( ( In ( the ( United States ) ) ) ( , ( weather ( ( most often ) ( ( ( ( moves ( west ( to east ) ) ) , ) ( because ( that ( is ( ( the direction ) ( the ( jet ( stream blows ) ) ) ) ) ) ) ) . ) ) ) ) )
(ROOT (S (PP (IN In) (NP (DT the) (NNP United) (NNPS States))) (, ,) (NP (NN weather)) (ADVP (RBS most) (RB often)) (VP (VBZ moves) (ADVP (RB west) (TO to) (RB east)) (, ,) (SBAR (IN because) (S (NP (DT that)) (VP (VBZ is) (NP (NP (DT the) (NN direction)) (NP (DT the) (NN jet) (NN stream) (NNS blows))))))) (. .)))
In the United States, weather most often moves west to east, because that is the direction the jet stream blows.
(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)))))))))) (. .)))