Synonyms for Alarming (same or very similar meaning)
WordNet sense 1 (causing extreme terror):
terrific, terrifyingWordNet sense 2 (so scary as to cause chills and shudders):
scaryWordNet sense 3 (causing fear or dread or terror):
dread, dreadful, frightening, terribleWordNet sense 4 (provoking horror):
frightfulFrom the ODE community, based on WordNetadd/edit