Niagara is also under a Special Weather Statement as the remnants of Hurricane Beryl are expected to bring heavy rain later this week.
Environment Canada says torrential downpours, with rainfall rates of 20 to 40 mm per hour at times, could hit us on Wednesday and Thursday.
Today, a heat warning was issued, with hot temperatures expected today and tomorrow.
It is expected to start raining late Tuesday night or Wednesday, and continuing into Thursday.
That will push the warm air mass out of Niagara.
"Heavy rainfall associated with the remnants of Hurricane Beryl may affect portions of southern Ontario beginning Tuesday night or Wednesday, and may persist into Thursday. Although confidence in the exact track of the weather system is low, these types of systems in the past have given very high rainfall rates in torrential downpours. "
Rainfall warnings may be issued as confidence in the track of the system increases.