Colorado Low Set To Bring Up to 40-75mm of Rainfall and Strong Wind Gusts to Southern Ontario Early This Week (Sunday - Tuesday)

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UPDATED MAP - Oct. 25 (12:30 AM)

UPDATE (Mon - Oct. 25): We have made a few adjustments to our map, but it mainly stays the same overall. The main changes are to reduce the rainfall totals for Eastern Ontario while increasing them for areas further to the north and west. This is due to a change in the track of the system which appears to put the bulk of the precipitation further west rather than locking in on Eastern Ontario.

We are still expecting total rainfall accumulation between 40-75mm for the Windsor, Sarnia, London and Hamilton/Niagara regions by the end of Tuesday. The rest of the GTA and Southwestern Ontario can expect between 25-50mm. Eastern and Central Ontario will range from between 15-30mm with higher amounts closer to Lake Ontario.

In addition to the rainfall, there will also be some strong wind gusts that are expected to develop early in the afternoon around Lake Huron and spread eastward throughout the rest of the day. At this point, they should remain below severe levels although it’s right on the line. We may see a few localized gusts come close to 90km/h particularly along the shorelines. Waiting for the data in the morning and if it appears we could see severe gusts we will issue a forecast map at that time.

OLD FORECAST

It has been a fairly quiet fall across Southern Ontario with a few heavy rainfall events and of course the unseasonably warm weather. But we’re slowly seeing the weather that we would normally see for this time of year after parts of Southern Ontario saw the first flurries of the season this weekend. Now we’re watching a strong Colorado low that’s expected to affect Southern Ontario starting late Sunday and continuing for much of the early part of the week. This low-pressure system will bring the threat of heavy rainfall along with some strong wind gusts. Some areas along the Lake Erie shoreline and into parts of Eastern Ontario could see rainfall totals between 40-75mm by the end of Tuesday.

We’ll begin to see the initial bands of precipitation reach Extreme Southwestern Ontario by the dinner hour on Sunday and it will slowly expand to the north and east throughout the overnight hours and into Monday. This will make for a wet Monday during the morning and afternoon as heavy rain encompasses most of the region from Windsor through to Ottawa. In addition to the rain, we will also see some strong wind gusts between 60-80km/h and maybe even near severe levels at 90km/h during the afternoon and evening on Monday.

As we head into the overnight hours and into Tuesday we should see the rain become less widespread except for Eastern Ontario which will continue to see heavy rainfall lasting through the early part of Tuesday. Some colder air will also flow into Central Ontario early Tuesday morning with near-freezing temperatures which may set the stage for the existing rain showers to transition over to some wet flurries particularly east of Georgian Bay. No accumulation is expected. Although confidence is low at this point so that’s not guaranteed. All of the precipitation should come to an end by late Tuesday as the system finally exits the province.

Total rainfall accumulation will be the highest through the Windsor, Hamilton and Kingston corridor where between 40-75mm of rainfall is possible. The rest of the GTA, Extreme Southwestern Ontario and into Eastern Ontario can expect around 25-50mm of rainfall. Those further north and west will see lower rainfall amounts with totals ranging from 15-30mm for the rest of Southwestern and Central Ontario. Less than 20mm is on tap for regions east of Georgian Bay and into Northeastern Ontario. Note that this is only a preliminary look at the expected rainfall totals and it may change as we get closer to the event - a final rainfall map will be issued by Sunday evening including a wind gust forecast if needed.

Southern Ontario: Fall Weather Hazards Outlook for Sunday, October 24, 2021

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Forecast Discussion

Temperatures are expected to drop to near the freezing mark throughout much of Southern Ontario during the morning on Sunday. This will bring the risk of frost to all areas away from the shorelines of Lake Ontario, Erie, Huron and Georgian Bay. Be sure to protect any sensitive plants that you want to save. The frost will melt away later in the morning after the sunrise.

Southern Ontario: Fall Weather Hazards Outlook for Sunday, October 24, 2021

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Forecast Discussion

Frost is possible throughout a big part of Southern Ontario including even Southwestern Ontario and the GTA. However, confidence is very low in exactly how cold it will get which will determine if frost occurs or not. We will have our frost outlook for Sunday sometime Saturday night once we get the latest information.

Southern Ontario: Fall Weather Hazards Outlook for Saturday, October 23, 2021

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Forecast Discussion

Near-freezing temperatures will once again be found across parts of Central and Eastern Ontario on Saturday morning. This will bring the risk of frost to the area so you should cover up and protect any sensitive plants that you want to save. In addition to the frost risk, we also could see some lake effect precipitation around Georgian Bay and into Northeastern Ontario and with the colder temperatures, it may come down in the form of flurries or wet snow. The confidence is low on this as any precipitation will be very scattered and weak so not anyone will see it and there will be little to no accumulation due to the ground still being relatively warm. Temperatures will rise later in the morning which will melt away any frost and turn any flurries over to rain showers.

Southern Ontario: Fall Weather Hazards Outlook for Saturday, October 23, 2021

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Forecast Discussion

Another chilly start to the day is expected for Saturday morning across Southern Ontario. This will also bring the risk for frost in some areas including Central and Eastern Ontario primarily along the Quebec border and higher elevations to the northwest of the GTA. A few days ago it looked like there might be the potential for the season’s first flurries in more northern parts of Southern Ontario. However, there is a lack of precipitation during the time when the temperatures are favourable for flurries. So right now it seems unlikely but we can’t rule it out if we get some lake effect precipitation around Georgian Bay.

Southern Ontario: Fall Weather Hazards Outlook for Friday, October 22, 2021

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Forecast Discussion

Near-freezing temperatures will flow into parts of Southern Ontario during the early morning hours of Friday. This will bring the risk of widespread frost throughout parts of Central, Eastern and higher elevations of Southwestern Ontario. Temperatures will moderate somewhat after sunrise so the frost risk should come to an end by mid-morning.

Southern Ontario: Fall Weather Hazards Outlook for Friday, October 22, 2021

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Forecast Discussion

Near-freezing temperatures will flow into parts of Southern Ontario during the early morning hours of Friday. This will bring the risk of widespread frost throughout Central, Eastern and higher elevations of Southwestern Ontario. Temperatures will moderate somewhat after sunrise so the frost risk should come to an end by mid-morning. Keep in mind this is only a preliminary map so the frost risk may extend outside of the zone as outlined on the map - we will have an updated map issued by Thursday evening if there’s any change in the data.

Northern Ontario: Fall Weather Hazards Outlook for Thursday, October 21, 2021

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Forecast Discussion

UPDATE (Wed. PM): There are no changes to our initial forecast although the rain/snow mix for Northeastern Ontario doesn’t appear as widespread in the latest data so locations like Sudbury and Elliot Lake might not see any snow.

Scattered flurries over regions near Hudson Bay will spread further south on Thursday to include locations like Sandy Lake, Fort Hope, Kapuskasing and Cochrane. Limited accumulation is expected with at most a few centimetres of accumulation. In addition to this, a system will track through the Great Lakes bringing heavy precipitation mostly in the form of rain. However, during the early morning hours on Thursday, there might be some snow that mixes in with the rain on the far northern edge of the system as it affects the Elliot Lake, Temiskaming Shores and Sudbury area. With temperatures near the freezing mark, we don’t expect any significant accumulation although it could result in some slushy road conditions.

Northern Ontario: Fall Weather Hazards Outlook for Thursday, October 21, 2021

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Forecast Discussion

UPDATE (Wed. PM): There are no changes to our initial forecast although the rain/snow mix for Northeastern Ontario doesn’t appear as widespread in the latest data so locations like Sudbury and Elliot Lake might not see any snow.

Scattered flurries over regions near Hudson Bay will spread further south on Thursday to include locations like Sandy Lake, Fort Hope, Kapuskasing and Cochrane. Limited accumulation is expected with at most a few centimetres of accumulation. In addition to this, a system will track through the Great Lakes bringing heavy precipitation mostly in the form of rain. However, during the early morning hours on Thursday, there might be some snow that mixes in with the rain on the far northern edge of the system as it affects the Elliot Lake, Temiskaming Shores and Sudbury area. With temperatures near the freezing mark, we don’t expect any significant accumulation although it could result in some slushy road conditions.

Southern Ontario: Fall Weather Hazards Outlook for Thursday, October 21, 2021

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Forecast Discussion

Moderate to heavy rainfall will work its way across Southern Ontario during the day on Thursday with embedded non-severe thunderstorms. The heaviest rainfall totals will be found through the Bruce Penisula and northeast of Georgian Bay where between 20-40mm of rain is possible. The rest of Southern Ontario can expect between 5-15mm of rain with higher amounts closer to Georgian Bay and Lake Huron. Rain will taper off by late Thursday evening as the system moves out over Quebec.