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A conditional freezing rain risk is expected to develop across parts of Southern Ontario during the evening on Thursday, with areas of drizzle continuing overnight into Friday morning. The greatest risk for icing currently appears to be around the western Greater Toronto Area and along the northern shoreline of Lake Ontario. However, there remains considerable uncertainty with the temperature forecast, which will ultimately determine how much ice is able to accumulate on surfaces.

If temperatures remain just above freezing, most of the precipitation will fall as plain rain or drizzle with minimal impacts. However, if temperatures dip even slightly below freezing, that drizzle could freeze on contact and produce slick road conditions across untreated surfaces.

In addition to the freezing rain risk, there are also indications that patches of fog could develop across parts of Southwestern Ontario overnight and linger into Friday morning. Fog can significantly reduce visibility on rural roads and highways, which may create hazardous travel conditions for the early morning commute.

When it comes to the potential impact on school buses, the timing of the freezing rain makes this a challenging forecast. Most of the precipitation is expected to taper off around or shortly after midnight for many areas. That would provide several hours for road crews to treat and clear surfaces before buses begin operating Friday morning.

However, localized icy patches could still linger, particularly in rural areas with untreated backroads. In addition, freezing drizzle is notoriously difficult for models to capture. Even after the main precipitation ends, light freezing drizzle could continue into the early morning hours, creating new icy spots that are difficult to anticipate in advance.

Because of these uncertainties, the highest probability for bus cancellations is focused on the more rural school boards north of Lake Ontario, where road conditions can take longer to improve.

At this time, we have assigned a toss-up probability of 50 percent for Northumberland County under the Kawartha Pine Ridge District School Board, as well as Belleville, Prince Edward County, South Lennox and Addington, and Kingston under Tri-Board Student Transportation Services. These areas are positioned close to where temperatures may hover near the freezing mark overnight, increasing the potential for icy conditions to develop on untreated roads.

A slight chance of 25 percent extends into surrounding regions, including Clarington and Peterborough County under the Kawartha Pine Ridge District School Board, and Central Hastings, Central Lennox and Addington, and South Frontenac under Tri-Board Student Transportation Services.

We have also assigned a slight chance for several regions within the Golden Horseshoe where some freezing rain may occur this evening. This includes North Niagara under the Niagara Region District School Board, Hamilton under the Hamilton-Wentworth District School Board, northern Halton under the Halton District School Board, Wellington County under the Upper Grand District School Board, and North Peel under the Peel District School Board.

These areas may experience some icing overnight if temperatures drop enough. However, their more urban nature, combined with the fact that precipitation is expected to end earlier in the night, makes it uncertain whether conditions will still be severe enough by Friday morning to warrant cancellations.

Because fog may also become an issue overnight in parts of Southwestern Ontario, we have included a separate slight chance for fog-related transportation disruptions. The highest probability we assign for fog events is 25 percent due to the localized nature of fog development.

This fog-related zone includes Oxford, Elgin and Middlesex counties under the Thames Valley District School Board, all regions within the Avon Maitland District School Board, Lambton County and Chatham Kent under the Lambton Kent District School Board, and Essex County under the Greater Essex County District School Board. If dense fog develops in these areas overnight, some rural transportation providers may choose to cancel or delay buses due to reduced visibility.

Across the remainder of Southwestern Ontario, Central Ontario, Eastern Ontario, and the rest of the Golden Horseshoe, we have assigned a widespread low to very low chance of cancellations.

While most areas in this zone are expected to have a normal school day on Friday, a few surprise cancellations cannot be completely ruled out. Localized freezing drizzle overnight, combined with falling temperatures, could allow earlier rainfall to freeze on road surfaces, producing isolated patches of black ice by the morning commute.

Ultimately, conditions will vary significantly from one community to another, and any decisions will come down to local road conditions early Friday morning.

Disclaimer: Instant Weather has zero authority when it comes to bus and school closures.

It is completely up to the school boards, bus companies, local authorities, and parents to decide what is best for their children. This is our best guess based on our forecast.