What does a snow day percentage mean?
By Peter, editor · Updated October 23, 2026 · How we research
In this guide (7)
The short answer
The percentage is how likely the calculator thinks it is that your district closes school tomorrow, given tonight's National Weather Service forecast for your area. A 70% chance means a closure is the likelier outcome, but on nights that look like this one, schools still open a fair share of the time. A 20% chance means a closure is possible, not expected.
It is an estimate, not a decision. Only your district closes school, and it can look at things the forecast can't see, like how the roads look at 4 AM.
The four chance bands
| Chance | What we call it | How to read it |
|---|---|---|
| 0–29% | Unlikely | Expect a normal school day. |
| 30–59% | Possible | Worth a plan B: check again before bed and in the morning. |
| 60–79% | Likely | A closure or delay is more likely than not. |
| 80–95% | Very likely | Most nights that look like this end in a closure or delay. |
Why the same snow gives different chances
The calculator compares the snow in the forecast with how much winter your area usually gets. Districts that see snow every week have the plows, budgets and habits to open in conditions that would close schools somewhere snow is rare. Here is the closure chance our model gives for overnight snow alone, with no ice, wind chill or alerts:
| Overnight snow | Atlanta | Washington | Boston | Buffalo |
|---|---|---|---|---|
| 2 in. | 95% | 4% | 1% | 1% |
| 4 in. | 95% | 38% | 5% | 3% |
| 6 in. | 95% | 91% | 21% | 8% |
| 8 in. | 95% | 95% | 57% | 23% |
| 12 in. | 95% | 95% | 95% | 77% |
Snow that usually closes schools: Atlanta about 1 in., Washington about 4.5 in., Boston about 8 in., Buffalo about 10.5 in.. From each city's NOAA 1991–2020 snow normals.
Why it never says 100%
The highest chance the calculator shows is 95%. Even in a big storm, a district can open late instead of closing, switch to a remote day, or decide the roads are fine. Saying 100% would turn an estimate into a promise only the district can make.
Closure chance vs. delay chance
The card shows two numbers. The delay chance is the chance school at least starts late (a delay or a closure), so it is never lower than the closure chance. A big gap between them, like 58% delay and 5% closure for 4 inches in Boston, usually means a storm that ends overnight: rough roads at dawn, fine by mid-morning. Two-hour delay calculator.
Why the number changes overnight
The calculator re-reads the National Weather Service forecast every hour overnight. When a storm trends wetter, colder or later, the chance moves with it, and when it has moved the card shows how. Check once in the evening and again before bed rather than trusting the first number you see.
How good are the percentages?
We don't claim an accuracy number yet. The chances come from our model's rules, not from a record of past closings. This winter we compare them with what districts actually do, from parent reports, and publish the results on the accuracy page once enough storm days are checked. The full method is on how it works.
Check your district
See tomorrow's chance of a closure or delay for your ZIP code. Open the calculator, or find your state or city.
Sources
- Snow Day Calculator, how it works (model and thresholds)
- NOAA NCEI, U.S. Climate Normals 1991–2020
- National Weather Service forecast data (NDFD)