A large swath of Eastern Canada is scorching under temperatures that, with the humidex, will feel like 40 C or higher on Friday and Saturday in some regions, in what is being called a "heat dome."

The main cause for a heat dome is a long lasting area of high pressure, combining with the lingering effects of La Niña, according to the National Oceanic Atmospheric Administration (NOAA). In laymen’s terms, it gets hot, and stays hot.

"When high pressure high up in the atmosphere is present it can trap warm air at the surface - it basically acts like a lid," said CTV News meteorologist Kelsey McEwen, adding that the current heat wave builds on weather patterns established from La Niña during the previous winter. La Niña is when sea surface temperatures in certain parts of the Pacific Ocean are cooler than normal.

The current "dome" is a very large scale, upper level high pressure system that is covering the eastern half of North America, McEwen said, stretching from north of Texas to Central Ontario, and east to the Atlantic.

"Tomorrow will be worse," she said, but added that it is not unusual to see hot events in July. The heat wave is expected to last until Sunday, when temperatures, while still hot, will ease below 30 C.

As temperatures fall back to slightly below the seasonal average next week, it will be Western Canada’s turn to face sweltering weather, McEwen said, noting that Calgary, for example, was sitting around 15 C on Friday, but was expected to see highs of 28 C by Tuesday.

"We’re entering into a major pattern reversal. The heat will come to an end in the east as we head into early next week, and the heat is on in Western Canada early next week."


In the meantime, Environment Canada issued a heat warning for a large part of Ontario, Quebec and the Maritimes on Friday, and cautioned that young children, pregnant women, people with chronic illness, older adults, and people working or exercising outdoors faced greater risks from the hot weather.

The heat alerts were put out Friday morning for Ontario and Quebec, and were followed several hours later by warnings for southern New Brunswick, Nova Scotia, and even part of the Northwest Territories.

If the heat wave continues, it could start breaking records. In Windsor, the hottest July 20 ever recorded was 35 degrees in 1977 -- meaning the 36 degrees projected in Windsor on Saturday could top that.

The public is advised to never leave people or pets inside a parked car, and to stay hydrated and find shade, especially if they feel dizzy, overly tired, unwell, nauseous, or have a headache.

“We can’t over emphasize that you need to take some regular breaks, drink lots of water and do find a cool place to sort of relax,” Lyle Karasiuk, national volunteer chair with the Canadian Red Cross' Canadian Council for First Aid Education, said in an interview with CTV News Channel.

Dr. Sohail Gandhi, president of the Ontario Medical Association, had similar advice, adding that it's important to drink water before you go outside at all, not just when you register thirst. Taking breaks from direct sunlight is particularly important between 10 a.m. and 4 p.m., when the heat is likely to be the most powerful, he told CTV News.

And while many people know that drinking water is helpful during a heat wave, it's important to remember also -- as the urge to have a cool beer or a cocktail on a patio rears its head -- that alcohol is a dehydrator, and could be a bad idea if you're outside for a prolonged period.

Karasiuk suggested the mall, a public library, or even a grocery store as options for staying cool, and said to call 911 immediately if someone appears confused, not acting normally, unconscious, and their skin is hot and dry.

“Just think of it like a pot on the stove. If you don’t keep water in there, it will eventually boil away. When you recognize that unconscious confusion you need to get them some immediate medical care,” Karasiuk said.

"Personality changes," are a bad sign particularly in young children playing in the sun, Gandhi added. If a child is acting very differently, it's probably time to get them out of the heat.

"We're Canadians, so we're just happy it's not snow," Gandhi said. But if you're not careful, fun in the sun can end up with you suffering from sunburns, lightheadedness, or even heat stroke.

For a full list of heat alerts, you can visit Environment Canada’s website.