Grey and Bruce counties may see the first accumulation of snow for the season.
A low-pressure system from the southwest will bring a significant rainfall with it through Thursday, with many areas receiving a total of 20 to 40 millimetres of rain by Thursday evening.
That means a cold wet Halloween night for trick or treaters, and low visibility for motorists, who will need to take extra care.
As the low passes by Thursday night, rain may change over to snow before tapering off Friday. Snowfall amounts of five to 10 centimetres m are possible, especially inland from the Great Lakes and over higher ground.
Residents may wake up to snow-covered grass for the first time this season Friday morning.
Environment Canada suggests motorists consider postponing non-essential travel as poor winter driving conditions are expected.