Windsor-Essex and Chatham-Kent are under two advisories issued Saturday by Environment Canada.
Along with a heat warning, there is a severe thunderstorm watch.
Environment Canada says conditions are favourable for the development of severe thunderstorms that may be capable of producing strong wind gusts, large hail and heavy rain.
Thunderstorms are expected to develop early Saturday afternoon and last until this evening with the potential for large hail, strong wind gusts, and heavy downpours.
Temperatures reached 31 degrees Friday and only dropped to near 20 degrees overnight. These temperatures are expected to repeat Saturday and overnight into Sunday. That has resulted in a heat warning.
Environment Canada says this two-day event meets heat warning criteria.
The agency advises people to watch for the effects of heat illness: swelling, rash, cramps, fainting, heat exhaustion, heat stroke and the worsening of some health conditions.
Drink plenty of water even before you feel thirsty and stay in a cool place.
And a warning to never leave people or pets inside a parked vehicle.
Heat warnings are issued when very high temperature or humidity conditions are expected to pose an elevated risk of heat illnesses, such as heat stroke or heat exhaustion.
