The GTA is waking up to one of the hottest — if not the hottest day on record.

On Thursday temperatures are expected to reach 38 C, just shy of the highest temperature ever recorded at Pearson International Airport: 38.3 C on Aug. 25, 1948. A temperature of 40.6 C was also recorded in July 1936 at a downtown Toronto station that is relied on less for records because it has moved around over the years.

And with the humidex figured in that 38 C could feel even hotter — maybe 48 C according to Environment Canada.

Senior climatologist, Dave Phillips, says the cause of the extreme weather is a heat dome, a hot, unmoving high-pressure area, that has settled over large swaths of the country, pushing the jet stream well to the north, and keeping cooler or wetter weather out.

"It tends to encourage weather from the south, from the Gulf of Mexico," he said.

"It's like a heat pump and it just sluggishly sits there and like an unwanted house guest, it just won't move."

Toronto is now under an extreme heat alert with seven emergency cooling centres open across the city.

"It's too hot to remain outside," said Patricia Anderson, the city's manager of the shelter, suppport and housing administration. "You've got to give your body a break and get into some air conditioning."

To help the most vulnerable, Red Cross volunteers have been fanning out across the city, offering bottles of water.

"This hot weather can be dangerous, particularly if you have young children, or elderly relatives," said Tom Windabank of the Red Cross. "Take it seriously."

Dr. Mark Bonta of Toronto General Hospital warns that heat exhaustion can sometimes be fatal.

"You need to be in a cool environment and maintaining fluids," said Dr. Bonta.

And with temperatures so high medical experts are warning that the risk for heat stroke is high, with symptoms you can't always see: dryness on the skin, an inability to sweat, dizziness or confusion, and a rapid heart beat.

Cooling centres open

Toronto has opened seven air conditioned cooling centres where people can rest and receive a cool drink. One of the cooling centres, at Metro Hall on John Street near King Street West, is open 24 hours a day.

The other six are open from 11 a.m. - 7 p.m.

Toronto has also extended the hours at nine of its outdoor swimming pools to help people beat the heat.

Late Wednesday the Toronto Blue Jays announced that they'd be getting into the act too by closing the roof of the Rogers Centre during Thursday's matinee game against the Seattle Mariners.

The club said it was making the move "in order to protect the health and safety of our fans, staff and players during a period of the extreme heat and humidity ..."

But some people take it all in stride.

Annabelle Villa-Real manages a bagel shop in Toronto, where she has been working for almost 16 years, and credits coming from the Philippines for being able to work among three ovens and no air conditioning.

"All the time people say, 'How do you survive, Annabelle?"' she said. "Think positive."

Villa-Real and her staff take plenty of breaks, drink cold water and snack on Popsicles, have ice at the ready, as well as towels and extra shirts.

"We're used to it and it's no big deal to us," she said. "The weather? Aw, go to the bakery. We're the last man standing."

With files from The Canadian Press