The Collaborative International Dictionary of English v.0.48:

Downpour \Down"pour`\, n.
   A pouring or streaming downwards; esp., a heavy or continuous
   shower.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

93 Moby Thesaurus words for "downpour":
   affluence, afflux, affluxion, brash, burst of rain, cascade,
   cataract, chute, cloudburst, collapse, comedown, concourse,
   confluence, conflux, course, crash, crosscurrent, current, debacle,
   declension, declination, defluxion, deluge, descending, descension,
   descent, down, downbend, downcome, downcurve, downfall, downflow,
   downgrade, downrush, downtrend, downturn, downward trend, drencher,
   drift, driftage, drop, dropping, fall, falling, flood, flow,
   flowing, fluency, flux, gravitation, gush, gushing rain,
   heavy rain, inclination, inflow, inundation, mill run, millrace,
   monsoon, onrush, onward course, outflow, plash, plummeting, pounce,
   pour, race, rainburst, rainspout, rainstorm, rapids, run, rush,
   scud, set, soaker, soaking rain, spate, spout, stoop, stream,
   surge, swoop, thunderstorm, tide, torrent, torrent of rain, trend,
   undercurrent, undertow, water flow, waterfall, waterspout