How the Snow Day Calculator Works

What the number means

The big percentage is the chance your school is closed on the next school day. It is built to mean what it says: in our tests on past winters, nights given about 30% ended in a snow day about 3 times in 10. Before 7 AM on a weekday it is about that day; otherwise it is about the next school morning, skipping weekends.

Below it you may see the chance of a 2-hour delay or an early dismissal, when those are realistic for your area, and the chances for the next few school days. Those further out are less certain, because the forecast is.

What goes into it

Your forecast

How much snow or ice is coming to your exact location, how cold it gets, and when it arrives. A storm that is over by midnight and one that peaks during the morning bus run are very different nights.

Winter weather alerts

Warnings and advisories from the National Weather Service. Alerts from Environment Canada appear with Canadian predictions.

Where you are

Two inches of snow closes schools across much of the South and barely registers in Minnesota. The same forecast means different things in different places, and the chance reflects how schools near you handle winter.

Your school

Public districts, private schools and colleges make the call differently. Colleges rarely close and almost never delay.

When to check

Each time you check, the chance is worked out fresh from the newest forecast published for your area. The fastest weather models behind those forecasts update every hour, and forecasters revise them whenever a storm calls for it, so the number can move between dinner and breakfast.

The evening before is the best time for a first look: by then the forecast for the next morning is about as settled as it gets. Look again in the morning if a storm is arriving overnight. Districts usually decide the night before when the forecast is clear, and otherwise between about 4:30 and 6:30 AM.

How we know it works

We test every version of the engine against what schools actually did. Run on the last three winters in 54 districts, on nights it gave 80% or higher, school closed 89% of the time (147 of 166), and on days it gave 0%, school was open on 9,987 of 9,989. It missed some too: of 324 snow days, 131 had a chance under 50% the night before. These are tests on past winters, not a record of live predictions.

What it can't know

The prediction only knows the weather. Power outages, a broken boiler, a shortage of bus drivers or a cautious new superintendent can all close a school on a clear day, or keep one open through a storm. Always confirm with your school or district: their website, phone line or official social accounts.