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]

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