Rain and storms are expected across most of Victoria on Wednesday night but are not forecast to reach the city until after 8pm.

But when the rain does arrive, the mercury is expected to drop by about five to six degrees to 20 degrees before falling to a minimum overnight temperature of 15 degrees.

For the rest of the week, Victorians would be wise to move their entire wardrobes to work as spring weather continues its wild fluctuations.

Wednesday's beach-worthy top of 31 degrees will be followed by a crisp and showery top of 20 on Thursday.

And it's not just the daily changes leaving Melburnians in a state of sartorial schizophrenia - there has been as much as 20 degrees between some days' recorded low and high temperatures.

Tuesday marked a morning low of eight before increasing to 29 in the afternoon.

Bureau of Meteorology senior forecaster Scott Williams said the weather was typical of spring, when winds pushed frigid blasts north from the Southern Ocean or brought hot air south from the continent's centre.

The fluctuations gradually decreased as southern waters warmed over summer.

Already in October, Melbourne has experienced a 30-degree day (on the 12th), followed by a top of 16.7 degrees and five consecutive days below 20.

The biggest drop in temperature recorded was on October 28 in 1977, when the mercury sank overnight from 34.5 to 14.6.

That October had six days above 30 degrees and four days that failed to reach 15, Mr Williams said.

"That was probably the most wild in terms of fluctuations," he said.

The highest-ever October maximum of 36.6 was recorded in 2006, while the lowest was nine in 1922.

This week's forecast continues the unsteady trend, with Thursday's predicted maximum of 20 followed by a top of 26 on Friday, a cloudy 22 for the Cox Plate on Saturday, 27 on Sunday and back to 23 and 20 on Monday and Tuesday.

with Nadia Wu