Southern Ontario: Thunderstorm Outlook for Tuesday, July 20, 2021

Forecast Discussion

There is a widespread risk of severe storms across Southern Ontario on Tuesday. A cold front will sweep across the province and may spark off a line of storms along the front starting during the early afternoon across Georgian Bay and northern parts of Central Ontario. It will move to the southeast during the afternoon bringing the potential for severe weather with strong wind gusts being the main threat although we could see some localized pockets of large hail if there are some isolated pop-up storms ahead of the main line of storms. Flash flooding could also be an issue as we’re seeing some indications of ‘training’ storms developing through parts of the GTA that are very susceptible to heavy rainfall within a short timeframe.

Compared to our previous forecast, the risk has become much more widespread and we’ve added a severe risk zone to parts of Central, Eastern and Southwestern Ontario along with the GTA and Niagara Region. Most of Southern Ontario has at least an isolated risk of severe storms, but keep in mind that not everyone will see the storms on Tuesday. Also, there is some disagreement in the data which means confidence in this forecast isn’t as high as we normally are the night before an event. Adjustment may need to be made by Tuesday early afternoon so check back for that.

As for the tornado risk, it isn’t super strong and nowhere near the level that we saw on Thursday, but we can’t rule out the potential for one or two tornadoes. The main risk zone from tornadoes includes much of the Golden Horseshoe, to the northwest into Orangeville, north through Lake Simcoe and eastward along the northern Lake Ontario shoreline. It’s possible we may see tornadoes outside of this zone as it isn’t really focused on a particular region and we had to be broad with the risk zone.