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

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000 FXUS64 KMRX 271915 AFDMRX Area Forecast Discussion National Weather Service Morristown TN 315 PM EDT Sat May 27 2017 .SHORT TERM /This Afternoon through Sunday/... Complex 12 to 24 hours across the eastern Tennessee Valley and Southern Appalachian region due to the threat for scattered to potentially widespread strong to severe thunderstorms. Morning cloud cover continues to break up as heating is rapidly breaking the capping inversion in place. Earlier convection across NW TN dissipated as it tracked into the Plateau, but has laid a couple of outflow boundaries along the I-40 corridor and across the Northern Plateau back into southern KY. Additionally, a weak front will be in the vicinity later on this evening across SW VA and southern KY. These boundaries will serve as a source and focus for convection later on by the late afternoon and evening as shown all of the mesoscale models. The peak timing of this activity will be 4 PM to 10 PM EDT, with coverage being scattered in nature at around 40-60 percent. In addition, an upper-level perturbation/shortwave that will drop of the mid- Mississippi Valley toward the Southern Appalachians later this afternoon and evening. This may provide enough forcing to overcome the lingering inhibition currently in place by around 20-22z later this afternoon. As noted in the morning discussion though, a favorable environment exist for any of this activity to quickly become strong to severe. The main threats will be large hail and downbursts. MLCAPE values are progged to be in the 2000-2500 J/kg range, with MUCAPEs over 3000 J/kg at times. Hail CAPEs (1500 J/kg and DCAPEs over 1000 J/kg) will create a very favorable thermodynamic environment for strong updrafts, capable of supporting the aforementioned threats. Bulk shear values are quite high in the 40-50 kt range across the area, though unidirectional in nature -- thus an ESE storm motion is expected. Straight hodographs in this shear environment would favor organized, multicell clusters and supercells (as well as numerous splitting cells). A low-end tornado threat will exist thanks to the presence of these boundaries -- though a less favorable 0-1 km shear profile limits this. Still, areas to watch will be supercells that can form along boundaries with a localized backed wind profile. The forecast for tonight into Sunday is less certain. Mesoscale models do, however, continue to show a much faster progressing MCS pushing out of NW and Middle TN into northern AL and the Central/southern Valleys of East Tennessee late tonight. The main question will be the exact timing and strength. Lower res models are slower, but may be underestimating the higher forward propagation that can occur in higher CAPE environments. Thus, favored the mesoscale models (HRRR/NSSL-WRF/3-km NAM) over the GFS/NAM-12 solutions. Damaging straight-line winds would be the main threat with this activity. Storms will weaken as they push across the area Sunday morning and may leave a lull in precipitation (thanks to a more stable air mass) for most of the morning and afternoon. If some heating can occur during the late morning thru mid afternoon, a third round of severe storms is possible late Sunday afternoon, evening and overnight -- along/near where the front stalls out. This part of the forecast remains fairly uncertain at this time. .LONG TERM /Sunday Night through Saturday/... Will remain in an active weather pattern early in the extended as a weakening cold front moves through the Southern Appalachian Region. A line of convection is expected to develop along/ahead of this frontal boundary...effecting the MRX CWA overnight Sunday into Monday morning. With recent convection...the air mass in place should be only weakly unstable and the chance of strong to severe storms is limited. The frontal boundary is expected to stall just south of the TN/GA boarder Monday night and could see scattered convection continue along our southern boarder Monday night and Tuesday. Models begin to diverge Wednesday through the end of the extended and confidence is low. Current trend is to remain in a wet...unsettle pattern with the boundary remaining in the vicinity Wednesday and Thursday and the next system moving in around the Friday/Saturday time frame. All of this will keep 20 to 40 POPs in place over our area through the last half of the extended. && .PRELIMINARY POINT TEMPS/POPS... Chattanooga Airport, TN 68 85 69 82 / 60 60 60 40 Knoxville McGhee Tyson Airport, TN 66 82 66 79 / 50 60 60 40 Oak Ridge, TN 66 82 66 79 / 60 60 60 40 Tri Cities Airport, TN 61 79 63 78 / 60 60 60 50 && .MRX WATCHES/WARNINGS/ADVISORIES... NC...None. TN...None. VA...None. && $$ AMP/MJB is the U.S. government's official web portal to all federal, state and local government web resources and services.