Stormy shift expected overnight across southern Ontario

A Stormy shift expected overnight across southern Ontario. Gary Archibald has the latest forecast.

Southern Ontario’s warm, spring‑like weather will give way to rain and possible thunderstorms overnight, CTV News meteorologist Gary Archibald says.

“We are in the midst of a big change,” Archibald said Monday, noting that showers will develop late Monday night and continue into Tuesday morning across southwestern Ontario and parts of the Midwest.

Temperatures reached 21 C, well above the seasonal high of 15, with areas along the 401 corridor and north toward Wiarton and Owen Sound nearing 20 C.

Strong southeast winds gusting above 50 km/h are expected to intensify through the evening and into Tuesday.

A low‑pressure system moving in from the west will spread cloud cover from Windsor‑Essex toward the GTA overnight. Archibald said rainfall amounts remain uncertain. “Worst‑case scenario, we get a blanket of rain, but some models show the leading edge breaking up with secondary waves behind it.”

Thunderstorms are possible between midnight and dawn, with another round late Tuesday morning.

Cooler weather arrives mid‑week, with highs in the upper single digits and low teens through the weekend. Overnight lows may dip below freezing Friday and Saturday.

Here’s a look at the rest of the forecast

Monday: Sunny. Wind southeast 20 km/h gusting to 40. High 20. UV index 7 or high.

Monday Night: Increasing cloudiness. 30 per cent chance of showers late this evening and after midnight. Showers beginning overnight. Risk of a thunderstorm overnight. Amount 5 mm. Wind southeast 20 km/h gusting to 50 increasing to 40 gusting to 60 late this evening. Low 12.

Tuesday: Showers ending in the afternoon then cloudy with 40 per cent chance of showers. Risk of a thunderstorm in the morning and afternoon. Amount 10 to 15 mm. Wind southwest 40 km/h gusting to 70. High 19.

Wednesday: Cloudy with 60 per cent chance of showers. High 9.

Thursday: A mix of sun and cloud with 30 per cent chance of showers. High 10.

Friday: A mix of sun and cloud with 30 per cent chance of showers. High 9.