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Soggy start to fall in southern Ontario, heavy rain brings flood risk

Tuesday, September 21st 2021, 10:44 pm - The first day of fall will not be pleasant in southern Ontario, with rainfall warnings in place for a widespread soaking rain that could dump as much as 100 mm of rain over parts of Ontario this week.

Wednesday is the official start to fall, and those in southern Ontario won't like what it has to offer. The day will be a washout, with a long-duration rainfall event that will continue through Thursday morning. This is thanks to a trough becoming stationary over Ontario as a result of a high pressure blocking over the Atlantic region. Far-reaching rainfall totals of 75+ mm are expected through Friday, with many areas potentially in line to see 100+ mm. This will raise the threat for flash and localized flooding in parts of southern Ontario, so motorists should be mindful of the conditions when heading out on the roads. More on the timing and impacts, below.

Visit our Complete Guide to Fall 2021 for an in-depth look at the Fall Forecast, tips to plan for it and much more!


As a cold front continues to move northeast, rain will ramp up Tuesday overnight in southern Ontario and will continue Wednesday, not letting up until Thursday afternoon, when a break in the precipitation is expected.


A second moisture-laden low-pressure system will trek in from stateside Wednesday afternoon, bringing more rainfall to what will already be a soaked region.

Rainfall warnings span across most of the south, alerting to widespread heavy totals through Thursday morning, with the highest amounts expected northwest and west of the Greater Toronto Area (GTA), particularly along the Lake Huron and Georgian Bay shorelines, portions of cottage country and southwestern Ontario.


Locally, some areas could be on the hook for totals surpassing 100 mm of rainfall since there may be embedded thunderstorms. This will raise the risk for flash and localized flooding where the heaviest rain occurs.

Motorists should exercise caution on the roads, as difficult conditions can be expected due to wet roads, periods of low visibility and ponding.

"Heavy downpours can cause flash floods and water pooling on roads," says Environment and Climate Change Canada in the warning. "If visibility is reduced while driving, turn on your lights and maintain a safe following distance. Localized flooding in low-lying areas is possible."

By the time the event winds down, rainfall amounts may actually approach, reach or exceed normal monthly values for September, potentially even topping totals received from the entire year so far in just a couple of days.


Winds won’t be too much of an issue Wednesday, but 40-60 km/h gusts can be expected, perhaps reaching near 70 km/h near the Lake Huron shorelines.

There may be some spotty showers lingering Thursday night and into the pre-dawn hours Friday morning before the system pulls away from the region, making for a fair day afterwards.


Temperatures will tumble to more seasonal values in behind the systems, remaining cooler through the weekend.


Another system will track into southern Ontario on Saturday with rain reaching the GTA during the afternoon. Rain will spread into eastern Ontario in the evening and overnight. However, some sunshine returns to southern Ontario for Sunday.

Seasonably cool for next week, but within a few degrees of seasonal. A warmer pattern (relative to normal) should return during the first week of October, but of course "seasonal" continues to steadily fall.

Be sure to check back for the latest updates on this slow-moving system that kicks off the fall season in Ontario.

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