Thunderstorm threat rolls into southern Ontario

That sticky, summer‑like humidity that seemed to vanish after Labour Day is making a slow comeback in southern Ontario. A strong surface high sitting over the southern Mississippi Valley is funneling hot, moisture‑laden air northward, and the result is a noticeably muggy feel across the province.

Temperatures will sit in the mid‑20s (Celsius) on Tuesday, but with the humidex they’ll feel more like the high‑20s. In the southwest the humidex climbs even higher – think low‑ to mid‑30s – so dress accordingly and keep a water bottle handy.

The real kicker comes later in the day. As the sun dips, a stalled frontal boundary starts to wobble north‑south, creating just enough lift for thunderstorms to pop up across southwestern Ontario. Those storms could linger into the overnight hours, and because the air mass is so moisture‑rich, localized flooding isn’t out of the question. If you’re out on the road or planning an evening barbeque, keep an eye on the sky and be ready to adjust your plans.

North of the front the weather stays relatively dry and warm, while south of it the conditions remain hot and muggy. That contrast adds a layer of uncertainty to the exact timing and intensity of any rain, so checking for updates from The Weather Network or local weather alerts is a good habit.

Looking ahead, southern Ontario is sliding into an active storm track for the rest of the week. A “heat dome” is expected to feed a series of showers through the weekend, while early next week may bring a brief cool‑down. By the second half of the week, though, temperatures are forecast to climb again, putting us back into above‑average warmth for the final days of September.

Bottom line: stay alert, keep your phone handy for any warnings, and maybe have a rain‑coat or an extra pair of socks on standby. Weather can turn on a dime, especially when that humid air decides to throw a thunderstorm your way.