Western Canada is about to be hit by the same weather system that has been scorching many parts of the American Midwest, while Central Canada is getting relief from record-setting temperatures.

A mass of warm air will settle in from Manitoba to British Columbia over the next five days, when temperatures are expected to be in the high 20s to low 30s.

Heat alerts and humidex advisories were issued across southern and central areas of Ontario the province and as far east as Montreal on Friday.

Thunderstorms across much of southern Ontario were in the forecast, a day after 11 temperature records were broken. (CBC )

Dr. Barbara Yaffe, acting medical officer of health for Toronto, declared the city's extreme heat alert over Saturday morning after it was issued Wednesday. Temperatures were to reach the high 20s and forecasters predicted thunderstorms could sweep through much of southern Ontario.

Environment Canada said the temperature hit 36.3 C at Toronto's Pearson airport on Friday, eclipsing the 1998 record of 34.7 C. It was one of 11 records that fell in Ontario.

More seasonal temperatures were in the forecast Saturday.

Meanwhile, a number of U.S. regions are still sweltering in extreme heat.

Forecasters say it will feel like 40 C or more in Manhattan on Saturday. People in Philadelphia will feel a high temperature, with humidex, of 44 C. Washington D.C. has a predicted high of 38 with a humidex of 49.

Heat warnings are also in effect for several Midwestern states, including Iowa, Indiana, and Illinois.

People in the U.S. Northeast and Midwest have endured nine consecutive days of oppressive heat. About 3,000 temperature records have been set in the U.S. in the past two weeks.

Scores of people are still having to tough it out without air conditioning. About 350,000 in a dozen states and Washington D.C. are still without power, a full week after violent storms fuelled by the heat.