Multiple Waves of Heavy Rainfall in Southern Ontario With Up to 40–80mm of Rain Possible by Tuesday
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The start of August has been marked by a wet trend in parts of Southern Ontario, and it looks like the pattern will persist as we enter the first full week of the month. Some of the heaviest rainfall totals are expected to affect the same areas that saw flash flooding last week. Those in the Lindsay area saw rainfall totals approaching 100mm!
And we are currently forecasting an additional 30-60mm of rain over the next few days for the same area. Anticipate several waves of precipitation moving across Southern Ontario starting Sunday afternoon and extending through Monday into Tuesday. The heavy rainfall will be accompanied by embedded thunderstorm activity, intensifying the overall rainfall totals.
The first wave has already begun to sweep through Southwestern Ontario as of Sunday mid-afternoon, expected to persist through the evening and overnight. The most substantial and prolonged rainfall will be centred off-shore over Lake Huron, but areas close to the shoreline and the Bruce Peninsula may also be affected.
Residents in the Golden Horseshoe should anticipate on-and-off rain beginning around the dinner hour on Sunday, with the intensity increasing after midnight, bringing the possibility of isolated non-severe thunderstorms. By early Monday morning, this precipitation will move into Central and Eastern Ontario.
There will be a temporary break in the rain for Southwestern Ontario by morning and for Eastern Ontario around noon on Monday. The timing of this lull between the two waves is crucial, as it will determine the potential for severe weather risk later in the day.
Some indications suggest the development of an environment conducive to supporting marginally severe storms during Monday afternoon and evening. The Golden Horseshoe and Eastern Ontario regions are particularly favourable for storm development, posing threats of heavy rainfall, strong wind gusts, and small hail.
A primary concern lies in the potential for flash flooding, as these storms may persist and bring extensive rainfall totals exceeding 100mm. Additionally, hail up to the size of nickels and wind gusts reaching 80-90 km/h are possible threats. Although the tornado risk is expected to be minimal, it can't be entirely ruled out.
Based on the latest data, we are also considering the possibility of adding a slight risk zone stretching from Lake Simcoe to Kingston, as we gain confidence in the potential for wind gusts of up to 100 km/h within these storms. An updated thunderstorm forecast will be posted late Sunday evening after reviewing the evening model data.
This storm risk will contribute to additional rainfall over the next 48 hours, with heavy rain expected late Monday and continuing into the early part of Tuesday in Eastern Ontario. The precipitation should finally clear out by the afternoon on Tuesday.
Overall, widespread rainfall totals over the next two days are projected to range from 20 to 60mm, with higher amounts in Eastern and Central Ontario due to the thunderstorm threat. Some models even suggest that a localized pocket around the Kingston region could receive in excess of 100mm by Tuesday afternoon.
