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Area Forecast Discussion
Issued by NWS Knoxville/Tri Cities, TN
Issued by NWS Knoxville/Tri Cities, TN
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951 FXUS64 KMRX 200516 AAA AFDMRX Area Forecast Discussion...UPDATED National Weather Service Morristown TN 116 AM EDT Sat Jul 20 2024 ...New AVIATION... .UPDATE... Issued at 950 PM EDT Fri Jul 19 2024 Just some mainly light showers around at the moment but an uptick in convection is still expected later tonight. Will lower PoPs and remove thunder early with this update, but then will trend back toward the earlier forecast for later tonight with higher chances. Will also tweak temps and dew points. && .SHORT TERM... (This evening through Saturday) Issued at 312 PM EDT Fri Jul 19 2024 Key Messages: 1. Some places will likely remain rain free, but rain chances continue to improve through the short term as multiple rounds of showers and thunderstorms are expected. 2. Continue below normal temperatures will continue. Discussion: Main concern for the short term period is timing of shower and thunderstorm activity. Overall, we`ll remain situated on the eastern side of a trough positioned over the lower and mid-Mississippi river valley region. Embedded in that trough will be shortwave activity lifting northeast out of the central Gulf states and moving through the Tennessee valley. This is expected to drive multiple rounds of convection over the next 24 hours or so. Currently have one of these impulses moving into the TN valley from the southwest, another is expected around or shortly before daybreak tomorrow, and finally we should see more diurnally driven convection tomorrow afternoon. Nothing looks terribly concerning in terms of severe weather or flash flooding. Forecast soundings do show deep layer shear, instability, or downdraft CAPE values that are supportive of severe type convection, with surface based CAPE values forecast to be only around 1000 J/kg, and DCAPE less than 500 J/kg. PWATs will be increasing tonight and tomorrow and storm motions are weak. Thus storm propagation will likely be focused on outflow boundary interactions. As such, if a storm maintains itself for a prolonged period of time it`s conceivable there could be a very isolated flash flooding threat this afternoon or tomorrow. But all in all that threat even appears low given the ongoing drought conditions. Otherwise, we`ll continue to get a break from the oppressive heat, with forecast highs remaining below seasonal norms tomorrow. Considered undercutting NBM guidance by a degree or two for tomorrow`s highs as upstream readings this afternoon have been stuck in the 70s. Given the expected cloud cover tomorrow as shortwave activity shifts north to the TN valley, I wouldn`t be surprised to see some valley highs nearer to 80 degrees versus norms of mid to upper 80s. Hard to say where that would be though, and any prolonged sunbreaks would allow for temps to climb quickly so the NBM guidance, which is already 4-6 degrees below normal, was fine. && .LONG TERM... (Saturday night through next Friday) Issued at 312 PM EDT Fri Jul 19 2024 Key Messages: 1. An unsettled weather pattern will persist through the long term with daily chances for showers and thunderstorms. 2. Slightly below normal temperatures expected through the extended forecast. Discussion: A pretty pronounced closed upper low will encompass eastern Canada much of the long term with troughing over our area of the country, while ridges dominate the desert SW and the western Atlantic. With this pattern, low level SWly flow will be the persistent/dominate direction. In other words, SWly flow and lower heights will mean increased humidity, lower than normal temperatures, and afternoon/evening convective instability. A stationary front draped to our southeast and another developing to our northwest next week, will also contribute to daily chances of showers and thunderstorms. This will also keep cloud cover around, limiting our daytime highs to the 80s. Even with decreased temperatures, dew points in the 60s and 70s will make it feel much hotter. Any rainfall will be welcome of course with the drought conditions we have in place. Precipitation totals for the entire long term show many locations receiving 1.5 to 2.5 inch range with the Appalachians possibly reaching 3 to 4 inches of accumulated rainfall. MRGL ERO`s are in place the next couple of days, but it would take a short- lived storm with heavy rain rates to reach any sort of flooding potential. PWATs will range between 1.5 to 2.0 inches, portraying how water loaded the atmosphere is and will support prolific rain makers. && .AVIATION... (06Z TAFS) Issued at 113 AM EDT Sat Jul 20 2024 Light showers will be around the area through the night, and cigs are expected to drop at all sites, mainly to MVFR. Most showers should exit by late morning, with cigs rising to VFR in the noon to 18Z time frame. Can`t rule out additional showers/storms in the afternoon at CHA and TYS, so VCTS will be mentioned at those sites. Winds will be light through the period. && .PRELIMINARY POINT TEMPS/POPS... Chattanooga Airport, TN 86 70 86 71 / 70 40 80 40 Knoxville McGhee Tyson Airport, TN 85 70 86 70 / 70 40 80 40 Oak Ridge, TN 84 68 86 69 / 70 30 80 40 Tri Cities Airport, TN 82 66 86 67 / 70 30 70 30 && .MRX WATCHES/WARNINGS/ADVISORIES... NC...NONE. TN...NONE. VA...NONE. && $$ SHORT TERM...CD LONG TERM....KS AVIATION...DGS