942 FXUS64 KMRX 060118 AFDMRX AREA FORECAST DISCUSSION National Weather Service Morristown TN 918 PM EDT Tue Oct 5 2021 .UPDATE... EVENING UPDATE. && .DISCUSSION... Increase in light returns on the radar during the last hour, but so far not much more than sprinkles in the northern/central valley downslope areas. Will adjust PoPs a bit to show these trends, but will also remove thunder for the first few hours of the night all areas and for the entire night in the northern/central valley. Will tweak temps and dew points as well with this update. Will leave the Flash Flood Watch as is for now with the start time of 12Z. && .AVIATION... 00Z TAF DISCUSSION. Will see showers and possibly a few thunderstorms increase again later tonight into Wednesday, with CHA having the best chance for MVFR or lower conditions as well as thunder. TYS/TRI will be influenced by downslope flow as the low level flow will be southeast over the mountains, so expect primarily VFR conditions both sites. However, there may be some brief lower conditions mainly with any showers/thunderstorms. LW && .PREV DISCUSSION... /ISSUED 331 PM EDT Tue Oct 5 2021/ SHORT TERM...(Tonight and Wednesday)... Key Points: 1. We remain warm and moist with scattered showers and thunderstorms producing locally heavy rainfall rates. 2. Any training convection will result in flash flood potential across the southern plateau, southern valley, and southwest North Carolina. Discussion: A stacked low pressure system across the Lower Mississippi Valley will slowly meander northward on Wednesday with deep southerly unidirectional flow across the forecast area. This will result in an anomalously moist airmass across the forecast area, especially across the southern third of the forecast area where downslope S/SE flow will not result in any localized drying of the boundary layer. In fact, upslope flow along the southern plateau into the Chattanooga area will likely enhance low-level convergence and convective initiation late Wednesday into Wednesday night and enhance rainfall totals for these locations. The environment is favorable for heavy rainfall. RAP and HREF guidance show PWAT values near 1.7 to 1.8 inch across the southern plateau and southern valley late Wednesday into Wednesday night with saturated forecast soundings showing a deep warm cloud layer of 12-13k ft. With the RAP indicating weak, skinny instability with MLCAPE around 500 J/kg across our southern zones, this will result in an environment favorable to heavy rain rates within developing convection. USGS stream gauges show streamflows already near the 90th percentile, so antecedent wet soil will favor quick runoff and enhance flooding risk from any heavy rain rates. At this time, the severe weather risk remains very low with SPC keeping the marginal risk to our southwest. The highest risk for training convection will be Wednesday afternoon and evening. At this time, the upper jet will move northeastward with our area within the diffluent exit region. This is the period of time where upper divergence will be at its strongest and convective initiation will be most likely with SE flow and low-level convergence along the southern plateau and into the Chattanooga area. This can be seen in some of the recent 12z CAM guidance such as the HRRR and hi-res 3km NAM which shows convection developing Wednesday afternoon and slowly moving NNW with training occurring as additional convection continues to develop along the convergent boundary within a favorable environment. Overall, this is a conditional threat. It depends on the convection developing and training in this area which may result in highly variable spatial distributions of rainfall totals. However, the environment is favorable for flash flooding, and with several CAMs showing very heavy rain rates on Wednesday afternoon near our southern plateau and southern valley counties, have issued a Flash Flood Watch for those locations. JB LONG TERM...(Wednesday night through Tuesday)... Key Messages: 1. A wet pattern continues Wednesday night through Thursday with drier conditions this weekend into next week. 2. Localized flooding a concern through Thursday morning across the southern Plateau, southeast Tennessee, and southwestern North Carolina. 3. Temperatures near to above normal through entire period. Discussion: Wednesday night through Friday night Starting Wednesday night the mid/upper closed low over the lower Mississippi River Valley will begin to lift northward overnight with a southerly/southwesterly flow bringing moisture north from the Gulf of Mexico. High precipitable water values and high streamflows across the southern part of the forecast area will keep the chance of flooding elevated through at least Thursday morning so Flash Flood Watch continues for that area. With southeast low level flow across northeast Tennessee rainfall amounts there will be light. As the upper low moves into IL and then western Great Lakes by Thursday night into Friday the bands of showers an occasional thunderstorms will be less frequent and shift farther northeast into central east Tennessee and northeast Tennessee. The Flood Watch in the southeast part of the area may have to be extended depending on the movement of the upper low. FFG values will likely be lower as well Wednesday night into Thursday based on rainfall amounts during the next 24 hours. Temperatures Thursday and Friday will be similar to the past few days in the lower to mid 70s. Saturday through Tuesday Starting Saturday a building ridge over the south central states will begin to influence the region with height rises and drying of the atmosphere. However a southerly flow mainly to the east of the Appalachians will still affect the northeast corner with clouds and a chance of showers. Saturday night and Sunday higher pressure will be over the entire region and skies will become mostly clear even in the northeast. A new fast moving trough and shortwave will push the ridge and surface high eastward Monday and so another slight chance of showers late Monday into Tuesday. This weak system looks like it will begin to weaken or dissipate as it moves into the eastern Tennessee Valley Tuesday. Temperatures Saturday through Tuesday will be above normal in the mid 70s to lower 80s. TD && .PRELIMINARY POINT TEMPS/POPS... Chattanooga Airport, TN 66 75 66 76 64 / 90 90 90 70 40 Knoxville McGhee Tyson Airport, TN 65 77 65 77 64 / 80 90 80 80 60 Oak Ridge, TN 65 76 64 76 63 / 80 90 80 80 70 Tri Cities Airport, TN 63 76 62 76 60 / 50 60 70 70 70 && .MRX WATCHES/WARNINGS/ADVISORIES... NC...Flash Flood Watch from 8 AM EDT Wednesday through Thursday morning for Cherokee-Clay. TN...Flash Flood Watch from 8 AM EDT /7 AM CDT/ Wednesday through Thursday morning for Bledsoe-Bradley-East Polk-Hamilton- Marion-McMinn-Meigs-Northwest Monroe-Rhea-Sequatchie- Southeast Monroe-West Polk. VA...NONE. && $$