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

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.