The Windsor Essex County Health Unit has issued a heat alert for the region Wednesday.
Environment Canada has also issued a humidex advisory for Windsor, Essex and Chatham-Kent.
Over Southwestern Ontario, temperatures in the low 30s combined with high humidity will result in humidex values of 40 C or more for a few hours in the afternoon.
People are urged to take steps to stay cool by doing drinking plenty of water, going to an air conditioned place and limiting outdoor activities to the coolest part of the day.
Environment Canada says relief will come later today as a cold front crosses the region.
Just before 11 a.m. Environment Canada issued a severe thunderstorm watch for all of southern Ontario. That was upgraded to a storm warning around 3 p.m.
The agency said there is the potential for severe thunderstorms during the afternoon and evening along a cold front.
There is the potential of severe thunderstorms with large hail, damaging winds or heavy rainfall.
A cold front will cross Southern Ontario from west to east this afternoon and evening.
"Conditions ahead of and along the front will be favourable for the development of severe thunderstorms. Wind gusts to 90 km/h are possible with these storms, as well as hail 2 cm in diameter and heavy downpours up to 50 mm in an hour," a statement on the agency's website read.
The front will affect Southwestern Ontario early this afternoon, the Greater Toronto area and South Central Ontario by mid afternoon, and Eastern Ontario late this afternoon and early this evening.
Environment Canada said it's closely monitoring this situation and will issue further watches and warnings as necessary.