How we calculate the chance of a snow day

Snow Day Tomorrow estimates the chance that schools close or open late on the next school day. It is a forecast, not an announcement: only your district can close schools.

1. The forecast

Every few hours we read the National Weather Service's official forecast grids (the National Digital Forecast Database) for every county in the country: how much snow is expected, how much ice, and how cold it will feel in the morning. We also check for active winter storm, ice storm, blizzard, winter weather and extreme cold alerts.

2. The school day that matters

Before 8 a.m. on a school day we look at that day; after that, at the next weekday. Snow that falls between 6 p.m. the evening before and 9 a.m. counts fully, because that is what decides closings. Snow later in the school day counts half, since it more often causes early dismissals than closings.

3. What it takes to close schools where you live

An inch of snow can close schools in Atlanta, while Buffalo or Minneapolis might stay open with six. We adjust for this using each county's NOAA 1991–2020 normals: places that get more snow and have colder winters are assumed to handle more before closing. Ice counts heavily everywhere, and even more where snow is rare. Very low wind chills can close schools on their own, with the threshold set by how cold a place usually gets.

4. The number

We combine the strongest hazard (snow, ice or cold) with the others and any active alerts into two chances: a full snow day, and a delay or closing. When there is no snow, ice or dangerous cold in the forecast, the chance is 0%.

Limits

We work at the county level, so a district in the mountains or near a lake can see very different weather from the rest of its county. We cannot know about school breaks, holidays or a district's own policies, and forecasts change. Treat the number as a guide, and always wait for your district's official word.

Sources

Forecasts and alerts: NOAA National Weather Service. Climate normals: NOAA National Centers for Environmental Information. School districts: U.S. Department of Education, National Center for Education Statistics, Common Core of Data (2022–23), via the Urban Institute Education Data Portal.