(S (S (ADVP (RB However)) (COMMA ,) (S (BASENP (JJ poor) (NN weather) (NNS conditions)) (VP (VBP have) (VP (VBN forced) (S (BASENP (DT these) (NNS aircraft)) (VP (TO to) (VP (VB return) (PP (TO to) (BASENP (NN land)))))))))) (PERIOD .))