The Collaborative International Dictionary of English v.0.48:

Extemporal \Ex*tem"po*ral\, a. [L. extemporalis, from ex
   tempore.]
   Extemporaneous; unpremeditated. [Obs.] --B. Jonson. --
   Ex*tem"po*ral*ly, adv. [Obs.]
   [1913 Webster]