The Collaborative International Dictionary of English v.0.48:

Cloud-burst \Cloud"-burst`\, n.
   A sudden copious rainfall, as if the whole cloud had been
   precipitated at once.
   [1913 Webster]

WordNet (r) 3.0 (2006):

cloudburst
    n 1: a heavy rain [syn: downpour, cloudburst, deluge,
         waterspout, torrent, pelter, soaker]

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "cloudburst":
   brash, burst of rain, deluge, downfall, downflow, downpour,
   drencher, flood, gushing rain, heavy rain, plash, pour, rainburst,
   rainspout, rainstorm, scud, soaker, soaking rain, spate, spout,
   torrent of rain, waterspout