Road work crews in the National Capital Region are expected to finish early and vulnerable residents are being urged to seek shade as the temperature is expected to hit 34 C in Ottawa and the humidex is expected to reach 47 C.

The city's medical officer of health, Dr. Isra Levy, issued a heat warning on Wednesday, saying extreme heat can cause dehydration, heat exhaustion, heat stroke and even death. He said infants, young children, the elderly, the chronically ill, and the homeless are at greater risk from extreme heat.

The heat warning is automatically declared when Environment Canada forecasts a humidex of 40 C or more for at least two consecutive days.

The temperature was 25 C and the humidex rating was 33 C at 7 a.m. Thursday morning, but construction crews continued road work on Bank Street in the Glebe neighbourhood.

But workers are being told to take frequent breaks and drink plenty of fluids and will stop work at 1 p.m., an hour before temperatures are expected to reach their peak.

Two schools in Ottawa will be closed Thursday because of the hot weather. The Adult High School on Rochester Street, and Immaculata High School on Main Street are not fully air-conditioned. Classes will resume on Friday.

On Wednesday two of the city's public beaches - Mooney's Bay and Petrie Island River Beaches - were allowing swimmers, but the remaining beaches were closed because of high e.coli levels.

Environment Canada also issued a warning for Gatineau and parts of the Outaouais, including Maniwaki and Chelsea, saying humidex values would be above 40 C on Thursday.

A thunderstorm watch was also in effect for the Pontiac region of Quebec, including Chelsea and Maniwaki, while forecasters at Environment Canada said there was a 40 per cent chance of a thunderstorm in Ottawa.

CBC Ottawa climatologist Ian Black said reports were already coming in of scattered showers throughout the region as of 8 a.m.