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

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000 FXUS64 KMRX 180702 AFDMRX Area Forecast Discussion National Weather Service Morristown TN 302 AM EDT Fri May 18 2018 .SHORT TERM...
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(Today and Tonight)... Seeing a few showers early this morning across NE TN and SW VA to the east of a broad upper level low. Current water vapor imagery and RAP upper air analysis indicates a broad closed low across the Mid- Mississippi River Valley into the Tennessee River Valley. Southerly flow throughout the atmosphere to the east of the system is leading to PW values across the area in the 1.4-1.7 inch range. These values are near the 90th percentile for mid to late May and have 40-80 PoP throughout the short-term forecast. Early this morning, most of the precipitation is on the North Carolina side of the mountains with downsloping across Tennessee with isolated to scattered showers currently. Expect there will also be a few areas of patchy fog early this morning with light winds and dewpoints well into the 60s. For today, expect isolated to scattered showers to continue throughout the morning with elevated PW values. Best chances for precipitation will once again be during the afternoon and evening hours. Models indicate some PVA this afternoon as a shortwave trough pivots around the upper level low across the Tennessee Valley. With the saturated atmosphere, lapse rates will be near moist adiabatic with SBCAPE values generally between 500-1000 J/Kg. Additionally, 0- 6 km bulk shear values will range from 20-30 kts. There could be a strong storm or two with gusty winds but do not anticipate any severe storms at this time. The continued rain could also lead to some localized flooding especially in areas of poor drainage. However, there will still be a downslope component to the low level winds today and expect the heaviest rainfall to be on the North Carolina side of the mountains. Model soundings remain saturated overnight as the main upper level low moves closer. Have PoPs decreasing slightly overnight with the lose of daytime heating but not as much as a typical summertime pattern with the additional lift from the upper level system. High temperatures today will generally range from upper 70s to low 80s with lows in the 60s for most locations tonight. .LONG TERM...(Saturday through Thursday)... Wet and warm will be the theme of the forecast through the weekend and much of next week. The slow moving low/trough should finally be north/northwest of the area but moisture will continue to be pumped in keeping shower and thunderstorm chances present through the weekend. Over the weekend in our northern counties closer to a trough/shortwave we could see some stronger thunderstorms with this upper level forcing. Currently the threat looks pretty minimal, but we could see some strong gusty winds or small hail with the strongest of the storms in SW Virginia towards NE Tennessee, but widespread severe weather is not expected at this time. Towards the beginning of the work week we start to see ridging building in across the SE United States. This will cause a slight decrease in precipitation chances, but enough moisture will be trapped under the ridge so at least some precipitation chances will need to remain in the forecast for next week. The ridge will slowly break down but we will still see a very slow build up of temperatures throughout next week, but the increased cloud coverage looks like it will keep away the widespread 90 degree temperatures. Another factor that will keep precipitation chances present is a frontal boundary that makes a run at the area but then is forecasted to stall out just to the north of the forecast area. Models typically struggle with how to handle quasi-stationary boundaries so precipitation chances could increase/decrease late next week depending on the frontal boundary location.
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Chattanooga Airport, TN 81 67 85 68 / 70 70 50 30 Knoxville McGhee Tyson Airport, TN 82 66 84 66 / 70 50 60 30 Oak Ridge, TN 81 66 82 66 / 70 60 60 30 Tri Cities Airport, TN 79 63 81 62 / 70 50 60 30
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