Torontonians may want to stay indoors with the air conditioning cranked this Canada Day long weekend as the humidex value across the GTA is expected to reach the mid-40s, making this upcoming heat wave the “most significant heat event” Ontario has experienced in the last few years.

Environment Canada issued a heat warning Thursday afternoon, saying that daytime highs are expected to reach the low-to-mid-30s starting Friday. With the humidex, the temperature could feel like it is in the mid-40s.

“Current indications suggest hot temperatures could persist through the next week,” the warning reads. “The risks are greater for young children, pregnant women, older adults, people with chronic illnesses and people working or exercising outdoors.”

The heat warning extends into the GTA and most of southern Ontario, including Hamilton, Waterloo, and Barrie.

Environment Canada also said that overnight low temperatures will only fall to the low-to-mid-20s, which means there will be no relief from the heat. The agency warns that residents should keep an eye out for symptoms of heat-related illness, which include swelling, rashes, cramps, fainting, and heat stroke.

Right now, Toronto is expected to see a high of 31 C on Friday, and 36 C on both Saturday and Sunday.

There is a 30 per cent chance of showers on both Sunday and Monday, but the rain is not expected to reduce the humidity. Tuesday is forecasted at 32 C.