819 FXUS64 KMRX 132306 AAA AFDMRX AREA FORECAST DISCUSSION...UPDATED National Weather Service Morristown TN 706 PM EDT Mon Sep 13 2021 .UPDATE... FOR 00Z AVIATION. && .SHORT TERM...(Tonight and Tuesday) Key Messages: 1. Continued above normal temperatures with mostly sunny skies. Isolated chance of diurnal shower/thunderstorm in the far SW TN Valley and Plateau Tuesday afternoon. Discussion: Current conditions/This evening Upper level ridging continues to prevail over the region, however, 500mb heights are beginning to fall as TS Nicholas continues its northward progression just offshore from eastern Texas. At the surface, high pressure and associated subsidence along with a lack of moisture will continue to keep conditions dry and skies mostly sunny for the remainder of Monday. Tonight through Tuesday Heading into the overnight hours, continued clear skies and associated radiational cooling will allow for the development of fog early Tuesday morning, especially in valley and river locations. The greatest possibility of radiational fog is going to be in the Tri Cities area and SW VA, decreasing to the south and west as model derived soundings suggest less favorable surface dewpoint depressions near the Chattanooga area. Overnight lows will drop into the 60s for valley locations, with mid to upper 60s in the southern TN counties cooling to the lower 60s in NE Tennessee and SW Virginia. By Tuesday afternoon, moisture advection will start to increase as TS Nicholas continues to meander northward. Overall, the increase in moisture will primarily be confined to the far SW Valley and Plateau locations where the potential for an isolated shower and an occasional rumble of thunder are possible. Otherwise, most locations should remain dry. Additionally, with the slow moisture return to the region and cloud coverage being limited, highs on Tuesday will resemble a similar pattern as Monday. Expect temps to reach the mid to upper 80s - roughly 3-5 degrees(F) warmer than seasonal normals. KS .LONG TERM...(Tuesday Night through Monday) Key Messages: 1. Warm and increasingly humid midweek through next weekend. 2. Chances for diurnal convection especially Wednesday through Friday. Rain chances will continue through the end of the extended due to the uncertainty of how far the tropical system moves east and how long before it gets absorbed into the high pressure ridge. Discussion: The track and impacts of now Tropical Storm Nicholas will be the main weather maker for the beginning of the extended forecast. By 00Z Wednesday, the remnants of this tropical system will be centered over southeast Texas and forecast to lift northeast across Louisiana and then into Mississippi by Thursday. An upper level disturbance will be over the Northern Plains and extend southwest into the Rockies. Large surface high over the eastern Atlantic will extend west across the southeast states and keep warm moist air streaming into the forecast area Tuesday night and Wednesday. With surface heating a few storms will may linger Tuesday evening across southeast Tennessee. Scattered showers and storms will increase Wednesday as a frontal system moves southeast from the Ohio Valley towards Tennessee and moisture from Nicholas moves into the region from the southwest. NAM and GFS models brings moisture from Nicholas into the forecast area late Wednesday and Wednesday night but keeps most of the rain to the south of Tennessee Thursday and Thursday night. The front to the northwest looks to wash out by Thursday but surface low associated with the remains of Nicholas will slowly move east northeast across Alabama and into the Tennessee Valley Friday. Scattered showers and storms will continue Thursday and again Friday. Saturday through Monday, moisture from the remains of Nicholas stays over the southeast states as upper level ridging rebuilds over the region. Troughing begins to develop over the southwest states and the building ridge sets up over the mid Atlantic. ECMWF is less pronounced with western trough but is more optimistic of disturbed weather north of the Bahamas and brings this system close to the Carolinas and develops a coastal low off the Carolinas Thursday and Friday. There could be decreasing chances of precipitation after Friday but uncertainty is high for where the remains of Nicholas goes and how long the moisture hangs around. The temperatures will stay above normal with highs in the 80s and lows in the 60s as now frontal system will move through and the Bermuda high being persistent over the southeast and mid Atlantic states. TD && .AVIATION... 00Z TAF DISCUSSION. All signs point to fog development again tonight, but the uncertainty is whether it will affect the TAF sites. Will lean the forecast toward climotology and the models, which favor TRI for the lowest vis, probably IFR or lower. Will have TEMPOs at TYS and CHA for MVFR vis, although CHA dropped to IFR this morning. That could happen again but will go with just MVFR for now. DGS && .PRELIMINARY POINT TEMPS/POPS... Chattanooga Airport, TN 67 88 68 83 67 / 0 20 30 50 30 Knoxville McGhee Tyson Airport, TN 67 89 68 82 66 / 0 10 10 40 30 Oak Ridge, TN 65 88 67 81 66 / 0 10 20 50 30 Tri Cities Airport, TN 63 88 60 83 64 / 0 10 10 40 30 && .MRX WATCHES/WARNINGS/ADVISORIES... NC...NONE. TN...NONE. VA...NONE. && $$