Methodology

What Snow Day Chance predicts, the data behind it, how we hold it to account, and where it is known to be weak. For a plain-language tour, see how it works.

What we predict

Snow day
The school or district closes for the day because of weather. In Ontario, where boards usually cancel buses and keep schools open, a bus cancellation counts too.
Delayed start
School opens late, usually by two hours. Shown for US schools and districts, but not for colleges or anywhere in Canada.
Early dismissal
School opens but sends students home early because weather arrives during the day. Early dismissals are rare and seldom recorded, so this chance is a rough guide, not a tested probability.
Next school day
Before 7 AM local time on a weekday the prediction is for that day; otherwise it is for the next weekday. School holidays are not on our calendar, so check your district calendar for those.

Closure and delay chances are probabilities in the plain sense: across many nights shown 30%, about 30% should end in a snow day. That is the standard we test against.

How we hold it to account

Before a new version of the engine goes live, we score it on every school day from November through March of up to 3 past winters in 54 US and Canadian school districts, using only the forecasts and alerts that were published at the time, and compare it with what each district actually did. A district is never scored by a version of the engine built with it.

Quiet days count as much as stormy ones. Calling a snow day on every big storm is easy; staying low on the nights that looked threatening and still had school is the harder part. The results are on the accuracy page.

Known limitations

  • We have no access to any district's internal decision, staffing or superintendent. The prediction reads the weather, not announcements.
  • Forecasts lose skill quickly beyond about 24 hours, so chances two or three school days out are rougher than the one for tomorrow.
  • Temperatures near freezing are the hardest case: a one-degree forecast error is the difference between rain and ice.
  • Closures that continue for days after a storm, while roads stay icy, are the most common miss. Districts decide those one day at a time.
  • Districts sometimes close for reasons unrelated to weather, such as power outages, water main breaks or building problems.
  • Districts differ in how cautious they are, and a new superintendent can change a district's habits from one winter to the next.

Treat the number as a well-informed estimate, and always confirm a closure with your school or district.

Data and credits

  • Forecasts and alerts: the National Weather Service (US) and Environment and Climate Change Canada.
  • Weather data by Open-Meteo.com, adapted, under CC BY 4.0.
  • School and district locations: the National Center for Education Statistics (US) and Statistics Canada.
  • Contains information licensed under the Open Government Licence - Canada.

No accounts and no personal information. See the privacy policy.

Back to Snow Day Calculator