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

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000 FXUS64 KMRX 261913 AFDMRX Area Forecast Discussion National Weather Service Morristown TN 313 PM EDT Wed Jul 26 2017 .SHORT TERM (This Afternoon through Thursday)...
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Not seeing much on the radar early this afternoon just a couple of isolated shower/storms across the Southern Plateau and Valley. Expect this will continue to be the main area for any showers and storms throughout the remainder of the afternoon. Current water vapor imagery and RAP upper air analysis indicates a ridge extending across the Central and Southern Appalachians. PW values across the Southern Plateau and Valley are above normal with drier air to the north and any afternoon convection should be limited to the southern zones. A mild night is forecast again tonight with lows in the mid 60s to low 70s. Most ongoing convection should come to an end after sunset with just a small chance for an isolated shower or two across the mountains. The ridge will breakdown on Thursday as a mid-level shortwave trough tracks through the region. There are still some minor timing issues between the models with this system. Deep layer shear will increase slightly as the trough approaches into the 20-30 kt range for the 0-6 km bulk shear. SBCAPE values will range from around 1000-1500 J/Kg. The PW values will slowly increase on Thursday into the 1.7- 1.9 inch range, which is in the 75th-90th percentile for late July. With this, expect numerous to widespread showers developing ahead of the trough tomorrow afternoon. Some of these storms could be strong to marginally severe with damaging winds and localized flooding being the main threats. .LONG TERM (Thursday night through Wednesday)... We will likely start the period with ongoing showers and thunderstorms across the area ahead of an incoming upper-level trough. Not too often do we see a trough of the amplitude this late in the year. This system has been difficult to pin down and models still not in the best of agreement on the timing and evolution. For now, the more likely scenario looks to fall more in line with the GFS. The GFS suggests ongoing showers and storms to begin the period with some of this activity lingering through the overnight hours but with decreasing coverage. It appears that a second wave will move in Friday morning and continue through early afternoon. The initial activity Thursday night looks to be pre- frontal and more diurnally driven, thus the expected decrease in coverage overnight. The wave Friday morning is in response to the trough beginning to dig into our area and the arrival of the upper-level jet. Because of the timing on Friday morning, it looks like the risk will mainly be for localized heavy rainfall and some isolated wind damage. Shear does pick up with the advancement of the trough and jet into the area but instability isn`t overly high due to saturated sounding profiles. Because of the better upper-level support can`t rule out an isolated tornado but this isn`t the main concern. PWs are at max values for this time of year with values between 2.00 and 2.3 inches. The potential for flash flooding will have to be monitored especially with any training storms due to the mean flow being somewhat parallel to the boundary. Showers and storms push out of our area through the day on Friday and things should be relatively calm by Friday night. The good news is that cooler and drier air move in for the weekend as we are under the influence of northerly flow and it`s looking real nice...real nice. Saturday and Sunday we will see highs in the low to mid 80s with lows in the upper 50s to mid 60s during the overnight hours. RH values will be in the 40s on Sunday! Next week is looking pretty nice as well. Temps slowly climb through the period but rain chances are slim. By Tuesday and Wednesday slight chances for terrain based convection return but most areas stay dry. By wednesday our highs and lows are closer to normal with highs back in the upper 80s and lows in the mid to upper 60s.
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Chattanooga Airport, TN 74 91 74 86 / 10 30 20 70 Knoxville McGhee Tyson Airport, TN 73 90 73 82 / 10 40 40 80 Oak Ridge, TN 73 90 73 82 / 10 50 50 80 Tri Cities Airport, TN 68 86 70 81 / 10 60 60 70
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