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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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841 FXUS64 KMRX 161758 AAC AFDMRX Area Forecast Discussion...UPDATED National Weather Service Morristown TN 158 PM EDT Fri Aug 16 2024 ...New AVIATION... .UPDATE... Issued at 949 AM EDT Fri Aug 16 2024 Main purpose for the morning update was to try and bring PoPs in line with current and expected radar trends. Currently have a ragged looking line of convection moving east through middle Tennessee. It should be approaching the plateau areas in the next couple of hours, then pushing into the valley areas during the early afternoon hours. PoPs from both the latest CAM guidance as well as the most recent hourly NBM breakdowns show increasing convective coverage during the afternoon hours. This isn`t out of line with what the previous shift had. Main adjustments for the morning update were to cut down PoPs outside of the plateau as our CWA is currently rain free and that trend likely continues for a few more hours before the incoming line/boundary plus diurnal trends assist in increasing convective coverage. Otherwise, bumped up afternoon highs in the south as they were a tad warmer than hourly forecast grids suggested, and they`re largely cloud free down there. && .SHORT TERM... (Today and tonight) Issued at 237 AM EDT Fri Aug 16 2024 1. A line of strong storms will move through the region this afternoon, with some severe storms and localized flooding possible. Damaging winds will be the primary threat with any severe storms. 2. Heat indices around 100 degrees in the Southern Tennessee Valley likely before storms move into the region. 3. Uncertainty is higher than normal for rain potential this evening and overnight. Some showers will be possible. Discussion: An unsettled pattern will be in place this period with a vertically stacked low moving into the Upper Midwest and Great Lakes today and tonight. There is increasing confidence that a MCS will move through the region this afternoon. CAMs are in pretty good agreement on timing (afternoon into early evening). SPC has the region outlooked in a marginal risk for severe which seems appropriate since the better forcing and shear is to the north. Although, CAPE will be decent around 1000 to 1500 J/Kg this afternoon. Effective shear is fairly low around 20 knots. Damaging winds will be the primary threat with the MCS this afternoon. The line of thunderstorms will move through fairly quickly but high rain rates are likely with precipitable water values near 2 inches. Some flooding issues will be possible but widespread flooding is not expected since the area has been dry. Heat indices around 100 degrees will be likely in the Southern Tennessee Valley this afternoon before storms move into the region due to increasing southerly flow and sunshine expected through early afternoon in the southernmost counties. This evening and overnight is more uncertain when it comes to rain chances. NBM seems too high on POPs since CAMs have little to no shower activity after sunset. Minor shortwaves moving through the pattern could spark some showers and storms overnight but confidence is low. Frontal passage will not occur during this forecast period. && .LONG TERM... (Saturday through Thursday) Issued at 237 AM EDT Fri Aug 16 2024 Key Messages: 1. Shower and thunderstorm chances continue through the weekend and into Monday. 2. High pressure begins to build in next week, with northwest flow expected leading to generally cool and dry weather. Discussion: Saturday night will see another chance for enhanced shower and thunderstorm activity as a boundary dives out of the north/northwest on the base of the slow moving low pressure system spinning through the Great Lakes region towards the northeast US. This system will bring additional heavy thunderstorms which could cause isolated flooding of poor drainage, and urban areas as the PWAT values will remain elevated. Outside of flooding, the main threat with these storms still looks to be straight-line winds right as the storms/line of storms moves through. Shower and thunderstorm chances will continue into Sunday as the wrap around moisture is able to continue to flow into our region. PWAT values and QPF amounts should both be lower for the second half of the weekend with less moisture in the atmosphere, hopefully limiting the flooding threat. Chances for showers and thunderstorms increase as you move further north (closer to to the low) with the best chances to see the most widespread storms being north of Interstate 40, and especially north of the Tennessee/Virginia state line. As the low continues to take it`s time moving off to the northeast we`ll eventually dry out on Monday as much drier and cooler air from the northwest filters in as a ridge continues to build in strength over the central/southwest US. This sustained flow of cooler and drier air will keep temperatures generally in the 70`s to 80`s and dew points in the 50`s. So most of the upcoming work week should be fairly pleasant weather-wise, especially for the month of August. && .AVIATION... (18Z TAFS) Issued at 142 PM EDT Fri Aug 16 2024 Still expect numerous SHRA/TSRA to develop across east TN through this evening. Previous timing was a little too quick so I bumped it back to the 20z-21z timeframe. Should see a definitive break in convective activity this evening and overnight. One caveat could be KTRI where some ISOLD SHRA may redevelop before daybreak as upper support draws near, but not confident enough to include that right now. Lastly, several guidance sources show fog or low clouds developing tonight north of the I-40 corridor. Think this largely depends on mid/high clouds clearing out and whether terminals see rain. Given at least some uncertainty there, will leave that out of TAFs for now as well. && .PRELIMINARY POINT TEMPS/POPS... Chattanooga Airport, TN 94 73 94 71 / 60 50 60 60 Knoxville McGhee Tyson Airport, TN 91 71 90 70 / 70 50 40 70 Oak Ridge, TN 89 71 89 69 / 70 50 40 70 Tri Cities Airport, TN 84 68 86 67 / 60 70 20 60 && .MRX WATCHES/WARNINGS/ADVISORIES... NC...NONE. TN...NONE. VA...NONE. && $$ SHORT TERM...CD AVIATION...CD