Rounds of rain from Hilary moisture in Ontario brings localized flooding

An active storm track has taken hold of southern Ontario, as tropical moisture seeps into the region, and periods of heavy rain and thunderstorms last into the weekend.

The same heat dome responsible for broiling much of the United States and steering Hilary’s historic rains in Southern California, is what's helped to bring Wednesday's moisture into Ontario.

Wednesday will also be watched closely by forecasters as the threat for late-day thunderstorms includes heavy downpours, which could lead to localized flooding in some areas due to their sluggish speed.

A drastically different air mass will spread over parts of Ontario, as well, with a near 20-degree difference in the humidex from Toronto to Windsor on Thursday.

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Wednesday

Areas: Southern Ontario

Timing: Wednesday

Weather: While a warm and dry high-pressure pattern develops stateside, Ontario’s weather this week will be anything but that. The boundary of the high will drive rounds of worrisome rainfall into southern Ontario through Wednesday and beyond.

The rain began across the region before dawn Wednesday morning, though with flooding concerns on the rise during round 2, which will likely impact the evening commute home.

The slow and narrow track of these early-evening storms is a setup known as training thunderstorms. That means one rainy storm is followed by another, and then another.

In a short period of time, rainfall totals may exceed 50 mm where the storms linger over the same region, risking localized flooding, pooling and ponding on the roadways. That will be possible between the west end of the Greater Toronto Area (GTA) to London.

The round of storms will also spill into the overnight hours, leading to round 3: Nocturnal storms in the southwest.

Enough thunderstorm energy for larger hail and severe thunderstorms may build into Windsor, Chatham and Sarnia by Wednesday evening and overnight. Backing winds mean rotating thunderstorms could be possible. However, the late timing of this environment lowers confidence greatly because of the limited daylight and diminishing dynamics.

Adding to the complexity, that region may lack a storm trigger with the larger boundary located farther north. Forecasters will monitor if a lake breeze or supercell can develop late Wednesday evening. If not, muggy and cloudy conditions are likely from Windsor to London until the boundary dips south very early Thursday morning.

Thursday

Areas: Southern Ontario

Timing: Thursday afternoon and evening

Weather: Another chance for rain showers and thunderstorms will develop for southern Ontario, with the risk for some severe storms popping up again in southwestern Ontario. There is ample amounts of energy for a few storms to become stronger, with large hail and strong wind gusts possible threats.

Along with these storms, muggy conditions will continue across the southwest, with a humidex value of 45 forecast in Windsor. A drastically different air mass will impact different parts of the region, however, with a daytime high of only 22°C expected in Toronto on Thursday, feeling like 26.

Rain chances continue into the weekend

Friday will be a much drier day, though with a small chance for some isolated rain showers in some areas. That will be short-lived as yet another low sets its eyes on Ontario for Saturday, likely interrupting weekend outdoor plans.

Prepare

Make sure to keep up-to-date on your local forecast information and alerts. If you are outdoors in an area with a severe weather threat, make sure you have a safety plan in place in case you need to get to shelter.

If a severe thunderstorm approaches your location, remain mindful of trees or tree limbs that loom near your home. Trees falling into buildings are a significant source of injury during strong storms. Try to avoid rooms where trees may cause damage during high winds.

The greatest danger in any thunderstorm is lightning. If you can hear thunder, you’re close enough to be struck by lightning.

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Be sure to check back for the latest weather updates across Ontario.