warning has 7 distinct synonyms spread across 2 of its 4 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a message informing of danger
“a warning that still more bombs could explode”
cautionary advice about something imminent (especially imminent danger or other unpleasantness)
“a letter of admonition about the dangers of immorality”
notification of something, usually in advance
“they gave little warning of their arrival”
Synonyms, antonyms and the broader and narrower relations for warning come from Princeton University’s WordNet. Full sources and licences.