( ( Most ( flash flooding ) ) ( ( is ( caused ( by ( ( slow-moving ( ( thunderstorms or ) thunderstorms ) ) ( repeatedly ( moving ( over ( the ( same area ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (JJS Most) (NN flash) (NN flooding)) (VP (VBZ is) (VP (VBN caused) (PP (IN by) (S (NP (JJ slow-moving) (NNS thunderstorms) (CC or) (NNS thunderstorms)) (ADVP (RB repeatedly)) (VP (VBG moving) (PP (IN over) (NP (DT the) (JJ same) (NN area)))))))) (. .)))
Most flash flooding is caused by slow-moving thunderstorms or thunderstorms repeatedly moving over the same area.
(ROOT (S (NP (DT A) (NN stream) (NN flooding)) (VP (VBZ is) (ADVP (RBS most) (RB likely)) (VP (VBN caused) (PP (IN by) (NP (DT a) (NN thunderstorm))))) (. .)))