid to upper 80s at most locations. Current water vapor imagery and RAP upper air analysis indicates an upper level ridge along the Eastern Seaboard with a closed low moving to the northeast across the Central Plains. At the surface, high pressure is currently in place over the Atlantic with a surface low near Kansas City. Another mild night is forecast tonight with weak southwesterly flow throughout the atmosphere with lows generally in the 60s. The upper level trough will pass well to the north across the Great Lakes on Thursday. Continued weak southwesterly flow will aid in advecting in slightly higher moisture values with PW values increasing into the 1.2-1.4 inch range, which is around the 75th percentile for mid-May. Additionally, the forecast area will be in the left exit region of a 50-60 kt 300 mb jet. This will help in providing some weak synoptic lift for isolated to scattered showers and storms during the day on Thursday. Increased PoPs slightly from the previous forecast with best chances across the higher elevations. Expect there could be a stronger storm or two with SBCAPE around 1000-1500 J/Kg and 0-6 km Bulk Shear of 20-30 kts. The main threats from any strong storm would be small hail and gusty winds. Temperatures on Thursday will be a couple of degrees cooler than today with increased cloud cover forecast. However, temperatures will still top out well above normal with highs generally in the mid to upper 80s. .LONG TERM (Thursday night through Wednesday)... To start the extended period, a weak boundary to our north will help trigger showers and thunderstorms for Thursday through Saturday. With plenty of moisture, marginal instability, and a bit of shear, SPC has placed our forecast area in a marginal risk for severe storms. A cold front will approach the area on Sunday. With with better forcing and a bit of instability and shear, we should see a better chance for thunderstorms on Sunday. Behind the frontal passage, surface high pressure will build in for early next week. This is also the point where models begin to vary significantly. The GFS keeps us mainly dry through early Wednesday with a cold front hung up across the Plains and the associated low almost into Canada. In contrast, the ECMWF is much wetter across our area, with the low centered further south and more progressive, resulting in a chance of showers and storms as early as Tuesday. To account for these differences, I have included low end PoPs for the timeframe. $$
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&& .PRELIMINARY POINT TEMPS/POPS...
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Chattanooga Airport, TN 68 87 68 88 / 0 30 30 40 Knoxville McGhee Tyson Airport, TN 67 86 68 86 / 0 20 20 40 Oak Ridge, TN 66 87 67 86 / 0 30 30 40 Tri Cities Airport, TN 60 85 63 81 / 0 20 30 50
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&& .MRX WATCHES/WARNINGS/ADVISORIES... NC...None. TN...None. VA...None. && $$ MA/EMH

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