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Mother Nature spoils Ontario with summer-searing heat, humidity climbs

Tuesday, May 18th 2021, 9:55 pm - Summer-like weather arrives in a hurry across southern Ontario as some places sizzle ahead of the May long weekend.

After short-changing southern Ontario earlier this month, Mother Nature is paying back the region and thensome this week. While summer is still about a month away, that hasn't stopped southern Ontario from seeing some July-like temperatures Wednesday and beyond, at least for a few days anyway. Daytime highs will climb into the mid-to-upper 20s, even pushing near the 30-degree mark. As is often the case in the summer, the humidity will come with the heat, so humidex values are expected to reach the low- to mid-30s by Saturday for some places. The long weekend may see some unsettled weather, but most of it will be dry, increasing worries for areas that are lacking rainfall recently. The warmth won't be here to stay, however, as there may be a quick shot of much cooler weather by the end of the long weekend. More on what we know for the unofficial kick off to summer this weekend, below.


Wednesday will see a carbon-copy pattern that has covered southern Ontario weather this week -- featuring more sunshine and high temperatures in the mid-20s.

A stationary ridge will form an arch of heat extending from southern Quebec to northwestern Ontario through Wednesday, causing daytime highs to soar near 30°C from Ottawa to Kenora.


Most of southern Ontario won't be quite as warm as the aformentioned areas, thanks to the cooling influence of the southerly winds over the lakes for places like Toronto, Kingston, Sault Ste. Marie and Sudbury.

Despite the impact, far-reaching temperatures in the high 20s will be felt across the region, with communities hitting 30°C and the humidex hitting the mid- to upper-30s by Thursday and Friday.

ONHumidexFri (May 18)

In addition to the searing temperatures, most of the week will be dry, but there is the chance for some scattered rain showers and thunderstorms near the boundary Thursday and Friday. However, there is no need to be worried about any days being a washout.


For the end of the week and into the long weekend, the big question regarding the warmth is where will the ridge be sitting?

As of now, it's looking like the backside will be holding over southern Ontario, allowing a southwesterly flow to drive temps near 30°C to end the week and start the long weekend.


As well, scattered showers and thunderstorms are expected on the weekend, but it will be mostly rain-free for southern areas.

"A cold front will sink south through the region during Sunday with falling temperatures, and cooler, more seasonal, weather for the holiday Monday," Gillham says, adding that changeable temperatures will continue into next week as summer-like warmth attempts to return.

"We can expect to see changeable temperatures for the final week of May and first week of June, with the potential for a more active pattern with some much needed rain."

Be sure to check back for the latest updates on the pattern change across Ontario.

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