Severe convection can also be outflow dominate. Outflow dominate severe thunderstorms can and often form a derecho and (MCS)'s Mesoscale Convective Systems. A derecho is a prolonged/wide-spread wind storm that lasts for hours and travels across several states. Bow-echo's, squall lines, and MCC's are all convective systems. Heck, even supercells can be embedded within a MCS. MCS's are notorious for torrential rains and damaging straight-line winds. MCS's usually form from storm mergers and surface cold pool interaction. This allows these systems to feed off warm (buoyant-air) as they progress east. They also usually have a well-defined shelf cloud out ahead of the rain/hail that can be seen rollin' as it passes overhead.
Here's an example of a (MCS) Mesoscale Convective System (above). This illustrates the rollin' motion of a shelf cloud.
Shelf clouds form when thunderstorms form a cold pool at the surface. Since warm air rises this allows the warm (buoyant) air to rise above the colder more dense cold pool. This in turn leads to the formation of the shelf cloud. Shelf clouds usually precede severe wind gusts and heavy rain. A few time-lapses can be found below:
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