451 FXUS64 KMRX 271942 AFDMRX AREA FORECAST DISCUSSION National Weather Service Morristown TN 342 PM EDT Thu May 27 2021 .SHORT TERM...(Tonight and Friday)... Hot and becoming more humid this afternoon across the eastern Tennessee Valley and southern Appalachians. A few showers have developed across the southern valley and plateau where low level moisture has increased. Dew points are a few degrees higher today than Wednesday...upper 50s northeast to lower to mid 60s southwest where there has been more cloud cover and showers. There is brief short wave ridging over the forecast area today but the main change coming is a short wave trough approaching tonight and cold front across the central plains extending out of low pressure across southern Iowa slowly shifting southeast toward the Tennessee Valley overnight and across the state of TN and KY during the day Friday. Short range models show the early activity this afternoon dissipate then develop more activity along the northern plateau and eastern mountains early this evening and move it across the northeast sections later in the evening. Have kept the slight chance pops through mid evening over the northern plateau and eastern mountains then drop rain chances until late tonight. Late tonight convection far to the west ahead of the front with major shortwave should reach the western sections of the area and be weakening. This convection should be breaking up across the forecast area in the morning but redevelopment is likely later in the day ahead of the front. The airmass will become somewhat unstable later in the day with dew points in the lower 60s. Also moderately strong mid level west to southwest flow moving into the Appalachians will help storm growth. A few strong storms are possible with damaging winds and marginally severe hail The severe risk will lower during the evening as the front shifts east of the Appalachians. Locally heavy rainfall will occur as well with any organized convection with up to about 1 inch of rain in spots. Kept high temperatures above guidance a few degrees based on the last few trends of being too cool. TD .LONG TERM...(Friday night through Thursday)... Key Messages: 1. Much cooler this weekend. 2. Warmer temps return next week with chances for thunderstorms increasing during midweek time frame. Discussion: Friday night through early Sunday morning an upper trough will linger over the Ohio river valley before ejecting off to the northeast late Sunday afternoon and evening. This feature will force a strong cold front through the forecast area Friday night. Widespread showers and thunderstorms, a few of which could be strong to severe, will be shifting into or east of the Appalachians during Friday evening along a pre-frontal trough. But the cold front likely doesn't arrive in the TN valley until midnight or so, and will be east of the Appalachians by daybreak Saturday. Despite widespread rainfall coming to an end Friday night, will keep at least some slight chance to chance PoPs in the mountains (especially northeast TN and into southwest VA) as vort maxes rotate around the upper trough and through our northern CWA. Temperature wise, for Saturday and Sunday I stuck close to NBM guidance which yields upper 60s to low 70s on Sat and basically low to upper 70s on Sun. It's worth noting here that I am on the warm side of the guidance envelope here with raw output from several operational models showing temperatures some 5-10 degrees cooler across the board for Saturday. Discrepancies are slightly lower on Sunday in the southern valley where clouds will be thinning out, but remain 5-7 degrees warmer than most guidance in the north. These high temperatures may need to be revisited significantly if this trend continues in guidance tonight. Heading into next week, guidance is in better agreement today than yesterday. Most solutions favor broad southwesterly flow from the mid-Atlantic into the southern plains as troughing sets up across the nation's midsection. The question really becomes how deep is the trough. The deeper GFS solution would favor increasing rain chances over eastern TN Tuesday through the end of the period as southwesterly flow aloft and return flow off the Gulf favor increased coverage of showers and thunderstorms. The ECMWF has a weaker central US trough, higher H5 heights over the southeast, and subsequently warmer/drier weather locally. Pattern recognition suggests that going completely dry is fairly bold so continued with some slight chances on Tue and Wed, and brought in chance PoPs heading into the end of the period when even the drier ECMWF is showing increased rain chances. Additionally, this WAA regime will promote a return to more seasonal temps after the cool weekend. CD && .AVIATION... 18Z TAF DISCUSSION. A few clouds will affect the east Tennessee area this afternoon and evening. Only isolated showers and a few storms will occur but chances at any TAF site will be too low to include in forecast. If storms develop near Chattanooga may have to add vicinity thunder in an amendment. Late tonight and Friday clouds increase with ceilings lowering. After 12Z have added prob30 groups for thunder at CHA and showers at TYS and TRI with MVFR ceilings and visibilities. Also late in the period thunderstorms could be near the terminals so have vicinity thunder just before 18Z Friday. TD && .PRELIMINARY POINT TEMPS/POPS... Chattanooga Airport, TN 68 86 55 72 52 / 30 80 50 10 0 Knoxville McGhee Tyson Airport, TN 67 84 55 67 50 / 20 90 70 20 10 Oak Ridge, TN 66 83 52 67 49 / 20 90 60 20 0 Tri Cities Airport, TN 61 82 55 67 49 / 20 90 80 30 10 && .MRX WATCHES/WARNINGS/ADVISORIES... NC...NONE. TN...NONE. VA...NONE. && $$