Sharp cooldown brings risk of frost to parts of Ontario

Temperatures could plummet 5 to 10 degrees below average, with some areas approaching freezing point.

A severe cold front is sweeping across Ontario, bringing a noticeable chill to parts of the province.

Daytime temperatures in several areas may struggle to exceed 20ºC, a remarkable dip for late August.

The shift will be particularly noticeable in southern and eastern Ontario, where thunderstorms struck Thursday afternoon and evening. 

Cold, Wind, Chill, Cool, Temperature, August 27, 2025. (The Weather Network)

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Big drop in temperatures

By Friday, temperatures in much of Ontario will be five to 10 degrees below seasonal averages. 

In Toronto and Ottawa, highs may reach 19ºC — a level not seen since mid-June. Windsor might drop into the upper teens, while Sudbury could reach just under 20ºC.

Friday, Toronto, Ontario, Forecast, August 27, 2025. (The Weather Network)

Overnight lows will fall much more dramatically. Parts of southern Ontario are expected to be in the single digits by Friday night. 

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Water along the Great Lakes will help to keep temperatures in the double digits. But farther inland, areas in northeastern Ontario may get their first frost, with some forecasts suggesting temperatures near freezing.

Looking ahead

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The chill will not stay forever. High pressure is expected to return next week, bringing temperatures closer to seasonal norms. 

However, the rapid swing serves as a reminder that September is approaching — and that summer's grip on Ontario is beginning to fade.

Toronto, Forecast, Weather, Rain, Sun, August 27, 2025. (The Weather Network)

Stay with The Weather Network for more information and updates on your weather across Ontario.