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

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000 FXUS64 KMRX 180722 AFDMRX Area Forecast Discussion National Weather Service Morristown TN 322 AM EDT Fri Aug 18 2017 .SHORT TERM...
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(Today and Tonight)... A weak cold front will approach and cross the area today. Scattered showers will push across the area this morning along and ahead of the boundary. Just a few rumbles of thunder, if any, will accompany these showers with very little instability to work with. With PWAT values of 1.75 to 2 inches, could see some brief heavy rain with this morning`s showers however their progressive nature should preclude any flooding issues. Clearing skies are expected this afternoon as drier air works into the area. Highs will be a few degrees cooler today due to the northwest flow, staying in the 80s areawide. For tonight, with clear skies and drier air overhead temperatures will dip into the 60s in most locations. .LONG TERM...(Saturday through Thursday)... We start the long term period with much drier air across the forecast area in the wake of a cold front. A shortwave trough will swing through the Ohio River Valley on Saturday, but with limited lift and dry air in place, expect no precipitation and only a few additional clouds. PW values will only be in the 0.8-1.0 range on Saturday which is near the 10th percentile for this time of year. An upper-level ridge begins to build in on Sunday increasing temperatures by 1-3 degrees compared to Saturday. With the drier low- level airmass and a building ridge, anticipate dry and pleasant conditions with highs in the mid 80s to low 90s and low humidity through the weekend. The ridge continues to expand on Monday (Eclipse Day) with 500mb heights around 594-596 dm which is near the 95th percentile for this time of year. Subsidence aloft will act to dry the air column and limit clouds and convection. Still cannot rule out about 10-20 percent PoPs, especially around the higher terrain where differential heating could play a role as a forcing mechanism for convection. However, effective deep layer shear is less than 10 kts which means any convective development would have a difficult time sustaining itself. It will also be interesting to see the effects of decreasing incoming solar radiation during the mid-afternoon on Monday. We may actually start to see a boundary layer profile similar to evenings around the path of totality where a stable or neutral thermal layer forms near the surface. This would help to limit convection and cumulus development. Thoughts are that between (1) the building ridge and associated subsidence, (2) weak shear, and (3) decrease in boundary layer heating between 17z-20z on Monday, it looks to be mostly dry with a mostly clear to partly cloudy sky leading to good eclipse viewing conditions. Temperatures will be very warm for this time of year, and without the eclipse, highs would be several degree warmer. The eclipse will act to drop temperatures between 2-8 degrees during max coverage with the largest temperature drops occurring along the path of totality. With dew points in the upper 60s to lower 70s, expect heat index values to be in the 90-95 degree range on Monday afternoon. Rain chances will increase through the middle of the week and peak on Wednesday/Wednesday night as the ridge is replaced by troughing and a cold front moves southward across the area. Long term models are currently in good agreement with frontal passage occurring late Wednesday and Wednesday night with likely PoPs along and ahead of this front. Behind the cold front, much drier and cooler air settles over the region late week.
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Chattanooga Airport, TN 89 69 90 70 / 30 0 0 0 Knoxville McGhee Tyson Airport, TN 86 67 88 67 / 40 0 0 0 Oak Ridge, TN 87 67 88 67 / 30 0 0 0 Tri Cities Airport, TN 82 64 87 61 / 30 10 0 0
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