Monitoring: BIG rain event on tap for Ontario, 75+ mm

Wednesday, May 3, 2017, 10:41 AM -

It might be good for the May flowers, but another round of heavy rain late this week could mean localized flooding for soggy southern Ontario.

Hot on the heels of the Texas low that brought nearly 70 mm of rain to some parts of the province on Monday, another slow-moving low pressure system has its sights on the region starting on Thursday.

Quick facts:

• Yet another slow-moving system will push out of the U.S. Midwest and into southwestern Ontario starting Thursday morning
• Heavy rain is expected in a large swath from Windsor to the GTA by midnight Thursday, with lighter rain stretching ahead to Ottawa
• Heavier rain reaches eastern Ontario, including Ottawa, by dawn on Friday
• Heavy rain starts to ease west to east late Friday morning
• Showers continue as the low ambles across southern Ontario through Friday and Friday night
• The main frontal system should clear eastern Ontario by Friday overnight
• Wraparound showers -- similar to Tuesday -- are again possible across the region for most of the weekend
• Environment Canada has blanketed southern Ontario with a special weather statement in advance of the system

SPRING HAS SPRUNG: How will the season play out for Canadians? Find out with The Weather Network’s 2017 Spring Forecast | FORECAST & MAPS HERE

Wednesday and Thursday

Many southern Ontarians get only a short break between systems this week, as high pressure will clear skies and allow temperatures to rebound somewhat on Wednesday. Some highs in the southwest will creep into the low-to-mid teens for Wednesday afternoon as sunny skies dominate the region.

The next system starts to move toward the province Wednesday night, arriving in the southwest Thursday morning. While the Golden Horseshoe sees increasing cloud cover and below-average temperatures before the rain arrives Thursday afternoon, highs in eastern Ontario - including the Ottawa Valley - will again nudge into the teens with another afternoon of mostly sunny skies and gusty winds.

Showers should reach the Quebec border by evening, however, meaning National Capital residents should plan on a patio lunch rather than dinner.

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Friday

Friday is forecast to be another wash-out for most.

Heavier rainfall continues across the region Thursday night into Friday, with temperatures stuck in the single digits for most. Strong and gusty northeast winds will continue, leading to another chilly, wet, and blustery spring day to end the work week.

In addition to flooding of rivers and streams after this week of heavy rain, coastal flooding along the shores of Lakes Erie and Ontario, as well as along the St. Lawrence, will be a concern, as water is driven onshore by strong east winds.

The Weekend

The surface low is likely to linger near the eastern Great Lakes into the weekend, bringing showers and unseasonably cool temperatures for Saturday and Sunday. Some elevated spots across southern Ontario, particularly the Dundalk and Halliburton Highlands, may even see wet snow mixing in at times during the weekend.

More widespread snow is likely on the northwest side of the system, including a swath of potentially significant accumulations for northern locations such as Timmins.

Blocking high pressure over the North Atlantic will mean this system will be slow to clear the region. Instead it will develop into a cutoff low that will bring unsettled weather to eastern Canada well into next week.

The forecast will continue to be refined as the event approaches, but widespread totals ranging form 25 to 50+ mm are likely from this slow-moving system through the weekend. Given the recent heavy rains and saturated conditions across the region, this will likely lead to additional flooding concerns as spring rainfall totals continue to mount up.

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With files from Weather Network meteorologist Michael Carter