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

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000 FXUS64 KMRX 190708 AFDMRX Area Forecast Discussion National Weather Service Morristown TN 308 AM EDT Sat Aug 19 2017 .SHORT TERM...
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(Today and Tonight)... Surface high pressure and a drying upper level northwest flow will result in calm, pleasant conditions today. Some patchy fog is possible around sunrise this morning, but it is not expected to become widespread. Otherwise, skies will be mostly sunny with light winds and highs generally in the 80s. .LONG TERM...(Sunday through Friday)... For the second half of the weekend, an upper-level ridge begins to build on Sunday increasing max 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 lower humidity. Taking a look at Monday (Eclipse Day) the 500mb ridge will continue to expand with heights expected to be around 594-596 dm which is near the 95th-98th percentile compared to climatology for this time of year. Subsidence aloft will act to dry the air column and limit clouds and convection. Cannot rule out about 10-20 percent PoPs around the higher terrain where differential heating could act as a forcing mechanism for convection. Models are showing the potential for convection along the mountains, and to a lesser extent, the plateau with the NAM being most vigorous with convective development. Taking a closer look at soundings, the 0z NAM sounding shows cooler mid-levels as opposed to the 0Z GFS which has a more typical sounding for a strong summer ridge with subsidence and associated warmer mid-levels. Based on the very strong ridge for Monday, believe that the NAM is over-doing convection and trended toward the drier GFS and ECMWF solutions. Further, effective deep layer shear is very low around 5 kts which means any ordinary convective development would not be able to be maintained for very long. As has been mentioned previously, it will be interesting to see the effects of decreasing incoming solar radiation during the mid-afternoon on Monday as the eclipse reaches its max for our area between 17z-20z. Hourly temperatures are expected to drop 2-8 degrees during maximum eclipse with the greatest drop in temperatures occurring within the path of totality. High temperatures may even be slightly cooler by a degree or two than on a similar day without a total solar eclipse. With near surface temperature reductions, may actually start to see a boundary layer profile similar to evenings near the path of totality where a stable or neutral thermal layer forms near the surface. This would likely further limit convection and cumulus development during the hours around the eclipse and this was reflected in the forecast. Overall, it is still looking like good viewing conditions for the area. After the eclipse has ended on Monday afternoon, temperatures will quickly rebound with highs in the upper 80s to lower 90s. Heat index values will be in the 90-95 degree range on Monday afternoon. It will be a very hot and mostly sunny day, so anyone spending a lot of time outdoors on Monday needs to stay hydrated, find shade, and use sunscreen with very high UV index values of 8-10 around midday before the solar eclipse even begins. The ridge will break down and rain chances will increase through midweek. PoPs increase late Tuesday and Tuesday night with likely PoPs on Wednesday as a cold front moves through the area and troughing deepens across the region. Long term models are actually in good agreement with this system. Drier air is expected to move in behind the front, but we may continue to see some additional clouds and rain showers on Thursday primarily across the mountains. Temperatures late next week expected to be cooler than average with troughing across the Eastern U.S.
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Chattanooga Airport, TN 91 72 93 72 / 0 0 10 10 Knoxville McGhee Tyson Airport, TN 88 69 91 70 / 0 0 10 10 Oak Ridge, TN 89 69 91 70 / 0 0 0 10 Tri Cities Airport, TN 86 64 89 65 / 0 0 10 10
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