( ( The ( polar ( jet stream ) ) ) ( ( ( ( ( was ( much ( farther north ) ) ) ( than usual ) ) , ) ( ( blocking ( cold ( air masses ) ) ) ( from ( ( moving south ) ( from Canada ) ) ) ) ) . ) )
(ROOT (S (NP (DT The) (JJ polar) (NN jet) (NN stream)) (VP (VBD was) (ADVP (RB much) (RBR farther) (RB north)) (PP (IN than) (ADJP (JJ usual))) (, ,) (VP (VBG blocking) (NP (JJ cold) (NN air) (NNS masses)) (PP (IN from) (S (VP (VBG moving) (ADVP (RB south)) (PP (IN from) (NP (NNP Canada)))))))) (. .)))
The polar jet stream was much farther north than usual, blocking cold air masses from moving south from Canada.
(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)))))))))) (. .)))