( But ( then ( ( the ( Chinook winds ) ) ( ( are unpredictable ) . ) ) ) )
(ROOT (S (CC But) (ADVP (RB then)) (NP (DT the) (NNP Chinook) (NNS winds)) (VP (VBP are) (ADJP (JJ unpredictable))) (. .)))
But then the Chinook winds are unpredictable.
(ROOT (S (NP (NN Chinook) (NNS winds)) (VP (VBP occur) (SBAR (WHADVP (WRB when)) (S (NP (NN air)) (VP (VBZ is) (VP (VBN forced) (PP (IN over) (NP (DT a) (NN mountain) (NN range)))))))) (. .)))