( ( The term ) ( ( ( is ( most commonly ) ) ( ( employed ( by ( the ( news media ) ) ) ) ( when ( ( such ( a ( frigid ( air mass ) ) ) ) ( moves ( into ( ( ( areas ( where ( it ( is ( relatively unusual ) ) ) ) ) , ) ( such ( as ( ( the ( Deep ( South ( or ( Eastern Seaboard ) ) ) ) ) ( of ( the ( United States ) ) ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT The) (NN term)) (VP (VBZ is) (ADVP (RBS most) (RB commonly)) (VP (VBN employed) (PP (IN by) (NP (DT the) (NN news) (NNS media))) (SBAR (WHADVP (WRB when)) (S (NP (PDT such) (DT a) (JJ frigid) (NN air) (NN mass)) (VP (VBZ moves) (PP (IN into) (NP (NP (NNS areas)) (SBAR (WHADVP (WRB where)) (S (NP (PRP it)) (VP (VBZ is) (ADJP (RB relatively) (JJ unusual))))) (, ,) (PP (JJ such) (IN as) (NP (NP (DT the) (NNP Deep) (NNP South) (CC or) (NNP Eastern) (NNP Seaboard)) (PP (IN of) (NP (DT the) (NNP United) (NNPS States)))))))))))) (. .)))
The term is most commonly employed by the news media when such a frigid air mass moves into areas where it is relatively unusual, such as the Deep South or Eastern Seaboard of 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)))))))))) (. .)))