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HIGH HEAT | Summer lovers rejoice

5 Things you NEED to know about Canada Day heat danger


Erin Wenckstern and Caroline Floyd
Meteorologists

Thursday, June 28, 2018, 5:01 PM - Set up the fans and freeze the popsicles, Ontario and Quebec. This is your official heads up: one of the most extensive heat waves of the summer is almost upon us, and it's set to spike over the holiday long weekend. Heat lovers may be rejoicing but this weekend's temperatures may be a shock even for those of us who soak up the sun like cold-blooded animals. Canada Day weekend's sky-high humidex values will be taking a stab at all-time summer records for cities like Ottawa and Montreal. We take a look at what to expect - and how to cope - below.


Record Heat Highlights

  • Temperatures in the 30s will drive 'feels-like' values into the 40s for many this weekend
  • Overnight temperatures will offer little relief 
  • Humidity peaks Canada Day, with only slight decrease in temperatures through next week

TROPICAL TEMPERATURES - 5 THINGS TO KNOW ABOUT OUR SCORCHING WEEKEND

1. We're about to be hotter than we were all last summer

Remembering the weather is a funny thing. Often, whether it rained or snowed on important events in our lives stands out, but overall impressions of things like 'what last summer was like'? Humans aren't great at that. So, as a refresher - this weekend, a lot of places from Windsor to Montreal are forecast to see daily high temperatures that top the highest point reached all last summer, from June to September. But it'll be close; last year Toronto topped out at 33.6 in September, while Ottawa hit 33. The heat was earlier for Windsor and Montreal, who topped out at 34.3 and 32.1 in June, respectively.

But speaking of short memories, do you remember the summer of 2016 being unusually hot? It was; in fact, although it might feel worse this weekend, thanks to the humidity, highs for this weekend will run 2 to 4 degrees colder than summer 2016's hottest days, when Toronto, for instance, topped out at 36.

2. Humidex values will be about as hot as they can get here

We all know the phrase "It's not the heat, it's the humidity", and most of us probably agree; high temperatures and high humidity are an uncomfortable - even dangerous - combination. While it's tempting to blame the humidity for all of our woes, it really does take a delicate balance of air temperature, humidity, sunshine, and cloud cover to make things really miserable. Average relative humidity - a measure of the amount of water vapour in the air - runs at about 55 per cent on a summer afternoon in southern Ontario. That might seem low, but it's about as high as we can go before excessive cloud cover starts blocking out our sunny skies. And, clearly, the more the cloud cover the less the heating from the sun.

Even on the day with the highest humidex reading on record for Toronto - July 14, 1995 - when the humidex value hit 50, the relative humidity was still only 55 per cent.

The difference between that day and this weekend was in the air temperature; peak heating on July 14, '95 took the thermometer to a blazing 38.3ºC.

With our average July high of about 27ºC, and humidity of around 55 per cent, a humidex reading of about 32 is the norm for most of southern Ontario and Quebec. Think of a really warm day, say, a high of 32, and drop in that 55 per cent humidity, and we get a humidex of 41. A cooler, cloudier, but more humid day? A 30 degree high with an RH of 65 still leaves us at about a 40 humidex.

3. Traveling to escape the heat? We'll be as hot as these places

Toronto and Montreal aren't exactly the top of the tropical getaways list, but this weekend, we'll be giving a few vacation destinations a run for their money.

If you're more of a fan of a dry heat, Phoenix, Ariz., and Riyadh, Saudia Arabia might be more to your liking. They're looking at highs of 40 and 44 C this weekend, respectively, but without a hint of humidity. Or you could try Newfoundland, where it SNOWED this week.

4. Cooling off in the lake? Play it safe in cold water

It might be hard to think about hypothermia when even thinking about going outside makes you feel sweaty, but if your holiday weekend plans include a dip in the lake, you might want to keep it in mind.

It's still early in the season, and while shallower lakes, like Lake Erie, are starting to warm up, most of our bodies of water are still on the chilly side. The dangerously chilly side, even.

While temperatures in the teens might not sound that cold, there's a big difference in comparing water temperatures and air temperatures.

According to the Canadian Safe Boating Council, our bodies lose heat about 25 times faster in water than they do in air at the same temperature. In fact, water temperatures below about 21ºC are considered 'dangerous' when it comes to cold water safety. It might sound soupy, but one of the reasons Olympic swimming pools are kept between 25-28ºC is to stave off any 'cold water shock' effects the swimmers might otherwise face.

This doesn't mean you should stay out of the water entirely, this weekend, of course; but it might be a good idea to brush up on your water safety procedures before heading out.


5. When is it an official heat wave?

Trick question - we don't have official heat waves in Canada!

Obviously that's not because it's always cold here; it's because, a few years ago, Environment Canada altered its warning criteria to issue heat warnings, retiring the concept of heat waves.

Heat warning criteria vary across the country, and generally reflect a combination of hot daytime temperatures with little relief through the overnight hours. For some places, including southern Ontario and Quebec, humidex values are also factored in.

TIPS FOR THE HEAT

While most of us enjoy the summer weather, it's important to remember the dangers and think of those who are most vulnerable: infants, elderly and pets. Here are some tips from Environment Canada:

  • Drink plenty of water, even before you feel thirsty and stay in a cool place
  • Reduce your heat risk by scheduling outdoor activities during the coolest parts of the day
  • NEVER leave people or pets inside a parked vehicle

(SUMMER FORECAST UPDATE: OUTLOOK ACROSS THE COUNTRY)

EARLY JULY: WHY THE HEAT

For early July, a robust, sprawling area of high pressure aloft or 'heat dome' will anchor itself over the continental U.S. and Great Lakes, reinforcing the pattern and boasting extreme heat over parts of southern Manitoba through Ontario, Quebec and the Maritimes, as well as the majority of the United States. Several days of hot weather will erupt across the region, leading to daytime highs into the 30s and even 40s stateside, leading to what may be our hottest stretch of weather for Ontario and Quebec this summer.



The looping video below shows the upper-level atmospheric pattern, where the oranges and reds depict ridging and where corresponding above normal temperatures will likely blossom.


Modeled 500 mb height anomalies from late June through early July. Image courtesy: WeatherBell Analytics.

Over the long weekend, the sun will be in full force over the southern reaches of the provinces, allowing for temperatures to reach their full potential and peak into the mid 30s. But with such high heat and humidity, the inevitable risk for thunderstorms will develop with even the slightest of triggers late in the weekend and into the holiday Monday.

The heat is forecast to ease somewhat during the middle of next week, but will rebuild for the weekend with hot weather expected to continue for the second week of July.

So whether or not you're a fan of the heat, it seems obvious that now is the time to prepare and of course befriend those with pools.

WATCH BELOW: HORSES COMPLETELY UNFAZED BY TORNADO


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