( ( Explain ( why ( ( the ( central ( United States ) ) ) ( ( is not ) ( a ( good ( air ( mass ( source region ) ) ) ) ) ) ) ) ) . )
(ROOT (S (VP (VB Explain) (SBAR (WHADVP (WRB why)) (S (NP (DT the) (JJ central) (NNP United) (NNPS States)) (VP (VBZ is) (RB not) (NP (DT a) (JJ good) (NN air) (NN mass) (NN source) (NN region)))))) (. .)))
Explain why the central United States is not a good air mass source region.
(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)))))))))) (. .)))