Environment Canada has added a thunderstorm watch to a variety of weather warnings for Thunder Bay and other areas of northwestern Ontario.

Late Monday afternoon, the agency issued thunderstorm watches for communities between Thunder Bay and the Manitoba border including Upsala, Shebandowan, Atikokan, Ignace, Dryden, Fort Frances and Kenora.

The Lakehead Region Conservation Authority released a flood outlook statement, saying high intensity rainfall could cause localized flooding overnight and into Tuesday.

The areas of Thunder Bay, Fort Frances, Rainy Lake, Atikokan, Upsala and Quetico have also been under heat warnings today.

The weather office said a warm and humid air mass would be in place over northwestern Ontario this afternoon, with the humidex climbing to nearly 40 C in some areas.

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Children enjoy a splash pad in Thunder Bay on a sweltering Monday afternoon. (Josh Lynn-CBC)

"For both the combination of very warm temperatures, temperatures expected to get up to around 30, 31 degrees Celsius today." said Geoff Coulson, a warning preparedness meteorologist with Environment Canada. 

"But, when we combine in the effects of humidity, it's going to feel more like 40 C."

What to watch out for

In these situations, Environment Canada says health risks are greatest for:

  • older adults
  • infants and young children
  • people with chronic illnesses such as breathing difficulties, heart conditions or psychiatric illnesses
  • people who work in the heat
  • people who exercise in the heat
  • people without access to air conditioning and
  • homeless people.

Experts advise drinking plenty of liquids especially water before you feel thirsty to decrease your risk of dehydration. Thirst is not a good indicator of dehydration.

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Geoff Coulson, a warning preparedness meteorologist with Environment Canada, said temperatures should return to normal by Tuesday. (Supplied)

Coulson said a front will move through the region tonight and tomorrow, bringing with it cooler temperatures, as well as the possibility of significant thunderstorms and rainfall.

"We could be looking at some significant thunderstorm activity for this evening, overnight and first thing in the day on Tuesday, as that front moves through."