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Rising heat, instability brings weekend storm risk in Ontario, Quebec

Friday, August 6th 2021, 8:21 pm - Building humidity and multiple systems will bring the chance for thunderstorms across southern Ontario and Quebec this weekend. A stretch of dangerous heat is possible next week as humidex values push 40.

Southern Ontario and Quebec are on the cusp of a hot and muggy weekend, with severe storm risk in the forecast for Saturday. Though that risk will subside Sunday, the humidity that will fuel Saturday's storms will stick around, and in fact, next week may feature the muggiest days of the summer, with multiple communities feeling close to 40 for several days. A closer look, below.

SATURDAY: THUNDERSTORM POTENTIAL SHIFTS INTO SOUTHERN ONTARIO, QUEBEC, SEVERE POSSIBLE

A cold front with origins in northern Ontario will finally reach southern Ontario and Quebec Saturday, and is expected to bring showers and thunderstorms in the afternoon and evening hours.

Some may turn severe, with widespread areas at risk of seeing potent storms developing -- stretching from Windsor to Ottawa, and cottage country, as well. With ample moisture in play for the storms to tap into, torrential rainfall will be the main hazard, in addition to strong wind gusts and small hail.

EastRiskSat (1)

High PWAT (precipitable water in the atmosphere) and weak shear will be a recipe for slow-moving storms with localized flooding, too.

Some of the storms may linger into the overnight hours in parts of cottage country, eastern Ontario and southern Quebec. Once they move out, fog is expected to move in.

Temperatures and humidex values won't change much from Friday, though the extreme southwest will see daytime highs a few degrees cooler.

A secondary system, a warm front, will be pushing into Lake Superior Saturday evening with showers, possibly reaching the Georgian Bay by Sunday morning with precipitation.

SUNDAY: STORM RISK SUBSIDES, TEMPERATURES CONTINUE THEIR SURGE

Aside from those showers Sunday morning around Georgian Bay, parts of eastern Ontario will be the focus of the day's storm risk, though any storms that do spark up presently look to remain non-severe, marked by heavy downpours, stiff winds, and some small hail.

ONRiskSun

It'll also be another hot day throughout the south. Parts of the extreme southwest will crack 30°C, but the rest of the region will be in the mid-to-upper 20s.

Humidity will continue to be a factor, however, so most places will be feeling like a few degrees above 30.

ONSun

LOOK AHEAD: MORE SHOWERS, COULD BE HOTTEST WEEK OF THE SUMMER

All this building humidity will gradually boost the region's PWAT, meaning any showers that do spring up will be heavier and longer-lived.

Monday and Tuesday look particularly likely to be a rainy one for many people, particularly from the afternoon into the evening.

PWAT ON (Aug. 6)

Meanwhile, the heat and humidity will continue to build to uncomfortable and even dangerous levels through the week.

In fact, next week should be the hottest week of the summer across southern Ontario and Quebec. Temperatures will remain locked into the upper 20s and lower 30s, with a humidex in the high-30s and low-40s for much of the week. It will be very humid.

The culprit will be a ridge of high-pressure parked by the southeastern U.S. In atmospheric terms, it will have the effect of ushering warmer southwesterly winds up into southern Ontario for several days.

ON Tuesday heat

When planning for next week, people should expect to have to limit outdoor activities and make active preparations to keep cool.

Check back as we continue to monitor the long-range forecast

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