The Collaborative International Dictionary of English v.0.48:

Forewarn \Fore*warn"\ (f[=o]r*w[add]rn"), v. t. [imp. & p. p.
   Forewarned (f[=o]r*w[add]rnd"); p. pr. & vb. n.
   Forewarning.]
   To warn beforehand; to give previous warning, admonition,
   information, or notice to; to caution in advance.
   [1913 Webster]

         We were forewarned of your coming.       --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

forewarn
    v 1: warn in advance or beforehand; give an early warning; "I
         forewarned him of the trouble that would arise if he showed
         up at his ex-wife's house" [syn: forewarn, previse]

Moby Thesaurus II by Grady Ward, 1.0:

20 Moby Thesaurus words for "forewarn":
   apprehend, bode, caution, croak, forebode, give advance notice,
   have a premonition, have a presentiment, look black, lower, menace,
   portend, preapprehend, precaution, premonish, prenotify, prewarn,
   tell in advance, threaten, warn