( ( Frontal storms ) ( ( ( ( form ( at ( ( the front ) ( of ( ( large ( , ( moist ( air masses ) ) ) ) ( moving ( across ( the country ) ) ) ) ) ) ) ) and ) ( can ( ( cause ( floods ( in ( ( the ( northern ( and ( eastern parts ) ) ) ) ( of ( the ( United States ) ) ) ) ) ) ) ( during ( the ( ( winter and ) spring ) ) ) ) ) ) . ) )
(ROOT (S (NP (JJ Frontal) (NNS storms)) (VP (VP (VBP form) (PP (IN at) (NP (NP (DT the) (NN front)) (PP (IN of) (NP (NP (JJ large) (, ,) (JJ moist) (NN air) (NNS masses)) (VP (VBG moving) (PP (IN across) (NP (DT the) (NN country))))))))) (CC and) (VP (MD can) (VP (VB cause) (NP (NP (NNS floods)) (PP (IN in) (NP (NP (DT the) (JJ northern) (CC and) (JJ eastern) (NNS parts)) (PP (IN of) (NP (DT the) (NNP United) (NNPS States)))))) (PP (IN during) (NP (DT the) (NN winter) (CC and) (NN spring)))))) (. .)))
Frontal storms form at the front of large, moist air masses moving across the country and can cause floods in the northern and eastern parts of the United States during the winter and spring.
(ROOT (S (NP (NP (DT The) (JJS best) (NN explanation)) (PP (IN for) (SBAR (WHADVP (WRB how)) (S (NP (NN air) (NNS masses)) (VP (VBP move) (PP (IN across) (NP (DT the) (VBN united) (NNS states)))))))) (VP (VBZ is) (SBAR (IN that) (S (NP (DT the) (VBG prevailing) (NNS westerlies)) (VP (VP (VBP move) (NP (NN air) (NNS masses)) (PP (IN from) (NP (NN west))) (PP (TO to) (ADJP (JJ east))) (PP (IN across) (NP (DT the) (VBN united) (NNS states)))) (CC but) (VP (MD may) (VP (VB be) (VP (VBN deflected) (PP (IN by) (NP (DT the) (NN jet) (NN stream)))))))))) (. .)))