The Collaborative International Dictionary of English v.0.48:

Prognosticator \Prog*nos"ti*ca`tor\, n.
   One who prognosticates; a foreknower or foreteller of a
   future course or event by present signs. --Isa. xlvii. 13.
   [1913 Webster]

WordNet (r) 3.0 (2006):

prognosticator
    n 1: someone who makes predictions of the future (usually on the
         basis of special knowledge) [syn: forecaster,
         predictor, prognosticator, soothsayer]

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "prognosticator":
   Cassandra, Druid, astrologer, augur, calamity howler,
   crystal gazer, divinator, diviner, divineress, forecaster,
   foreknower, foreseer, foreshower, foreteller, fortuneteller,
   geomancer, haruspex, palmist, predictor, prefigurer, presager,
   prophesier, prophet, prophet of doom, prophetess, psychic,
   pythoness, religious prophets, seer, seeress, soothsayer, vates,
   weather prophet