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

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000 FXUS64 KMRX 200722 AFDMRX Area Forecast Discussion National Weather Service Morristown TN 322 AM EDT Sun Aug 20 2017 .SHORT TERM (Today and Tonight)...
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Main concern early this morning and early Monday morning will be patchy fog, some locally dense, near area lakes and streams due to steam fog development. Upper and surface ridging will be the rule for Today and Tonight. Forecast soundings show a fairly strong mid-level inversion which will essentially cap convection from developing today. A fairly hot day is in store with a warming temperature profile. .LONG TERM... We should start the week hot and mostly dry. On Monday (Eclipse Day), a strong 500mb ridge will expand with heights expected to be around 594-596 dm which is near the 97th-99th 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 percent PoPs around the higher terrain where differential heating will create a valley-mountain circulation with rising air across the higher elevations and sinking air across the Tennessee Valley. Models is good agreement tonight with only very limited convection anticipated across the higher terrain features. Effective deep layer shear is very weak around 10 kts or less which means any ordinary convective development would not be well maintained and would dissipate quickly. The total solar eclipse will have some unique impacts on our diurnal weather on Monday. Decreasing incoming solar radiation during the mid-afternoon as the eclipse reaches its max for our area between 17z-20z will have several effects. 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 expected to be slightly cooler by a degree or two as well. 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, especially near the path of totality. Even for local winds, may see some brief mesoscale wind patterns due to the total solar eclipse where a weak divergent flow occurs near the path of totality. Overall, it is still looking like good viewing conditions for the area with mostly clear skies across the valley and additional cumulus/partly cloud skies across the higher terrain. At this point, only few, to at most scattered, cirrus would be expected. 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. With the hot and sunny weather, it`s important for anyone spending a lot of time outdoors on Monday to stay hydrated, find shade, and use sunscreen because of very high UV index values of 8-10 around midday before the solar eclipse even begins. The ridge breaks down on Tuesday as an upper-level trough approaches. Model solutions in good agreement with a cold front moving through the area on Wednesday with increasing PoPs late Tuesday night and likely PoPs on Wednesday. May have a few lingering showers across the mountains on Thursday before high pressure builds in late week with dry weather expected. With troughing in place, expect cooler, below normal temperatures late next week behind the cold front.
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Chattanooga Airport, TN 93 73 92 73 / 0 0 10 10 Knoxville McGhee Tyson Airport, TN 90 70 89 71 / 0 0 10 10 Oak Ridge, TN 91 71 91 71 / 0 0 10 0 Tri Cities Airport, TN 89 65 89 66 / 10 10 10 10
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