Brockville, Ontario, is set to feel the heat this week as Environment Canada forecasts sunny conditions and a high of 30°C on Tuesday, June 9, 2026, with a humidex of 33 and a very high UV index of 9. That is about 7 degrees above the normal early June high of 23°C.
No heat warning was in effect for Brockville as of Monday afternoon. The province’s warning system for Southern Ontario kicks in when the high reaches at least 31°C and the overnight low stays at or above 20°C, or the humidex hits 40, for two or more days in a row.
The Environment Canada forecast shows the heat will be short-lived. After Tuesday’s peak, a 60 percent chance of showers arrives Wednesday and the high drops to 23°C, near the seasonal normal. But temperatures are expected to climb again to 29°C on Thursday and 30°C on Friday.
The early-season heat can hit harder because bodies are still used to spring’s cooler temperatures, according to Environment Canada meteorologist Kristina Kretchman. The sudden shift to midsummer conditions may feel dramatic even though a hot spell at this time of year is not unusual.
The Leeds, Grenville and Lanark District Health Unit reminds residents that heat illnesses are preventable. Warning signs include headache, nausea, dizziness, thirst, dark urine, and intense fatigue. Heat stroke is a medical emergency. Anyone experiencing confusion, fainting, or hot dry or damp skin should get help right away and cool down.
Those especially at risk include older adults, people living alone, those with chronic health conditions, and individuals without stable housing. The United Counties of Leeds and Grenville has recently offered a warming centre at 166 Pearl St. E. for vulnerable residents.
Brockville’s hottest day on record remains 39.4°C, set on June 4, 1919.
