060 FXUS64 KMRX 080158 AAA AFDMRX AREA FORECAST DISCUSSION...UPDATED National Weather Service Morristown TN 958 PM EDT Wed Apr 7 2021 .UPDATE... EVENING UPDATE. && .DISCUSSION... The forecast timing of the onset of showers will be adjusted slightly later with this update. There is good agreement among the high-res rapid refresh models that showers will begin to reach our SW counties around 09Z, so will drop PoPs before that time. All other parameters appear on track this evening. The potential for severe storms on Thursday is looking a little more concerning with the latest model runs, particularly in the afternoon when models show redevelopment occurring. MLCAPE values in the NAM rise to 1500 J/kg while wet bulb 0 heights drop below 10 kft. Deep layer shear is supportive of discrete supercells. Recent HRRR runs are pointing in that direction as well. The main threats in this environment appear to be primarily large hail with damaging wind gusts as a secondary threat given the dry air aloft that builds in during the afternoon. DGS && .AVIATION... 00Z TAF DISCUSSION. VFR conditions will persist through the night. Toward morning, showers are expected to spread into the area from the SW, bringing MVFR cigs to CHA in the morning, with a chance of MVFR vis with thunderstorms. These should spread into TYS late in the morning, although cigs are expected to stay higher due to a southerly downslope flow off the mountains. The downslope flow will help weaken showers and keep cigs VFR at TRI in the afternoon. DGS && .PREV DISCUSSION... /ISSUED 330 PM EDT Wed Apr 7 2021/ SHORT TERM...(Tonight and Thursday)... This afternoon mid to upper level ridging is beginning to shift east of the forecast area as a low pressure at the surface approaches the Mississippi Valley and a mid to upper level low lies over the central Plains states. A cold front was moving into the western part of Arkansas with a line of showers and thunderstorms developing along and ahead of the cold front at mid afternoon. A few showers were occurring across middle TN and the Cumberland Plateau at this hour. At mid afternoon temperatures were about 10 degrees above normal in the mid 70s to lower 80s. Skies were most sunny east and partly cloudy west. Expect a mostly clear evening after clouds gradually decrease west. isolated showers and a possible thunderstorm possible plateau counties next few hours before dissipating before sunset. Overnight moisture will continue to increase ahead of the cold front moving into the Lower Mississippi Valley. Temperatures will be mild with lows dropping into the 50s to near 60. The line of storms are expected to weaken as they move away from the upper level support over Missouri late tonight moving into middle Tennessee. The steady precipitation with this system may become more scattered as it moves onto the plateau late tonight. Rain showers and a few thunderstorms are possible before sunrise Thursday or about 12Z. The stacked low will then move slowly north to northeast toward the Great Lakes Thursday. Showers and thunderstorms will increase during the day as heating increases. The convection should weaken in the morning and then strengthen during the afternoon. Some strong storms possible later in the day with gusty winds and marginally severe hail, especially if significant heating occurs. Temperatures will stay mild in the 70s, possibly closer to 80 across the eastern valley near the mountains with enough early clearing. TD LONG TERM...(Thursday night through Wednesday)... We start the period with a large stacked low pressure system drifting north across the upper Mississippi Valley/western Great Lakes region. This system will drag a weak front though our area Thursday night. Will carry a few showers/possibly some thunder for the first part of the night, but forcing looks to be minimal and PoPs will be no higher than low chance. We will be in southwest flow aloft behind the front, with no significant cooling. We should be between systems on Friday, and the model soundings indicate northern sections may be capped with little or no convection. Soundings look a bit more favorable for convection south. Models have been inconsistent and given the uncertainty will keep low chance PoPs for showers/thunderstorms all areas on Friday with highest PoPs south. Much higher confidence for a round of precipitation Saturday, as models in decent agreement that a strong short wave will move southeast out of the northern Plains into the Mississippi Valley before lifting northeast into the eastern Great Lakes region. Surface low pressure will develop to our west and track northeast, dragging a cold front through our area Saturday night. The low level jet will increase out of the south Friday night into Saturday, bringing significant moisture. Downslope flow may inhibit precipitation early on Friday night/early Saturday in the northern/central valley areas, and while direction is still uncertain it looks marginal for some mountain wave enhancement of the winds for late Friday night and/or Saturday. This will bear watching as it gets closer. Right now it looks like the rain begins moving in late Friday night and continues into Saturday, and convective energy looks to be quite low so will go no more than chance thunder despite likely/categorical PoPs. There will be considerable shear present with this system, so we will need to monitor for any signs of an increase in available convective energy which would lead to the possibility of strong/severe storms. Drier air will move in behind the front late Saturday night into Sunday with surface high pressure building in from the southwest and heights rising as a ridge aloft begins to build in. Monday will be dry and warm, but there remains significant uncertainty for the Tuesday/Wednesday time frame as models show considerable spread. Right now another short wave and moisture starved front looks to move through sometime in that time frame, and will carry low PoPs both days for showers. LW && .MRX WATCHES/WARNINGS/ADVISORIES... NC...NONE. TN...NONE. VA...NONE. && $$