Significant Severe Thunderstorm Outbreak Could Bring Tornadoes to Southern Ontario Wednesday
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The calendar may say September, with the first day of meteorological fall arriving on Tuesday, but the next 24 hours are shaping up to bring potentially the most volatile severe weather risk we’ve seen all summer across Southern Ontario.
Multiple rounds of thunderstorms are possible beginning Tuesday evening and continuing into Wednesday morning. However, the main concern will come Wednesday afternoon and evening, when a much more significant severe weather threat is expected to develop.
Current indications suggest the environment on Wednesday could support an outbreak of supercells across portions of Southwestern and Central Ontario, extending into the Greater Toronto Area and Golden Horseshoe.
These storms could bring several tornadoes, very large hail up to the size of golf balls and widespread damaging to destructive wind gusts.
It is important to stress that we are still about a day away from the main event, so there is plenty of time for the forecast to change.
However, confidence continues to grow in the potential for a high-impact severe weather event on Wednesday. There is substantial agreement between the latest models on the strength of the environment, although some uncertainty remains regarding exactly when and where storms will develop.
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Given the potential severity of Wednesday's setup, now is the time to make sure you have a reliable way to stay informed.
Download the FREE Instant Weather app for the fastest source of Environment Canada watches and warnings, along with our own custom storm notifications that can provide an early heads-up when we identify potentially dangerous weather.
We’ll also be live throughout the event on our YouTube channel, providing the most comprehensive storm coverage powered by the largest storm reporting community in Canada.
With real-time reports from across the region combined with radar and our own analysis, our goal is to help you be the first to know when dangerous weather is heading your way.
Make sure you have notifications and location permissions enabled in the app, and if a tornado warning is issued for your location, take it seriously and seek shelter immediately!
DATA FROM WEATHERBELL
The severe weather risk actually begins Tuesday evening with a conditional threat across parts of Southern Ontario.
By conditional, we mean that the atmosphere appears capable of supporting severe thunderstorms, but it remains unclear whether storms will actually be able to develop and take advantage of that environment.
Looking at the supercell composite, which combines several atmospheric ingredients to give us an idea of how favourable the environment is for severe thunderstorms, there is a fairly potent setup across Southwestern Ontario Tuesday evening.
This favourable environment extends northward into portions of Central and Northeastern Ontario.
The environment is expected to persist beyond midnight and into the overnight hours, meaning any thunderstorms that manage to develop could remain strong well after sunset.
The big question is storm development.
Most of the latest models show fairly limited thunderstorm activity Tuesday evening, aside from the possibility of a few isolated pop-up storms across Deep Southwestern Ontario.
Additional thunderstorms are expected to move into the region from Michigan overnight, with further scattered development possible across Southern Ontario.
Any storms that can become established in this environment could quickly reach severe levels. However, confidence is fairly low regarding exactly where these storms will develop and how widespread they will become.
DATA FROM WEATHERBELL
The favourable environment is expected to continue into the predawn hours of Wednesday, with some of the strongest conditions focused around the Lake Huron shoreline.
Once again, storm coverage will be a major question mark.
These types of overnight setups can be difficult for models to handle, especially when there is no obvious trigger to initiate widespread thunderstorm development. The lack of daytime heating also adds another layer of uncertainty.
DATA FROM WEATHERBELL
One area where we are seeing better consistency is Deep Southwestern Ontario.
Several models continue to show a strong cluster of thunderstorms moving through the Sarnia and Chatham-Kent areas around sunrise Wednesday.
If this develops as currently shown, damaging wind gusts could become a significant concern.
Some of the more aggressive model guidance is even showing a localized pocket of wind gusts approaching 130 to 140 km/h. It is too early to say whether winds of that magnitude will actually occur, but it highlights just how intense this potential morning round could become.
Additional thunderstorms may also develop around Georgian Bay and track toward the Lake Simcoe area Wednesday morning.
There is also some potential for storms around Sudbury and North Bay tracking southeastward toward Algonquin Park.
However, the latest data has backed away somewhat from this scenario, so confidence in morning storm development across Central and Northeastern Ontario is lower than it is across Deep Southwestern Ontario.
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For the Tuesday night and Wednesday morning portion of the forecast, we have gone with a strong severe risk across Deep Southwestern Ontario.
This is where we currently have the highest confidence in seeing thunderstorms, both from potential development Tuesday evening and the stronger round that could arrive around sunrise Wednesday.
This strong risk includes Windsor, Leamington, Chatham, Sarnia, London and St. Thomas.
A fairly expansive widespread severe risk extends from Southwestern Ontario through Central Ontario and into the Sudbury and North Bay areas.
The threat in this zone is more conditional because confidence in storm development is lower.
However, if storms do develop, the environment would support the potential for several of them to become severe. Damaging wind gusts and large hail appear to be the primary threats, although an isolated tornado cannot be completely ruled out with any stronger rotating storms.
This widespread severe risk includes Niagara Falls, Hamilton, Woodstock, Kitchener, Guelph, Goderich, Owen Sound, Collingwood, Barrie, Muskoka, Parry Sound, Algonquin Park, North Bay and Sudbury.
For the Greater Toronto Area and into the Ottawa Valley, we currently have an isolated severe risk.
The environment in these areas is generally less favourable during the overnight and morning hours. However, any mature thunderstorms arriving from farther west could maintain severe potential as they track into the region.
DATA FROM WEATHERBELL
Once the morning activity clears out, attention will quickly turn toward a much more potent severe weather environment expected to take shape Wednesday afternoon.
This is the part of the forecast that concerns us the most.
Current indications suggest the strongest severe weather environment will stretch from the Lake Huron shoreline eastward toward Lake Simcoe, before extending southeastward into the Greater Toronto Area and Golden Horseshoe.
Models also show a potentially favourable environment developing in the Ottawa Valley. However, storm development is much more questionable there at this point, which could ultimately limit the severe threat.
DATA FROM WEATHERBELL
There is still some uncertainty regarding exactly when thunderstorms will begin developing Wednesday afternoon.
Earlier model runs were pointing toward a main complex of storms developing over or near Lake Huron sometime during the late afternoon.
Under that scenario, the storms would track through Southwestern and Central Ontario before reaching the Golden Horseshoe during the early evening.
There would also be the potential for isolated thunderstorms to develop ahead of the main complex during the afternoon.
Any isolated storms that can develop ahead of the main line or cluster will need to be watched particularly closely. With storms having more room to remain discrete, they could take advantage of the favourable environment and become supercells capable of producing tornadoes, very large hail and destructive wind gusts.
However, the latest data is beginning to suggest a somewhat earlier start.
Some models now develop the main complex during the early afternoon, with storms already well inland by the late afternoon.
From there, the storms would continue southeastward through the Golden Horseshoe and toward Lake Ontario, eventually crossing into the United States during the evening.
The exact timing will be extremely important because even a difference of a few hours could significantly change which areas experience the greatest severe weather threat.
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For our preliminary Wednesday afternoon and evening forecast, we have gone with a strong severe risk extending from the Lake Huron shoreline through portions of Southwestern and Central Ontario and into the Golden Horseshoe.
This is currently the area where we see the greatest potential for widespread severe thunderstorms.
Several tornadoes could occur if the environment develops as currently projected, particularly with any discrete supercells that form ahead of the main complex.
Very large hail up to the size of golf balls will also be possible, along with widespread damaging wind gusts.
There is also the potential for pockets of destructive winds with the strongest storms, especially if thunderstorms eventually organize into a powerful line or cluster.
The strong severe risk includes Toronto, Hamilton, Niagara Falls, Woodstock, London, Sarnia, Goderich, Kincardine, Owen Sound, Collingwood, Kitchener, Guelph, Newmarket, Barrie, Orillia, Peterborough and Belleville.
Surrounding areas, including Deep Southwestern Ontario, northern sections of Central Ontario and southern portions of Eastern Ontario, could also experience severe thunderstorms Wednesday.
However, the environment is currently expected to be somewhat less favourable for a more significant severe weather event in these areas.
This includes Windsor, Chatham, Muskoka, Parry Sound, Bancroft, Kingston and Brockville.
Isolated severe thunderstorms are also possible across the Ottawa Valley.
There are signs that the atmosphere could become favourable enough to support severe thunderstorms around Ottawa, but whether storms actually develop in this environment remains questionable.
Right now, this forecast remains preliminary and is subject to change as new data comes in.
There are several moving pieces that still need to be resolved, particularly the evolution of Tuesday night's storms and how much they affect the atmosphere heading into Wednesday afternoon.
The exact timing of Wednesday's storm development will also be critical in determining where the greatest tornado, hail and destructive wind threat ultimately sets up.
What is becoming increasingly clear is that Wednesday has the potential to be a significant severe weather day across parts of Southern Ontario.
We will be reviewing the latest data over the coming hours and will put together a more detailed forecast later tonight or early Wednesday morning.
That update will focus more heavily on timing and the areas at greatest risk once we have better consistency in the latest data.
