Soaking Start To Fall Across Southern Ontario With Up to 75–125mm of Rainfall Accumulation by Thursday

The calendar might show the start of fall as of Wednesday afternoon, but it will seem like Mother Nature has mistaken it for the start of spring with expected significant rainfall across the region over the next few days. This rainfall event will get going late Tuesday evening as bands of heavy rainfall and embedded non-severe thunderstorms set up from Extreme Southwestern Ontario and into parts of Central Ontario. It will continue overnight and throughout the day on Wednesday encompassing almost the entire region.

The rainfall will come in waves on Wednesday with isolated heavier pockets where embedded thunderstorms could deliver locally heavier rainfall amounts. There might be a few hours of clearing during the afternoon on Wednesday particularly around the Golden Horseshoe, but it won’t last long as more rounds of heavy rainfall move into the region from south of the border. This round will linger past the evening and overnight for the second night of heavy rainfall with a focus on Central Ontario and the GTA.

We will also see some strong wind gusts developing along the Lake Huron and Georgian Bay shoreline during the evening on Wednesday and continuing overnight. Wind gusts of between 50-80km/h are possible especially in areas close to the shoreline, but they shouldn’t reach severe levels.

By early Thursday, we’ll finally see the light at the end of the tunnel as the flow of heavy rainfall slowly breaks apart in the west and shifts to the east. Central and Southwestern Ontario should see the rain taper off by the late morning or early afternoon. Rain will continue for Eastern Ontario through the day on Thursday and into the overnight hours but it will be less intense than what we saw the previous night. The system will fully exit Southern Ontario by Friday morning although it will be short-lived with more rainfall possible over the weekend.

In terms of accumulation, we expect the heaviest rainfall totals will be found in a zone extending from Windsor, through regions east of Lake Huron and into portions of Central Ontario including Muskoka and Algonquin Park. Totals here will generally range from 75-125mm with isolated pockets that see thunderstorm activity picking up over 125mm. The rest of Southern Ontario except for Northeastern and Eastern Ontario will see widespread amounts between 40-75mm with locally up to 100mm. Eastern Ontario should see around 25-50mm, but most of that rainfall will come on Thursday instead of the first two days.

It’s important to note that this rainfall won’t be coming all at once as it will be spread out over the next 72 hours. Flooding will still be a major concern with this event, but it won’t be anywhere close to the impacts if we saw the same amount of rain from a stationary thunderstorm in just a few hours. Most of the rainfall will come on Wednesday and into early Thursday morning except for Eastern Ontario which will mostly stay dry until late Wednesday and Thursday.