ONTARIO: Thunderstorm Forecast for Wednesday, August 23, 2023
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As August winds down and cooler nights settle in across Southern Ontario, there's a noticeable touch of fall in the air, with some areas experiencing single-digit overnight temperatures. This shift to colder air also signifies the winding down of our primary severe thunderstorm season. While the risk of severe thunderstorms can persist into September and even October, historically, the most intense severe events tend to occur during the months of June, July, and August in our region.
However, there's no quiet exit in sight for summer just yet. The threat of severe thunderstorms is looming over parts of Southern Ontario for Wednesday and Thursday. The latest data indicates a potential storm threat in two separate waves over the next 48 hours.
The first wave is expected to develop late on Wednesday and continue into Thursday morning, bringing with it a potential risk of strong nocturnal storms in Deep Southwestern Ontario. A second severe risk is forecasted for Thursday afternoon and early evening, affecting a similar area as on Wednesday.
The overall weather pattern is intricate, with multiple elements in play. Bands of heavy rain are projected to develop across Central and Southwestern Ontario during Wednesday morning, resulting in a prolonged period of steady rainfall in the GTA and areas around Lake Huron and Georgian Bay. There's also the possibility of embedded thunderstorm activity within the band of precipitation.
Simultaneously, colder air will accompany the area of rain, leading to notable temperature differences across Southern Ontario. For instance, while the GTA and regions around Georgian Bay struggle to reach daytime highs in the mid-teens on Wednesday, the Ottawa Valley and Deep Southwestern Ontario are expected to enjoy temperatures ranging well into the mid to upper twenties.
In the southwestern part of the province, a pocket of significant instability is anticipated to form across Michigan and extend into Deep Southwestern Ontario by Wednesday evening. Models suggest that the line of ongoing precipitation will gradually shift southwestward and intensify as it taps into the more robust environment. By the late evening hours, this line of storms could potentially become quite intense as it moves through the London, Sarnia, Chatham, and Windsor region between 10 PM and 2 AM.
The overall storm risk is subject to some uncertainty primarily due to the timing of this event. If it were to occur a few hours earlier, it would likely fall within a significant severe risk based on the expected intense environmental conditions. While we have opted for a ‘strong’ severe risk due to the later timing. We can’t rule out a potential upgrade to ‘significant’ if our confidence grows in a more widespread event.
These late-night storms could potentially bring strong wind gusts of up to 110 km/h as the primary threat. Although large hail up to the size of ping pong balls is also possible, the linear nature of the storm may mitigate the likelihood of very large hail. The atmospheric conditions could also support the development of a few tornadoes, with the possibility of an isolated strong tornado.
In addition to the damaging wind threat, there's a notable risk of flash flooding on Wednesday. The slow movement of the storm line could result in prolonged heavy rainfall over specific areas, particularly in Southwestern Ontario, potentially leading to rainfall totals exceeding 100mm. Urban areas, which are more vulnerable to significant rainfall in short periods of time, could experience localized flooding.
Thunderstorm activity is expected to clear out of Southern Ontario by Thursday morning. Later on Thursday, the environmental conditions could allow for more storm development in Deep Southwestern Ontario during the afternoon and early evening. However, the models present varying scenarios, so we'll wait for the initial round of storms to unfold before focusing on the specifics of the second round. Keep an eye out for our forthcoming forecast for Thursday afternoon/evening!
