rain bomb

This is the common name for what meteorologists would call a wet microburst. Macquarie Dictionary defines microburst as:

a form of windshear in which a jet of air is propelled downwards from a high convective cloud system, suddenly changing direction as it nears ground level.

 A rain bomb usually occurs in the middle of a thunderstorm associated with a cold front, so it pulls down a lot of water.  It is usually no wider than three or four kilometres. The rain and hail in it can travel at speeds up to 120 kph.  The whole event normally lasts no longer than 15 minutes.  So now among the terrifying things that the weather can bring, the rain bomb is beginning to feature.