When thunderstorms go linear, they form squall lines. With supercells, this is not good for tornado potential, except the southernmost storm in the line may stil produce one under the right conditions. The tail-end charlie, as they call it, still may be drawing inflow to sustain it's updraft, while the others in the line have become outflow dominant. Chasing linear storms can be fun. Outflow from these storms is known to sometimes produce dark, black, turbulent shelf clouds, accompanied by gusty and sometimes damaging winds. I got a rather photogenic shelf cloud in Nebraska last Tuesday.