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

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000 FXUS64 KMRX 271906 AFDMRX Area Forecast Discussion National Weather Service Morristown TN 306 PM EDT Thu Jul 27 2017 .SHORT TERM...
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(This Afternoon through Friday)...A humid airmass across the area this afternoon with dewpoints in the low 70s. Current water vapor imagery and RAP upper air analysis indicates that the ridge axis has now pushed to the south with a trough to the northwest across the MId-Mississippi River Valley. PW values continue to increase ahead of this approaching system with values currently in the 1.6-1.8 inch range. Also, seeing a few more shower/storms this afternoon than the past few days thanks to the increased moisture. The ongoing cloud cover should keep SBCAPE values in the 1000-2000 J/Kg range this afternoon and 0-6 km shear is only around 10-20 kts. Therefore, expect there could be a storm or two that becomes strong to marginally severe with damaging winds being the main threat. Models indicate a sharp drop off in the coverage and intensity this evening and overnight with the loss of daytime heating. Another mild night is forecast with lows in the upper 60s to mid 70s. The forecast is rather challenging tomorrow and there are still some slight timing differences in the models. A cold front will track southeastward across the Mid-Mississippi and Ohio River Valleys as a closed upper low moves southeastward across the Great Lakes region. Closer to home a weak shortwave ahead of the front will move through during the morning hours. Models indicate that PW values during this timeframe will be up into the 2.0-2.3 inch range, which are near the maximum values for late July. Most of the models suggest that a shortwave will come through in the morning leading to numerous to widespread showers and storms. In this scenario there would be less time for instability to build and may see slightly lower chances for severe weather. The front is not forecast to move through until later in the day and 0-6 km bulk shear values will increase into the 30-40 kt range Friday afternoon. However, thanks to the cloud cover it may be hard to get SBCAPE values above 1000 J/Kg. The shear is decent for a late July event and will continue with the threat for severe weather in the HWO with damaging winds and localized flooding being the main threats. Rainfall amounts between 1-2 inches will be common across the area as the event comes to a close. .LONG TERM...(Friday night through Wednesday)...Lingering showers/thunderstorms will be possible across the area Friday night until the front has completely moved through during the overnight hours. We then move into a period of prolonged northerly to northwesterly flow over the area. This will lead to a drier atmosphere with lower dew points and clearer skies. Temperatures will start the weekend out about 5 degrees cooler than normal for this time of year. Temperatures will slowly climb through the weekend and through next week until the end of the work week when temperatures are expected to be back or slightly above normal. Otherwise there`s not much in the way of notable weather to talk about in the forecast. The GFS tries to zip across some energy in the second half of the week bringing isolated showers and thunderstorms to the area, but will only go with low end precipitation chances over the southern/western portions of the forecast area. More widespread weather will likely move in just beyond this 7-day forecast, next weekend, with increasing shower and thunderstorm rain chances.
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Chattanooga Airport, TN 75 83 71 85 / 30 80 60 10 Knoxville McGhee Tyson Airport, TN 73 81 68 81 / 30 80 60 10 Oak Ridge, TN 72 79 69 82 / 40 80 50 10 Tri Cities Airport, TN 69 82 65 78 / 40 80 60 20
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