The Collaborative International Dictionary of English v.0.48:

Provable \Prov"a*ble\, a. [See Prove, and cf. Probable.]
   Capable of being proved; demonstrable. -- Prov"a*ble*ness,
   n. -- Prov"a*bly, adv.
   [1913 Webster] Provand

WordNet (r) 3.0 (2006):

provably
    adv 1: in an obvious and provable manner; "his documentary
           sources are demonstrably wrong" [syn: demonstrably,
           provably, incontrovertibly]