At least 13 people have been killed as a string of violent thunderstorms battered middle-America, with several tornadoes touching down in Oklahoma and high winds pounding rural Kansas.br br Tuesday's 'supercell' storms came just two days after a massive tornado tore through the southwest Missouri town of Joplin, killing at least 122 people and injuring hundreds more, according to authorities.br br Rob Reynolds reports from Joplin.