Be prepared for yet another wave of hot and humid weather as Environment Canada issued a heat warning for Toronto and parts of the GTA on Monday.

“This may be the longest and most significant heat wave of the summer so far,” Environment Canada said in a news release just after 3:20 p.m.

Temperatures are expected to reach a high of 28 C Monday afternoon but it will get even hotter and more humid as the week continues, with Tuesday’s UV index expected to have a rating of 8 (very high) and the temperature soaring to 31 C.

“The air mass will become considerably more humid Tuesday night, which will keep overnight temperatures near 20 in most areas,” Environment Canada said.

And the hot weather won’t let up on Wednesday or Thursday either. It is expected to be sunny with cloudy periods, according to Environment Canada, with temperatures reaching 34 C.

“The warm and muggy nights will not provide much of a break from the almost relentless heat,” Environment Canada said, “even in the odd location where temperatures may briefly dip to the upper teens shortly before sunrise.”

There might be some relief from the heat in “a few locales near the shorelines of the Great Lakes if a breeze of the lake develops,” they said.

Toward the end of the week and into the weekend, Toronto will have a 30 to 40 per cent chance of rain and thunderstorms. This could offer a little relief from the heat and humidity, Environment Canada said.

Heat warnings are issued when it’s possible for the hot, humid conditions put people at a higher risk of heart illnesses, like heat stroke or heat exhaustion.

Environment Canada suggests checking on at-risk people, such as the elderly and young children, and rescheduling outdoor activities for cooler parts of the day. They also said, if possible, prepare meals that don’t have to be cooked in an oven.