Early arrivals to Flemington are expected to cop a drenching as heavy rain and thunderstorms lash Melbourne.
Bureau of Meteorology senior forecaster Michael Efron says five millimetres of rain had already fallen on Flemington Racecourse before the gates opened at 8.30am on Tuesday and the wet weather is set to continue.
Melbourne Cup goers are urged to pack an umbrella and maybe some not-so-fashionable gumboots as the city braces for a deluge, with temperatures to top at a humid 20C.
"We will start to see showers easing in the afternoon but are still likely to see showers during the main race as well," Mr Efron told AAP.
It might also be a good idea to pack a warm coat, with southerly winds forecast to bring cold temperatures in the late afternoon, as punters begin heading home.
"People heading home from Flemington are likely to have a pretty cold trip," he said.
It could be particularly cold for flamboyant British billionaire Marwan Koukash who has vowed to accept the trophy in his G-string if his horse Magic Circle wins the race.
"That's the lace that's going to stop the nation," he told AAP at Monday's Cup Parade.
A severe weather warning is in place for Melbourne and eastern parts of the state, as a slow moving low pressure trough brings heavy rain and thunderstorms.
Those storms could lead to flash flooding in some parts.
Australian Associated Press