Area Forecast Discussion
Issued by NWS Knoxville/Tri Cities, TN
Issued by NWS Knoxville/Tri Cities, TN
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606 FXUS64 KMRX 160232 AFDMRX Area Forecast Discussion National Weather Service Morristown TN 1032 PM EDT Thu Aug 15 2024 ...New UPDATE... .UPDATE... Issued at 1023 PM EDT Thu Aug 15 2024 Forecast is on track tonight with mostly clear sky conditions. The main impacts will be on Friday when a line of showers and thunderstorms is expected to move across the area. Exact timing is uncertain, but recent CAMs are showing additional convective initiation associated with the convective cluster currently across Missouri. This cluster would develop into an MCS and move towards our area mid-day to early afternoon on Friday. While this will need to be monitored, the overall severe risk continues to be marginal. The upper shortwave and PVA will be focused north of our area across the Ohio Valley with limited to no height falls for the East Tennessee area. This will result in poor mid- level lapse rates and limited instability with HRRR forecast soundings showing MLCAPE around 1000 to 1500 J/Kg with highest values across our western counties near the Cumberland Plateau. Effective bulk shear is also weak at around 15 to 20 kt. While the severe potential is limited, there is still a low risk. Even with sub- severe winds, some downed trees will be expected. In addition, RAP forecast PW values around 2.0 to 2.2 inches will result in heavy rain rates and localized flash flooding potential on Friday. && .SHORT TERM... (This evening through Friday) Issued at 300 PM EDT Thu Aug 15 2024 Key Messages: 1. Isolated showers through this evening over parts of higher terrain. 2. Scattered to numerous showers and storms Friday, with some strong to severe storms and localized flooding possible. Damaging winds would be the primary threat with any severe storms. Discussion: The rest of this evening and tonight should be mostly quiet, with some isolated shower development over the plateau and mountains. At least a couple hundredths has been recorded as of this time in some areas. Activity should wane with the loss of day time heating. Patches of fog may also be possible due to the influx of moisture funneling our way under calm conditions overnight. Friday: our mostly dry spell will come to an end this weekend with an approaching system and attendant cold front. A mid to upper level trough is forecast to slide east across the northern tier of the country. The low center will pretty much be stacked underneath it. Pre-frontal showers and thunderstorms will impact the area as early as just before or around sunrise from the west and northwest. SPC currently has us in a MRGL risk for severe weather and they mention the possibility of the morning convection diurnally strengthening as we approach Friday afternoon. 20 to 30KT effective shear along with CAPE in the 1000 J/Kg range is enough to support multicellular storms. Exact timing, evolution, movement, and the possibility or not of multiple rounds is still pretty unclear even with CAMs less than 24 hours out. All in all we can expect the possibility of strong to severe storms with damaging winds the primary threat. Heavy rainfall could also lead to localized flash flooding. Hopefully the latter will hold off with how dry we have lately been. && .LONG TERM... (Friday night through next Thursday) Issued at 300 PM EDT Thu Aug 15 2024 Key Messages: 1. Scattered to numerous showers and storms Friday evening and Saturday, with some strong to severe storms possible. 2. More scattered showers and storms possible Sunday and Monday, with better coverage across the northern areas. 3. Higher pressure building south with a more northwest flow and drier Tuesday and Wednesday with cooler high temperatures and low temperatures dropping into the 50s. Discussion: Models in general agreement with upper low over Wisconsin and Michigan to start the extended forecast period Friday night. The upper low will drift, move southeast through Saturday and Sunday opening up by Sunday night into the northeast states and digging an upper trough across the eastern states into the TN Valley and southeast through Sunday night. Multiple shortwaves will progress through the NW flow and across the Tennessee Valley Friday night and continuing through Saturday and Sunday. A cold front will be approaching the area as well on Saturday. The trough looks to shift eastward Monday into Monday night with improving conditions Monday night and continuing through Wednesday. Models are still different in the details with the timing of the shortwaves coming through but are showing Friday evening and late Saturday afternoon and evening as the best times for significant rainfall and thunderstorms. Sunday the trough and cold front will have shifted farther south with more convection south of the Tennessee state line in MS, AL, GA and the Carolinas with the GFS and NAM12 showing that will be where more significant convection develops. Model runs have been showing differences so stay tuned. With the threat for heavy rainfall some isolated flooding is possible Friday night but a better chance will be Saturday night if Friday night is wet too. Sunday and Sunday night into Monday expect storms to more directly effect the northern half of the area with another significant shortwave moving through the Ohio Valley Sunday. Overall the atmosphere looks supportive of more organized convection with some strong to severe storms possible Friday evening. The slow- moving upper trough/low continues east across the Great Lakes/OH Valley Saturday. The weak surface low over southern MI is forecast to gradually develop into southern Canada/Ontario. The weak front extending southward from the low should slowly move east-southeast across the OH Valley and Mid-Atlantic into the Southeast through Saturday night. The boundary, although not well defined and having only modest low-level convergence, should serve as a focus for strong to severe convection. Diurnal heating of a moist airmass along/ahead of the front should bring weak to moderate destabilization Saturday afternoon, even though mid-level lapse rates should remain modest. Enhanced mid-level flow associated with the upper trough/low should focus storms over parts of the lower OH Valley into the TN Valley and Mid-South. But, sufficient deep-layer shear to support some updraft organization should be present over much of the broad warm sector. Multiple clusters may develop along the length of the front Saturday afternoon and pose primarily an isolated strong to damaging wind threat as they spread east- southeast through Saturday evening. So Marginal risk is across the forecast area. Damaging winds will be the primary severe threat with these storms. In addition, any areas that see repeated and/or prolonged rounds of heavy rainfall may experience localized flooding. Cooler temperatures will be moving in behind the front Sunday and Monday, with temperatures at or a little below normal Sunday and below normal on Monday. Highs Monday only in the 60s to lower 70s higher elevations and 70s to the lower 80s. On Tuesday and continuing into Wednesday a cooler surface high pressure will build into the eastern Tennessee Valley and southern Appalachians. Dewpoints will lower into the more comfortable 50s at most sites with actual lows in the 50s Tuesday into Thursday mornings. A weak surface trough is expected to move through the upper flow Thursday and may trigger a few showers or storms. Temperatures will be warming back into the mid to upper 80s Thursday. && .AVIATION... (00Z TAFS) Issued at 724 PM EDT Thu Aug 15 2024 VFR conditions continue tonight with a low probability of fog near the TRI terminal. Rain chances increase on Friday with VCSH from mid-morning throughout the afternoon. Have included prob30 TSRA at all terminals as a thunderstorm complex is forecast to move across the region, but timing is still uncertain. SW winds remain mostly 5-10 kt with higher gusts near any thunderstorms. && .PRELIMINARY POINT TEMPS/POPS... Chattanooga Airport, TN 72 94 73 93 / 0 50 50 70 Knoxville McGhee Tyson Airport, TN 72 91 71 90 / 0 50 70 70 Oak Ridge, TN 71 89 70 89 / 10 60 70 70 Tri Cities Airport, TN 67 86 69 85 / 10 50 60 50 && .MRX WATCHES/WARNINGS/ADVISORIES... NC...NONE. TN...NONE. VA...NONE. && $$ SHORT TERM...KS LONG TERM....TD AVIATION...JB