Spring thunderstorms marched across the Plains, hurling hail, whipping up 60-m.p.h. winds and prompting flash-flood warnings. Showers and thunderstorms stretched from Texas through Oklahoma and into Iowa, Kansas, Nebraska and South Dakota, the National Weather Service said. Showers also extended from Mississippi into Alabama and from Illinois into Wisconsin. High winds, which authorities say may have been a tornado, damaged a barn in Crawford, Tex., near Waco, where winds were clocked at more than 60 m.p.h. During a six-hour period, rainfall was measured at two inches at Kansas City, 1 1/2 inches at Grand Island, Neb., and nearly an inch at Concordia, Kan.