Southern Ontario will feel a late-summer surge in humidity by midweek as a hot, moisture-laden air mass moves north from the central United States. The change will lift daytime temperatures into the low to mid-20s Celsius for many communities and push humidex readings into the upper 20s across parts of the Greater Toronto Area and southwestern Ontario.
Why conditions will be unsettled
A surface high pressure region over the southern Mississippi Valley will help funnel warm, humid air toward Ontario. That moisture, combined with a slow-moving frontal boundary parked over the region, will increase the chance of showers and thunderstorms on several days through the middle of the week.
Where the front stalls, showers and thunderstorms are most likely. The front’s tendency to wobble a little north and south will make exact timing and locations uncertain, but forecasters warn that repeated rounds of rainfall are possible for areas that lie beneath the boundary.
Impacts to expect
- Daily chances of showers and isolated thunderstorms through midweek for parts of southern Ontario.
- Periods of hot, humid conditions away from the frontal zone; muggy weather where the humid air mass builds in.
- Risk of localized flooding in areas hit by heavy downpours from thunderstorms.
Temperatures should generally reach the low to mid-20s Celsius during the day. Humidex values are expected to climb into the upper 20s in some locations, making conditions feel noticeably muggier than recent days. Once the front lifts away, drier and somewhat cooler air is likely to follow, with a trend toward more comfortable conditions heading into the week that marks the official start of autumn.
| Area | Daytime temperature | Approx. humidex |
|---|---|---|
| Southwestern Ontario | Low–mid 20s °C | Upper 20s |
| GTA | Low–mid 20s °C | Upper 20s |
| Areas north of the front | Generally warm and drier | Lower humidex |
Practical advice
- Keep an eye on local weather watches and warnings and heed guidance from municipal emergency services if heavy rain or flooding is reported.
- Avoid driving through flooded roadways and be prepared for sudden downpours that can reduce visibility and road traction.
- Stay hydrated and limit strenuous outdoor activity during the hottest, most humid parts of the day.
Forecasters stress that the frontal boundary’s slow movement will create uncertainty in the short-term forecast. Residents should check updated forecasts for their specific area for the latest timing and impacts.
