Synonyms for Forewarn:

n.

whistle, lecture, sound/raise the alarm, tip off, portend, warn off. alarm. foresight. forewarn (noun)
previse.

v.

caution (verb)
exhort, warn, enjoin, advise, caution, alert, admonish.
caution that something may happen (verb)
alarm, admonish, tip off, alert, advise, portend.
threaten (verb)
scare, threaten, intimidate, frighten, browbeat, harass, torment, bully, forebode, hint, bulldoze.

Other synonyms:

portend, warn off. tip off. lecture, whistle. warn
alert.
Other relevant words:
alarm, previse, whistle, portend.

Usage examples for Forewarn

  1. " It happens," says he, " to be an advantage that at present I may own to have been the first so happy to foresee, or honest to forewarn our Court here, of the absolute necessity of our then breaking with the Czar, and shutting him out again of the Baltic." – Secret Diplomatic History of The Eighteenth Century by Karl Marx
  2. The native messenger, to whom I gave a small gratuity, immediately started; thus I should be able to forewarn my people in the event of trouble. – Ismailia by Samuel W. Baker