Here are the thoughts of NWS Forecaster Kevin Laws:
Quick update on mesoscale situation…
My biggest and most obvious concern is areas east and southeast of where the initial line of showers and thunderstorms are slowing to a stop. In general…areas from Marengo to Calhoun counties.
I really feel like the areas back to the northwest of this line will struggle to rebuild instability.
I am a bit on the fence about canceling those counties out of the original watch, mainly because the surface low center is expected to track right through those counties later this afternoon.
This really keeps the shear numbers very high…but again instability is low.
There are just of few cloud breaks over the original watched counties, so i suppose there is a small window of opportunity to regenerate thunder along and northeast of the cold pool being generated in central and north central MS.
To the east and southeast (new watched counties), surface based instabilities will continue to sky rocket through the bulk of the afternoon, but best shear is lagging back with the surface low.
Also, the derecho is starting to outrun best upper dynamics…hence it has weakened over the past hour.
Even considering the fact of outrunning upper support, the environment downstream is extremely juicy.
In addition, the new boundary setup across the central AL from southwest to northeast (see surface obs and visible sat image) will likely begin to interact as well…hence the shape of the watch.
