( ( When ( ( the ( jet stream ) ) ( ( dips ( to ( the south ) ) ) ( in ( the ( Northern Hemisphere ) ) ) ) ) ) ( , ( it ( ( ( ( creates ( ( a trough ) ( of ( low pressure ) ) ) ) , ) ( ( allowing ( cold ( polar ( air masses ) ) ) ) ( to ( invade ( the ( United States ) ) ) ) ) ) . ) ) ) )
(ROOT (S (SBAR (WHADVP (WRB When)) (S (NP (DT the) (NN jet) (NN stream)) (VP (VBD dips) (PP (TO to) (NP (DT the) (NN south))) (PP (IN in) (NP (DT the) (NNP Northern) (NNP Hemisphere)))))) (, ,) (NP (PRP it)) (VP (VBZ creates) (NP (NP (DT a) (NN trough)) (PP (IN of) (NP (JJ low) (NN pressure)))) (, ,) (S (VP (VBG allowing) (NP (JJ cold) (JJ polar) (NN air) (NNS masses)) (S (VP (TO to) (VP (VB invade) (NP (DT the) (NNP United) (NNPS States)))))))) (. .)))
When the jet stream dips to the south in the Northern Hemisphere, it creates a trough of low pressure, allowing cold polar air masses to invade the United States.
(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)))))))))) (. .)))