Environment Canada says a fog advisory issued for Hamilton, Brantford, Burlington, Haldimand County, Norfolk County and Niagara Region will end Sunday morning.
The national weather agency says there are areas of dense fog, which may reduce visibility to near zero.
It said drivers should be prepared for areas with almost no visibility.
Environment Canada says Sunday’s forecast will see a high of 23 C and when factoring in the humidex, it’ll feel like 27 C.
As day turns to night, the weather agency says it’ll become cloudier, with a low of 17 C and a 60 per cent chance of rain and a thunderstorm overnight.
Monday’s daytime forecast calls for a high of 21 C, with showers or thunderstorms ending in the afternoon, followed by cloudy weather with a 40 per cent of more rain and another thunderstorm.
Tuesday and Wednesday will be cooler, with highs of 18 C and 16 C respectively, and also have a 40 per cent of seeing some rain.