[UPDATED] Ontario: After chilly days, heat starts building in

Sunday, October 7, 2018, 11:46 AM – Rumbles of thunder accompanied rising temperatures back into southern Ontario Saturday into the early hours of Sunday, in a subtle warm up after a freezing Friday morning.

But warmer temperatures — and near-record humidex values — are still ahead, as Ontarians may be giving thanks for a blast of summer-like warmth by the end of the holiday long weekend. We take a look at the complicated temperature trend, and who still has a risk for storms, below.

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


  • Overcast and slightly cool Sunday morning for many after overnight storms
  • Temperatures rising into early part of new week
  • Humidex rising substantially; record levels for early October possible by Tuesday
  • BE PREPARED: Monitor weather alerts for your area, here


After overnight storms in some areas, Sunday looks to be a mostly cloudy day, with the threat for showers lingering here and there — mainly across the Niagara Peninsula — but most of the day will be rain-free.

The next major storm system with its eyes on Ontario will lift north into the Nickel Belt and beyond, taking most of the rain and thunder threat well north with it. That doesn’t mean we won’t see any effects in the south, however, as warm, humid air floods back into the region, starting on Thanksgiving Monday.

“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.”




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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