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

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306 FXUS64 KMRX 170700 AFDMRX Area Forecast Discussion National Weather Service Morristown TN 300 AM EDT Thu Aug 17 2017 .SHORT TERM (Today and Tonight)... An upper-level ridge centered across the Gulf Coast will remain in place for much of the day Thursday resulting in a typical summer pattern across the region. The combination of forcing mechanisms due to terrain based differential heating and upper-level divergence due to a 200-300mb jet streak will act to position highest PoPs across northeast Tennessee, southwest Virginia, and across the TN/NC mountains. Modest MLCAPE of 1000-2000 J/Kg and effective bulk shear of 10-15 kts means that the risk of severe weather is low with mainly ordinary convection expected. However, the atmosphere remains very moist with PWs of 1.8-2.0 inches, so some rain showers and thunderstorms may produce heavy rainfall. High temperatures are once again expected to be several degrees above normal. The upper-level ridge will finally start to break down Thursday night in response to an approaching trough and surface cold front with chance PoPs across the area. Highest PoPs on Thursday night expected to be across western portions of the CWA after midnight. With increasing cloud cover and a very moist boundary layer, low temperatures are forecast to be 5-8 degrees above normal on Thursday night. .LONG TERM (Friday through Wednesday)... A cold front will be moving into the forecast area on Friday as a shortwave trough passes to the north across the Great Lakes. PW values ahead of the boundary will be in the 1.9-2.1 inch range. These values are in excess of the 90th percentile for mid-August. The timing of this front continues to be late morning into the early afternoon. Due to the timing of the front, SBCAPE values will only increase into the 750-1500 J/Kg range with 0-6 km Bulk Shear values in the 10-15 range with the upper level trough passing well to the north. Therefore, not expecting any strong to severe storms with this frontal passage. PW values will crash behind the boundary with much drier air advecting in from the northwest. Chances for showers and storms will come to an end from northwest to southeast during the afternoon and evening on Friday. PW values by Saturday morning will be in the 0.7- 0.9 inch range, which are around the 10th percentile for mid-August. Another mid-level shortwave trough swings across the Ohio River Valley on Saturday, but with the lack of lift, only expect a slight increase in cloud cover. Overall, the weekend is forecast to be dry with highs in the mid 80s to low 90s both Saturday and Sunday with sunny to partly sunny skies. An upper level ridge across the Atlantic builds westward on Sunday. Temperatures will be 3-5 degrees warmer on Sunday than on Saturday with the increasing heights aloft. On Monday (Eclipse Day) start to see some weak southerly return flow with surface high pressure across the Atlantic. In the upper levels, the ridge continues to expand across the area with upper level heights in the 594-596 dm range. This is a typical late summer time pattern across the area. The main questions continue to be about cloud cover and precipitation chances. Based on the latest model trends with the further amplification of the ridge these chances have gone down to around 10-20 percent. The lack of insolation from the sun during the Eclipse will act to decrease these chances as well and beginning to think based on the latest trends that viewing will be good throughout the forecast area. Temperatures will warm quickly into the early afternoon and beginning to plateau as the eclipse begins and will decrease by 2-8 degrees during the peak of totality. Without the eclipse expect highs would be in the low 90s areawide with the strength of the ridge but have forecast highs in the mid to upper 80s across most of the area at this time. Chances for showers and storms will increase on Tuesday and Wednesday as the ridge begins to dampens with an upper level trough passing to the north. Temperatures during this timeframe will be near normal. && .PRELIMINARY POINT TEMPS/POPS... Chattanooga Airport, TN 92 75 90 69 / 30 40 40 0 Knoxville McGhee Tyson Airport, TN 89 73 87 67 / 30 30 50 0 Oak Ridge, TN 90 73 87 67 / 30 40 40 0 Tri Cities Airport, TN 87 70 84 63 / 50 40 40 10 && .MRX WATCHES/WARNINGS/ADVISORIES... NC...None. TN...None. VA...None. && $$ JB/MA

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