Victorian towns are facing a major flood threat with the arrival on Wednesday of a "nasty" storm front which could dump up to 200 millimetres of rain into already-soaked catchments and rivers.

The Bureau of Meteorology has forecast severe thunderstorms in the state's west today and tomorrow, leading to a day of torrential rain on Wednesday, when areas in the Victoria's north and north-west could see rivers break their banks and towns come under threat.

Wild weather hits Melbourne last Thursday. Credit:Michael Clayton-Jones

"There is potential for widespread rain on and north of the ranges in the 50 to 100 millimetre bracket," senior forecaster Scott Williams said.

"Some isolated falls are probably going to be 125 to 150 millimetres, especially about the north east highlands.