The Collaborative International Dictionary of English v.0.48:

Observable \Ob*serv"a*ble\, a. [L. observabilis: cf. F.
   observable.]
   1. Capable of being observed; discernible; noticeable. --Sir.
      T. Browne.
      [1913 Webster]

            The difference is sufficiently observable.
                                                  --Southey.
      [1913 Webster]

   2. Worthy of being observed; important enough to be noted or
      celebrated; as, an observable anniversary.
      [PJC]

   3. Noteworthy; remarkable.
      [PJC] -- Ob*serv"a*ble*ness, n. -- Ob*serv"a*bly, adv.
      [1913 Webster]

WordNet (r) 3.0 (2006):

observably
    adv 1: in a noticeable manner; "he changed noticeably over the
           years" [syn: perceptibly, noticeably, observably]
           [ant: imperceptibly, unnoticeably]