SPC Mesoscale Discussion: Severe Potential… Watch Unlikely

| August 7, 2019 @ 1:24 am

SUMMARY…Isolated damaging gusts, some near severe limits, may occur with a southeastward-moving band of thunderstorms during the next few hours. A watch is not currently expected.

DISCUSSION…A loosely organized, multicellular thunderstorm cluster was evident at 06Z crossing from southern TN into portions of northeastern MS and northern AL along an instability gradient. The thermodynamic environment was characterized by rich low-level moisture — manifest in low/mid 70s F surface dew points, though modified 00Z BMX RAOB and RAP soundings indicate only about 1.3- 1.5-inch total PW thanks to a dry layer above the surface. Nocturnal stabilization, and the presence of that dry layer, has offsetting effects: 1. Rendering MUCAPE unrepresentative on the high side while minimizing MLCAPE, with the latter ranging from around 1000 J/kg in central MS to only around 250 J/kg in northeastern AL, and 2. Support evaporative cooling and downdraft acceleration in that layer just above the surface. It is uncertain how much such downward parcel acceleration can overcome a gradually cooling near-surface layer (with associated increase in static stability) to render severe wind at the surface, but damaging gusts cannot be ruled out in the most intense cores. Kinematically, low-level flow should remain very weak, with west-northwesterly to northwesterly 850-mb winds minimizing storm-relative flow in the inflow layer. Regardless, the forward-propagational component of convective motion should be sufficient to sustain some inflow for a few more hours, amidst about 30-40 kt ambient effective-shear magnitudes. While a severe (50+ kt) gust cannot be ruled out, the potential currently appears too isolated and marginal for a watch.


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