Southern Ontario faces Yellow Watch - Severe Thunderstorm Tuesday

Southern Ontario faces a yellow watch - severe thunderstorm Tuesday as a powerful cold front could trigger damaging thunderstorms before dawn Wednesday. The first storms are expected in the mid-to-late afternoon near Lake Huron and Georgian Bay, including Barrie, before a more organized line reaches the Highway 401 corridor in the evening.

Highway 401 corridor

The strongest storms are most likely to hit southwestern Ontario, where the line is expected to move from the GTA to London, Windsor and Sarnia. That corridor is the most organized part of the setup and the one most likely to see frequent lightning, damaging wind gusts and large hail.

Localized flooding could develop quickly after repeated heavy downpours. The chance for a tornado cannot be ruled out, adding to a storm threat that is broader than Monday’s more scattered activity.

Southern Ontario temperatures

The cold front should bring the summer-like warmth to an abrupt end later Tuesday. Some communities may see temperatures drop by more than 20 degrees in less than a day as much cooler air arrives overnight.

Barrie is forecast to fall from around 28°C Tuesday afternoon to near 6°C by early Wednesday. London could drop from near 30°C to roughly 7°C overnight, with gusty winds making Wednesday morning feel much closer to spring than to the start of summer.

By early Wednesday morning, the cold front is expected to move into Quebec and the severe weather threat should shift mainly south of the border and toward Atlantic Canada. For people along the Highway 401 corridor, the practical window is short: storms build late Tuesday, the worst line moves through in the evening, and the sharp cool-down follows overnight.