( PLH ( ( ( ( overwinter ( as ( adults ( in ( the ( southern states ) ) ) ) ) ) and ) ( ( move northward ) ( in ( summer ( air masses ) ) ) ) ) . ) )
(ROOT (S (NP (NNP PLH)) (VP (VP (VBP overwinter) (PP (IN as) (NP (NP (NNS adults)) (PP (IN in) (NP (DT the) (JJ southern) (NNS states)))))) (CC and) (VP (VBP move) (ADVP (RB northward)) (PP (IN in) (NP (NN summer) (NN air) (NNS masses))))) (. .)))
PLH overwinter as adults in the southern states and move northward in summer air masses.
(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)))))))))) (. .)))