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

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000 FXUS64 KMRX 271445 AAB AFDMRX Area Forecast Discussion...UPDATED National Weather Service Morristown TN 1045 AM EDT Sat May 27 2017 .DISCUSSION...
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Complex weather situation unfolding today, with the **potential** for some strong to severe thunderstorms from early this afternoon through late this evening **if** the current capping inversion in place now, weakens. As might be expected in this situation, there have been plenty of fluctuations within the models (particularly the mesoscale ones) on if this will occur and how this day will play out. The NAM-3/NAM-12 and NSSL-WRF have been most consistent showing the current activity over NW TN weakening this morning (due to the cap) and then redeveloping late this afternoon across the region once the cap breaks. The HRRR (as is sometimes the case) has been much more inconsistent with this afternoon`s activity. Some runs have shown the CAP holding, while others have shown robust convective signals in the 21-02z time period. In addition to gradual boundary layer heating, a feature that will aid in overcoming any inhibition will be a an upper- level perturbation/shortwave that will drop of the mid-Mississippi Valley toward the Southern Appalachians later this afternoon and evening. This may provide enough forcing to overcome any lingering inhibition in the early to mid afternoon and getting things going later today. The upstream convection will weaken (as mentioned) as it moves into the area by midday. However, it still may lay out a outflow boundary or two somewhere -- which could also help to initiate/focus convection later on. If/when the cap does break, several environmental parameters are in place that will allow storms to quickly go severe. Particularly, with respect to large hail and downbursts. MLCAPE values are progged to be in the 2000-2500 J/kg range, with MUCAPEs over 3000 J/kg at times. Hail CAPEs (1500 J/kg and DCAPEs over 1000 J/kg) will create a very favorable thermodynamic environment for strong updrafts, capable of supporting the aforementioned threats. Bulk shear values are quite high in the 50-60 kt range across the area -- though generally unidirectional in nature (ESE). Straight hodographs in this shear environment would favor organized, multicell clusters and supercells (as well as numerous splitting cells). The 0-1 km shear is not as favorable for tornadoes as other days, but the presence of a couple boundaries may enhance that some -- especially if a localized backed wind profile is realized. A surface cold front may approach SW Virginia around 00z, also serving as a source for severe weather in and around the northeast zones around sunset (00-03z). One final note (though this is outside of the today period). Mesoscale models are showing an earlier arrival time for the MCS overnight. This is something to watch very closely as we head into the afternoon forecast package. MCS features can tend to propagate faster in higher CAPE environments than the lower res models sometimes realize.
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Chattanooga Airport, TN 83 69 87 69 / 60 50 60 60 Knoxville McGhee Tyson Airport, TN 80 65 83 66 / 60 60 60 60 Oak Ridge, TN 81 65 83 67 / 60 50 60 60 Tri Cities Airport, TN 78 62 80 63 / 60 70 70 60
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