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

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000 FXUS64 KMRX 220909 AFDMRX Area Forecast Discussion National Weather Service Morristown TN 409 AM EST Wed Feb 22 2017 .SHORT TERM...
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(Today and Tonight)...Upper low over Mississippi will move slowly southeast into the northeast Gulf today. Isentropic lift will continue this morning and afternoon and trigger light showers across much of the forecast area. A weak surface trough over Kentucky into middle Tennessee will also act to enhance the shower activity. Rainfall amounts will be light...heaviest near the Tennessee border and across southwest Virginia. Skies will stay mostly cloudy. Mild temperatures this morning in the mid 50s will rise to the mid and upper 60s this afternoon. Tonight showers will end as upper low moves southeast. Low level moisture will remain with mostly cloudy skies and mild temperatures in the lower to mid 50s. .LONG TERM...(Thursday through Tuesday)... The main focus for the long-term forecast will be on the severe weather expected to impact the area late Friday into early Saturday morning. On Thursday, an upper level trough will move out of the Rocky Mountains with lee cyclogenesis. A warm front will extend eastward from the surface low in Colorado to the Ohio River Valley. This will put the forecast area in the warm sector. PW values will be around an inch on Thursday. These values are around the 90th percentile for late February. There is a weak mid-level trough that moves through but not seeing much in the way of lift. Therefore, went ahead and trimmed back PoPs on Thursday to mainly just the higher elevations and only have around 20 percent there. Temperatures will continue to be well above normal with highs in the upper 60s to mid 70s. Mid- level ridging will build in Thursday night into Friday morning ahead of the next approaching trough. The small PoPs will come to an end and expect dry conditions during the overnight period. Southerly flow will begin to pick up overnight as the pressure gradient tightens as a cold front races across the Central and Southern Plains. These southerly winds will keep overnight lows up in the 50s across most of the forecast area. On Friday, the focus will shift to the possibilty of severe storms. Currently, it appears that most of the day Friday will be dry with the ridge still providing some influence. In addition, model soundings indicate a cap throughout much of the day with drier air in the mid-levels. Temperatures will once again top out near record values with highs in the 70s. Southerly winds will also be strong throughout the day and approach advisory level criteria. Expect the cap aloft and the linear forcing will keep most of the convection along a line just ahead of the front. The NAM and GFS indicate a surge in the low level jet around 03-06z Saturday up into the 45-55 kt range. In addition, the strong warm air and moisture advection will produce SBCAPE values in the 500-1000 J/Kg range during the overnight hours. The strong upper level flow aloft will lead to 0-6 km bulk shear values of 50-60 kts. Strong jet dynamics will also setup at this time as a speed max rotates through the base of the trough. The low level 0-1 km SRH will range from around 200- 300 m2/s2. Some model soundings indicate that the boundary layer will decouple and produce lower instability values, however, this is unrealistic given the strong southerly flow. Therefore, have sided more towards the NAM since it shows less decoupling. At this time it appears the highest risk for severe weather would be from roughly 10 pm - 6am late Friday night into early Saturday with damaging winds being the main threat. Much drier and colder air will advect into the Southern Appalachians on Saturday into Saturday night. So cold in fact, models indicate some light snow possible across the higher elevations Saturday night across the higher elevation. With the recent warm weather, flowers and plants have been blooming and it looks like a widespread freeze will occur Sunday morning. The cooler conditions will be short-lived as a warm front lift across the area early next week. Rain showers will be possible Monday night into Tuesday as the front lifts northward with highs on Tuesday in the mid to upper 60s.
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&& .PRELIMINARY POINT TEMPS/POPS...
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Chattanooga Airport, TN 70 54 73 55 / 60 10 10 10 Knoxville McGhee Tyson Airport, TN 67 52 72 53 / 60 10 10 10 Oak Ridge, TN 67 52 72 52 / 60 10 10 10 Tri Cities Airport, TN 63 48 71 48 / 70 20 20 20
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&& .MRX WATCHES/WARNINGS/ADVISORIES... NC...None. TN...None. VA...None. && $$ TD/MA

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