A severe thunderstorm has caused fearsome flash flooding, leaving cars underwater and downing trees which caused minor damage to buildings.\r<br>\r<br>Geelong, 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.\r<br>\r<br>Social media has been overrun with pictures and footage of the rain and damage, showing cars underwater and flash flooding pour down central streets.