NEW : Tornado watch in effect for parts of northwestern Mississippi . Law enforcement official reports tornado damage in Tupelo , Mississippi . Storm damages trees , power lines , and 18-wheeler on Interstate 20 . No injuries reported in Mississippi town .
NEW : Tornado watch in effect for parts of northwestern Alabama . Law enforcement official reports tornado damage in Tupelo , Mississippi . Storm damages trees , power lines , overturns 18-wheeler on U.S. 45 . No injuries reported in Mississippi town .