Fashions will be challenged on the field at Derby Day by the wild weather. Credit:Angela Wylie

The weather forecast for Derby Day is definitely not fascinator friendly: heavy rain, damaging winds and possible flash flooding.

The Bureau of Meteorology is expecting up to 30 millimetres of rain, but warned that could be exceeded in some areas in just three hours.

Melbourne will most likely see 10 to 20 millimetres of rain, soaking Derby Day at Flemington Racecourse, destroying footwear and challenging the waterproofing of all but the sturdiest marquees.

Wind gusts of up to 100km/h will buffet Melbourne on Saturday, and the city will be drenched by “bursts of really intense rainfall” as a tropical air mass moves across the state.