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Severe storm risk includes supercell potential in Ontario

Monday, July 12th 2021, 7:40 pm - The heat and humidity will build across parts of Ontario this week, bringing a daily thunderstorm risk. Tuesday could feature supercell development in parts of the province.

The humidity has made a return to southern Ontario this week, alongside a boundary parked over the Great Lakes region, fuelling an active storm track. The setup is quite favourable for daily threats for thunderstorms, but the week won't be a writeoff as there will be plenty of sunshine in between the unsettled weather. Tueday, however, looks to feature the most widespread and potent thunderstorm risk, including the potential for supercell development, so a tornado can't be ruled out. With the daily chances of torrential rainfall, localized flooding will be possible. While there's a possibility of drier conditions by the weekend, the rain and thunderstorm risk will remain. More on the timing and potential impacts, below.


Constant southerly winds usher in heat and humidity near the surface and a steady stream of atmospheric moisture that has pushed in from the Gulf of Mexico has set the stage for a soupy pattern that will persist over southern Ontario for much of the week.

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Temperatures will be around seasonal, but the high humidity will make for an uncomfortable week across the region. Daytime highs will hit the mid-20s each day, though parts of southwestern and eastern areas may push the 30-degree mark by Thursday. The temperatures will be accompanied by humidex values jumping into the mid-30s.

If you're looking for relief overnight, conditions won't improve much, either, as high humidity levels will keep lows on the warmer side.

Each day also brings about a chance for thunderstorm, with Tuesday yielding the most widespread thunderstorm risk -- with heavy rain, strong winds and large hail being the main threats.

ONriskTues (July 12)

Although the threat has diminished somewhat, forecasters will be closely monitoring for the potential for supercell formation for a line extending from Parry Sound to Bancroft, stretching to the Ottawa Valley in the afternoon and evening hours. A tornado can't be ruled out . Residents across the region are reminded to monitor the forecast closely and prepare to take shelter when severe weather strikes.

The good news is that the day won't be a complete washout, but storms that do develop in the areas outlined on the map above will have the potential to bring torrential downpours and localized flooding.


Most places look to stay dry again on Wednesday, though a passing shower or thunderstorm can't be ruled out completely, with a higher risk in eastern areas. As well, there is a threat for for strong to severe storms to develop by late week, thanks to a cool front sinking south.

At this point, it looks like the weekend will become less humid and the storm threat will push south and east of our region, but we will not be able to completely shake the risk for showers and thunderstorms.

By next week, near seasonal or slightly above seasonal temperatures return. There will be a risk for passing thunderstorms at times, but at this point, it looks like next week will not be as stormy as this one.

Thumbnail courtesy of Beny Guenther, taken in Leamington, Ont.

Be sure to check back for the latest updates on the forecast in Ontario.

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