'Heat dome' to lift Friday but hot humid weather will return

A "heat dome" that has been sitting over Ontario since Friday will lift later this week, giving residents of Waterloo region a breath of fresh air, but weather experts predict more hot, humid weather ahead.

"This is something that has only typically happened in our area maybe once every 10, 15 years," said Frank Seglenieks, who co-ordinates the University of Waterloo's weather station.

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Seglenieks, who has decades of weather data at his disposal, said the seven days that are forecast above 30 C are "pretty unique, in that way."

The heat began June 29, when a hot, humid air mass moved up from the Gulf of Mexico and got trapped over Ontario.

"It's really the length that is unusualbecause we're talking about a heat wave that's forecast to last for seven days," said Gerald Cheng, meteorologist with Environment Canada.

The hot weather is expected to break briefly Thursday night, when a cold front will "flush" the hot air to the east.

But Cheng warns the cooler temperatures won't last, and residents of Ontario can expect a return to hot, humid weather next week, and above seasonal temperatures for the rest of July.

Impact on hydro

Despite the hot weather, Waterloo region residents have not experienced wide-spread power outages.

"Our supply isn't being taxed," said David Wilkinson, vice president of operations with Waterloo North Hydro.

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He added the system is designed to handle peak demand and, as a result, is not having "any operating difficulty."

"Our biggest challenge is when we have very, very hot weather combined with humidity and thunderstorms," he told CBC News.

"Typically, those thunderstorms include either high winds or lightning strikes that damage equipment ... [and] some customers experience a localized outage while that equipment is being replaced."

Calls to paramedics

While it hasn't affected hydro operations, the heat wave did have an impact on the health of Waterloo region residents.

According to a statement emailed to CBC News, paramedics responded to "approximately 15 calls for service at the various Canada Day Celebrations throughout the region" on July 1.

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Public health said the emergencies "were due to weather-related illnesses or alcohol misuse," but only represent a fraction of the true story.

"We know that extreme heat and humidity can worsen and even cause many types of medical emergencies and conditions," public health said in a statement.

"While it's difficult to determine which calls were directly related, we believe the weather played a significant role in the higher than usual call volumes for paramedic services this past weekend."