ONTARIO: Thunderstorm Forecast for Thursday, July 20, 2023
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FORECAST DISCUSSION
Confidence is growing in what could be the strongest severe weather event we've seen thus far across Southern Ontario on Thursday. The most significant area of concern will be along the Lake Huron shoreline and into Deep Southwestern Ontario during the late afternoon into the evening.
Locations like Windsor and Chatham particularly stand out in the data, with the risk of destructive wind gusts and an isolated potential for a strong tornado. Severe potential carries eastward into the GTA and Central Ontario with a slight to isolated severe risk for storms arriving during the evening hours.
A line of strong severe storms is expected to form over Central Michigan just after the lunch hour and slowly track towards our region, reaching Lake Huron by the mid-afternoon. Ahead of this line, we are seeing the potential for isolated storms to pop up around Southwestern Ontario, which could quickly become quite severe with all severe threats possible.
As this line arrives in Southwestern Ontario, widespread destructive wind gusts are expected from Kincardine down into Windsor, with gusts reaching upwards of 90-110 km/h and pockets even higher. As a result, we have put this area in a “significant” severe threat on the map as the wind damage will be widespread.
The line could also feature embedded rotation, leading to a moderate tornado risk. We believe at least one tornado will occur on Thursday somewhere. Hail will also be a threat with locally up to golf ball-sized hail, and more widespread hail ranging from quarter to toonie-sized.
There is some uncertainty as the line tracks further into Ontario towards the evening hours. The data seems to show an environment that could sustain this storm for quite a distance, primarily posing a wind gust threat. It should reach Central Ontario into the GTA sometime around the 8 - 11 PM timeframe, but that could shift depending on the speed of the line. Localized wind gusts here could reach around 100 km/h with up to quarter-size hail, particularly around the Niagara/Hamilton region and Western GTA.
Thunderstorms will continue eastward into Eastern Ontario by the pre-dawn hours, but they should be mostly non-severe. Some models suggest some re-intensification during the morning hours, which could bring a questionable severe threat mainly for strong wind gusts. More on the Friday morning risk will be covered in our forecast for Friday.
TORNADO RISK
The highest tornado risk for this event is expected to be mainly focused on Deep Southwestern Ontario where the environment is capable of supporting rotating storms. These tornadoes may spin up quickly without warning and could be rain-wrapped, making them hard to see coming. A strong tornado can’t be ruled out somewhere in the Windsor, Leamington, Chatham, and Sarnia regions.
One or two tornadoes are possible in much of Southwestern Ontario, including London, Goderich, and Kincardine. There is an isolated tornado threat extending to the Golden Horseshoe and Central Ontario, but this is very conditional based on timing and the strength of the storms once they reach this area. The tornado threat is questionable but still worth including.
Northern Ontario
Isolated thunderstorms could develop around the Georgian Bay shoreline in Northeastern Ontario during the late afternoon and early evening on Thursday. A few of these storms could become marginally severe with threats including 90 km/h wind gusts, up to quarter-sized hail and heavy rainfall. An isolated tornado can’t be ruled out.
