I think part of the reason for the extreme damage in the Jarrell tornado was because it was moving so slowly (10-15 mph I believe). It was propagating southwestward along a boundary rather than hauling ass with the westerlies. The WCM at WFO Austin/San Antonio told me a few years ago that several cars were never found. The only place they think they could have gone was Lake Travis ~40 miles away, which seems unlikely.