Ontario: Cooldown and storms, but RECORD HEAT next

Saturday, October 6, 2018, 11:29 AM – Feel that chill? Temperatures in Ontario took a steep and steady tumble to close out the week, hovering around FREEZING Friday morning, and it’s little warmer Saturday.

But despite the plunge, near-record warmth returns by early next week. More on this complicated temperature trend PLUS a looming weekend storm threat, below.

(A look at the next three months: Your Official 2018 Fall Forecast)


  • Up-and-down temperature pattern, with alternating surges of warmth and chillier air
  • Severe thunderstorm threat on Saturday with strong winds, small hail and heavy rain at play
  • BE PREPARED: Monitor weather alerts for your area, here



A tight temperature gradient will remain over the province this weekend, with below seasonal air dominating the north and above seasonal and even muggy weather in the south. Periods of showers and embedded thunderstorms can be expected across the province, hindering the full warm up potential.

A low pressure system will track through southern Ontario and the Great Lakes Basin Saturday. This system will bring a warm, humid, and unstable airmass, but there will still be a relatively tight gradient along the warm front.

The placement of this warm front is key as to where a thunderstorm risk will develop. For now, latest guidance indicated that the warm front is expected to clear southwestern Ontario and setup either through or just north and east of the GTA — meaning that the more unstable airmass could setup across southwestern Ontario, the GTA, as well as Hamilton and Niagara.

Through the mid to late afternoon hours, the cold front associated with the low is expected to cut through the unstable air leading to the threat of thunderstorms mainly for the region mentioned above. With the current trend, sufficient instability and wind shear are expected to be in place Saturday afternoon to lead to the develop of vigorous thunderstorms and perhaps even a few severe storms, with the main threats being strong winds, small hail, and heavy rain.

The main caveats to this setup are that if the warm front does not push into southern Ontario, then the storm threat will be rather limited to nil. The other caveat is if any morning cloud cover will clear out enough to allow for enough daytime eating to build up the instability. Stay tuned for updates as we continue to monitor this evolving forecast.

The Temperature Swings:

Sunday will be mostly cloudy with the threat for showers, but most of the day will be rain free although several degrees cooler than Saturday.

On Thanksgiving Monday, a warm front will slowly lift north, but it looks to stall near the west end of Lake Ontario.

“For most of the GTA this means that a southeast wind off of Lake Ontario will hold temperatures in the upper teens,” says Weather Network meteorologist Dr. Doug Gillham. “Meanwhile, summer-like weather will spread into southern Niagara and into areas west of the GTA with temperatures reaching into the mid 20s with a humidex in the low to mid 30s.”


While a cooler and more typical fall pattern was expected to stick for the first half of October, by Tuesday there’s confidence for more summer-like weather with all time humidex recordsin jeopardy. High temperatures are expected to reach the mid to upper 20s feeling closer to the mid 30s with the humidity.

What is happening here?

The reason for this trend and the delay of a cooler more autumnal pattern is due to an upper level ridge across the southeastern portions of the continent, which continues to have much more fight to it than what long range guidance expected. This will be the case for next week as another anomalously strong upper level ridge sets up along the eastern seaboard. This results in a surface pattern, as shown below, with a strong area of high pressure over the eastern portions of the continent and generally lower pressure across the central portions. This allows subtropical air to push up from the Caribbean spreading into southern and central Ontario.

By Tuesday and through the remainder of next week, temperatures across much of the province are expected to run anywhere from about 5°C to nearly 12°C above average for this time of year. The warmest air is expected to be focused across southern portions of the province through the 401 corridor. Tuesday and Wednesday are expected to be the two mildest days overall with highs forecast to be into the mid to high twenties with humidex values running into the low to mid-thirties.

As we head into next weekend this pattern looks to change back over to a more autumnal pattern as long range guidance starts to weaken the ridge in the east and the trough pushes eastward. This will result in a fall storm setting up to end next week and will usher in a more autumnal pattern with temperatures getting back to seasonal or even below. The timing of when the fall storm pushes through is still yet to be determined as long-range guidance varies on this.

The question is, does the cooler more seasonal air stick around? Some teleconnection patterns and overall trend setting up does lean toward yes, but the upper level ridge across the southeast still lingers. This could give the odd push of milder air into extreme portions of southern Ontario at times, typical of fall, but overall the trend does look cooler for the province.

With files from meteorologist, Brad Rousseau

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