WordNet (r) 3.0 (2006):

rainstorm
    n 1: a storm with rain

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "rainstorm":
   brash, burst of rain, cloudburst, cyclone, deluge, dirty weather,
   downfall, downflow, downpour, drencher, flood, foul weather,
   gushing rain, heavy rain, hurricane, line squall, plash, pour,
   rainburst, rainspout, rough weather, scud, snowstorm, soaker,
   soaking rain, spate, spout, squall, storm, stormy weather, tempest,
   tempestuous rage, thunderstorm, tornado, torrent of rain, typhoon,
   waterspout, windstorm