30-75+mm: Soggy Sunday in Ontario with high rain totals from thunderstorms

With August now here, summer is turning the page on the 2026 season with the end of it slowly creeping up on the horizon.

A surge of atmospheric moisture will fuel heavy rain and a thunderstorm risk across Ontario on Sunday. Localized flooding will be possible with the rainfall and associated thunderstorm risks.

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Stay alert for rapidly changing conditions in your area, and be sure to keep up with the latest watches and warnings as you go about your day.

Rainy, stormy interruptions on Sunday

A moisture-laden low slides across the Ohio Valley on Sunday, bringing a moist, frontal boundary across southern Ontario throughout the day.

As a result, widespread showers and embedded thunderstorms will continue across southern Ontario on Sunday.

The increased moisture will lead to higher rainfall totals through the day as multiple rounds of showers and thunderstorms are expected.

Rainfall warnings and special weather statements have been issued along the 401 corridor and lower lakes for more than 50 mm of rain, and in some cases, up to 80 mm.

Projected rainfall forecasts:

• Southwestern Ontario: 15-30+ mm

• Toronto-Greater Toronto Area (GTA): 20-40 mm

• Niagara region: 30-50+ mm

• Eastern Ontario and Quebec: 50-75+ mm

• Storm maximum in southern Ontario will likely approach or exceed 100 mm of rainfall

The primary hazard with Sunday’s storms will be the high rainfall amounts, especially in eastern Quebec, Niagara Peninsula and the National Capital Region.

A few breaks in the rain will be likely Sunday evening, but the western GTA and cottage country won’t dry out completely until Monday--when the sun makes a return.

Eastern Ontario (Kingston-Ottawa corridor) may still see showers linger into Monday afternoon.

Stay with The Weather Network for the latest updates for Ontario and Quebec.