What this calculator does
In most of Canada, the weather event that changes your morning is a bus cancellation, not a school closure. The number on this page is the chance of either one. It covers the next school day, Monday to Friday. The window runs from 3 pm the day before through 9 am on the target day. Snow between 4 am and 9 am counts as falling during the commute.
This is an estimate for a typical public school board. Boards decide for themselves, and they often differ. Check your board's own channels for the final call.
Where the forecast comes from
You enter your postal code. The site uses only the first three characters, called the forward sortation area (FSA). There are 1,643 of these areas in Canada. Statistics Canada's 2021 boundary file places each one on the map.
The forecast comes from Environment Canada's GeoMet API. That is the same city page forecast you see on weather.gc.ca. The site picks the city page nearest your FSA's centre and names it on the page. Results refresh at most every 30 minutes.
Snow amounts come from Environment Canada's forecast periods. The night before the target day counts in full. About 35 percent of the target day's total counts as falling before 9 am. When the forecast gives a range like "Amount 5 to 10 cm," the site reads 7.5 cm. "Flurries" reads as 1 cm. "Periods of snow" with no amount reads as 3 cm. "Blizzard" or "heavy snow" reads as 15 cm.
Freezing rain in the text counts as ice: 2.5 mm, or 6 mm if the forecast says significant, heavy or prolonged. Ice pellets or freezing drizzle count as 1.25 mm. Wind chill comes from the forecast field or from the text, such as "Wind chill minus 38."
What moves the number
Each province has a snow amount at which boards have about a 50 percent chance of cancelling buses or closing. These are our judgements, not measured statistics.
| Province or territory | Closing point |
|---|---|
| British Columbia | 5 cm |
| Nova Scotia, Prince Edward Island | 10 cm |
| Ontario, New Brunswick, Newfoundland and Labrador | 15 cm |
| Alberta, Saskatchewan, Manitoba, Quebec | 20 cm |
| Yukon, Northwest Territories, Nunavut | 25 cm |
The chance rises along an S-curve centred on that amount. At half the closing point, the chance is about 15 percent. At the closing point, it is 50 percent. At double, it is about 97 percent. Zero snow gives zero.
Other rules apply on top of the snow total:
- Ice: from 1.25 mm of ice accumulation, the chance rises sharply. About 6 mm of ice alone gives roughly a 60 percent chance in any province.
- Cold: a wind chill of minus 29 C or lower raises the chance. Minus 37 C or lower raises it strongly and puts the delay chance at 70 percent or more.
- Timing: 1.25 cm or more of snow falling between 4 am and 9 am raises the chance by 15 percent of itself. Snow that ends before midnight, with none in the commute, lowers it by 15 percent of itself.
- Wind: 48 km/h or more with at least 2.5 cm of snow raises the chance by 10 percent of itself.
The result is capped at 98 percent. Temperatures show in Celsius, snow in centimetres, wind in km/h.
A worked example
Suppose you live in Ontario. The forecast shows 15 cm of snow in the window. Ontario's closing point is 15 cm, so the base chance is 50 percent.
Now add the other rules. If 2 cm of that snow falls between 4 am and 9 am, the chance rises by 15 percent of itself, to about 58 percent. If the wind chill hits minus 30 C, the chance rises again. If the wind reaches 50 km/h with at least 2.5 cm of snow, it rises once more. The final number lands in the "Likely a snow day" range.
Change one input and the number moves. Snow that ends before midnight, with none in the commute, drops the chance by 15 percent of itself. A forecast with no snow gives zero.
The full model is published at how it works.
What the number cannot know
The site does not know your board's holiday calendar. It targets the next weekday, Monday to Friday. If tomorrow is a holiday, the page may still show a number for a day with no school.
Private schools, universities and individual boards decide for themselves. They often differ from the typical public board this model describes. The number is an estimate, not a promise.
Your board makes the final call. Check its website, app or social channels the night before and again in the morning.