A severe thunderstorm has caused fearsome flash flooding, leaving cars underwater and downing trees which caused minor damage to buildings.\rbr\rbrGeelong, an hour-and-a-half south-west of Melbourne, was smashed by the storm on Wednesday afternoon, dumping 24mm of rain in the 20 minutes leading to 3.22pm.\rbr\rbrSocial media has been overrun with pictures and footage of the rain and damage, showing cars underwater and flash flooding pour down central streets.