Area Forecast Discussion
Issued by NWS Knoxville/Tri Cities, TN
Issued by NWS Knoxville/Tri Cities, TN
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922 FXUS64 KMRX 242004 AFDMRX Area Forecast Discussion National Weather Service Morristown TN 404 PM EDT Tue Sep 24 2024 ...New SHORT TERM, LONG TERM... .SHORT TERM... (This evening through Wednesday) Issued at 330 PM EDT Tue Sep 24 2024 Key Messages: 1. A few strong to severe storms are possible through early evening, with the best chances across the northern Cumberland Plateau. A Severe Thunderstorm Watch is in effect for Morgan, Scott, and Campbell through 8 PM EDT. The main threats are damaging straight line-winds, locally heavy rainfall, and a very low probability of an isolated tornado along with 1 inch hail. 2. Upper low taps into tropical moisture tomorrow, with a moderate to heavy precip axis setting up across our area. This will allow for the training of showers and storms which will lead to an increasing flood threat. Discussion: A few strong to severe storms are possible through early evening. The best chances will be across the northern Cumberland Plateau where a Severe Thunderstorm Watch is in effect until 8 PM EDT. SPC`s meso page currently shows roughly 40 to 50kts of 0-6km shear in place with 30kts at 0-3km and 20kts at 0-1km. Instability is moderate with 1500 J/kg MLCAPE. The main threats with these storms will be damaging straight-line winds, locally heavy rainfall, and a very low probability of an isolated tornado and 1" hail. The severe threat should lessen later this evening but a few strong storms are possible through the overnight hours due to elevated instability remaining in place along with elevated shear values. Tomorrow into tomorrow night brings another concern. A frontal boundary associated with the upper trough will move into the region and stall. The upper trough will then begin to tap into Gulf moisture and pull it northward into the area. Strong upper divergence, mean southerly flow aligned parallel to the boundary, and ample moisture, will allow for the training of showers and storms. This means a moderate to heavy rainfall axis will likely setup somewhere across our area. As of now, the most likely area will be along I-75 and east towards the east TN mountains. However, out of caution, a Flood Watch will go into effect at 15Z tomorrow for the majority of the area. Because of the heavy rainfall potential, WPC has upgraded the ERO to moderate across portions of our area. 12Z HREF means paint a good swath of 2 to 3 inches of rainfall within the moderate ERO. Because of this, flooding will begin to become a concern. && .LONG TERM... (Wednesday night through next Tuesday) Issued at 330 PM EDT Tue Sep 24 2024 Key Messages: 1. Remnants from Tropical Storm Helene expected to impact the region Thursday and Friday. Flooding possible as well as gusty winds. Flood Watch in effect for southeastern Tennessee, including all southern Appalachian counties, and southwest North Carolina. 2. Unsettled weather will continue throughout the long term period with temperatures near seasonal norms. Discussion: The period starting Thursday morning will feature a closed upper low to our west and what will eventually become Hurricane Helene approximately due west of the Keys. Before impacts are felt by Helene, this upper low to our west will help feed moisture into our area well before rains from Helene arrive. Which may pose some issues for SE Tennessee, SW North Carolina, and the southern Appalachians with several inches of accumulating rainfall. Which yes, is good for many areas that are dry, but this will be too much in such a short period of time. Flood Watches are in effect for the aforementioned areas that will begin Wednesday morning and go through Friday. Not including amounts in the short term forecast period, but accumulations beginning Thursday morning and ending on Saturday, place many areas with 3 inches total rain. Right now, the rainfall hot spots are SW North Carolina, Polk, Monroe, Blount, Sevier counties in Tennessee. Unicoi and Carter Counties may feel the impacts of orographic lift as well. Other areas will receive a few inches of rainfall, as well, but track of the storm and downslope drying similar to what mountainwave events do, will make rainfall amounts more uncertain. Winds will be the next impact element with those expected to increase as we go through the day on Thursday. Peak winds and gusts will be late Thursday and Friday morning when the center of Helene is just to our south. Higher elevation spots will likely see gusts in the 40 mph range and into the 50 mph range for the highest peaks. Lower elevation spots such as the valley will commonly see gusts in the 25 to 35 mph range. No wind hazards have been issued yet, but most definitely will as we get closer. The main impact from Helene without question will be the high rainfall amounts and what that will do once the soils saturate. Once Helene further weakens and deteriorates later Friday, through the weekend and into next week, our area will remain under a closed low aloft which will keep temperatures cooler and unsettled weather in the forecast. Of course pinning this down beyond a tropical system that hasn`t really gotten itself together yet, is not possible, so slight to isolated chances of showers paint the last few days of the long term. && .AVIATION... (18Z TAFS) Issued at 125 PM EDT Tue Sep 24 2024 An unsettled TAF period with isolated to scattered showers and storms over the next 24 hours. A mix of VFR and MVFR conditions are mostly expected. && .PRELIMINARY POINT TEMPS/POPS... Chattanooga Airport, TN 69 78 66 75 / 70 80 80 90 Knoxville McGhee Tyson Airport, TN 67 76 65 75 / 80 90 80 70 Oak Ridge, TN 66 76 64 75 / 80 90 80 90 Tri Cities Airport, TN 65 76 64 75 / 60 80 70 60 && .MRX WATCHES/WARNINGS/ADVISORIES... NC...Flood Watch from Wednesday morning through Friday afternoon for Cherokee-Clay. TN...Flood Watch from Wednesday morning through Friday afternoon for Bledsoe-Blount Smoky Mountains-Bradley-Cocke Smoky Mountains-East Polk-Hamilton-Johnson-Marion-McMinn-Meigs- Northwest Monroe-Rhea-Sequatchie-Sevier Smoky Mountains- Southeast Carter-Southeast Greene-Southeast Monroe-Unicoi- West Polk. Flood Watch from Wednesday morning through Thursday morning for Anderson-Claiborne-Grainger-Hamblen-Hancock-Hawkins- Jefferson-Knox-Loudon-North Sevier-Northwest Blount- Northwest Carter-Northwest Cocke-Northwest Greene-Roane- Sullivan-Union-Washington TN. VA...Flood Watch from Wednesday morning through Thursday morning for Lee-Russell-Scott VA-Washington VA-Wise. && $$ SHORT TERM... AVIATION...