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):

forewarning
    n 1: an early warning about a future event [syn: forewarning,
         premonition]

Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "forewarning":
   admonition, advance notice, apocalyptic, augural, auguring,
   caution, caveat, divinatory, fatidic, foreboding, forecasting,
   forerunning, foreseeing, foreshadowing, foreshowing, foretelling,
   foretokening, fortunetelling, haruspical, heavy with meaning,
   indicative, intuitive, mantic, meaningful, monition, monitory,
   oracular, plenty of notice, portent, portentous, precautional,
   precautionary, precautioning, precursive, precursory, predictional,
   predictive, predictory, prefigurative, prefiguring, preindicative,
   premonition, premonitory, prenotice, prenotification, presageful,
   presaging, presentiment, presignificative, presignifying,
   prewarning, prodromal, prodromic, prognostic, prognosticative,
   prophetic, sibyllic, sibylline, significant, vaticinal,
   vaticinatory, warning, weather-wise