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Ontario: Rains will gradually ease off as potent low slowly moves out

Monday, May 18th 2020, 8:30 am - Rain will gradually ease off in the south Monday, but parts of southwestern Ontario could get another 20-30 mm through Tuesday morning

A sluggish low will continue to bring rain in southern Ontario Monday morning, but will taper off through the day, with it moving out of the GTA by late afternoon. Heavy rains will linger longer in the southwest, where most of the moisture will be centred. Another 20-30 mm of rain could fall, which could lead to isolated ponding and pooling. There may also be embedded thunderstorms. Temperatures will remain well below seasonal and the cool air will linger through most of the week before a warmup by the weekend. Details and timing, below.


  • Rainfall will be heaviest in southwest Monday, gradually tapers off in the south
  • Potential for an additional 20-30 mm of rain in southwest
  • Temperatures remain cool until late week, warming up for the weekend
  • Keep on top of active weather ALERTS here.


The forecast rainfall totals for the potent system that is tracking through southern Ontario have shifted considerably over the past 24 hours.

Weather Network meteorologist Jessie Uppal explains that a sprawling ridge of high pressure centred over northern Ontario slowed the incoming system. An overall westward shift of an upper level low has also forced the heaviest rainfall across southwestern Ontario and further stateside.


The heaviest rains in southern Ontario fell overnight, but rain will continue through the morning before tapering off during the day gradually, east to west. The GTA will see precipitation end in the late afternoon as the rain continues to pull westward.

The western GTA, London, Sarnia, Niagara and areas further west will pick up another 5-10 mm, while parts of the Lake Huron shores may see 15 mm.

Because most of the moisture associated with the low will be centered over the extreme southwest, heavy rains will persist through the overnight hours, giving them additional totals of 20-30 mm before tapering off Tuesday morning. There may be isolated ponding and pooling because of the rains. There is also the risk for embedded thunderstorms.

Thanks to a raw easterly wind, temperatures will hover around 11-13°C Monday, which is well below the normal high of 20°C for this time of year.


Dry weather will return Tuesday with a mix of sun and clouds. However, an east wind off Lake Ontario will remain, keeping temperatures below seasonal, especially around the west end of the lake.


Wednesday will feature similar conditions and near seasonal temperatures across much of southern Ontario, but the east wind off Lake Ontario will continue to keep temperatures quite cool across the GTA.

The persistent easterly wind is the regult of a large area of low pressure near the East Coast of the U.S. and high pressure over Quebec, which will stall through the end of this week. Showers could return Friday.

"Several days ago it looked like the end of next week would bring a taste of summer weather to Ontario and Quebec. The warm weather is still on the way, but a persistent easterly flow off Lake Ontario will keep temperatures cooler than originally expected across the Golden Horseshoe through Friday," says Weather Network meteorologist Dr. Doug Gillham.


At this point, indictations are that the east wind will relax for the weekend, allowing for temperatures to climb into the 20s across the region.

Check back as we continue to monitor the forecast.

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