After a humid morning where the temperature reached a top of 31, a cold front saw the mercury drop by 7 degrees in just one minute at Geelong about 12.30pm.

In Melbourne, dark clouds and rain moved in at 1.30pm, where temperatures fell from 31 degrees to 22 degrees within half an hour.

Shortly before 3pm, the mercury had dropped further to 20.6 degrees.

Thunderstorms and damaging winds are predicted to batter Victoria throughout the afternoon.

The speed limit on the West Gate Bridge has been reduced to 60 km/h while in central Ballarat traffic lights are out at multiple intersections due to high winds.

Commuters also experienced minor delays on the Pakenham line after a tree branch came across the tracks near Narre Warren about midday.

On Sunday night, the city sweltered through an average of 20 degrees, just dipping to 19.4 degrees at 11pm, before rising again to 24.2 degrees at 6am.

"We're forecasting wind gusts around the 90km/h mark, possibly as hight as 100," Bureau senior forecaster Dean Stewart said.

"Winds at that strength are enough to do some damage to trees and blow loose items around."

Those winds will rapidly ease an hour or two after the 2pm cool change.

The Bureau has issued a severe weather warning for strengthening northerly winds, saying there may also be thunderstorms in central and eastern parts during the afternoon.

The strongest winds will be felt in Melbourne, Ballarat, Geelong, Warrnambool, Seymour, Maryborough, Traralgon and Bairnsdale.

Severe Weather Warning for Damaging Winds for southern and mountain parts of Victoria for Monday. #VicWeather https://t.co/HLs2UYFQyQ pic.twitter.com/8EfdGndJbn??? BOM Victoria (@BOM_Vic) March 26, 2017