How accurate is the snow day calculator?
Short answer: it is only as accurate as the weather forecast, and it is most reliable the evening before. Below is the honest detail, then every step of the method, so you can judge the number yourself. Want to try it first? Use the snow day calculator.
The honest answer
Honestly: no snow day calculator is fully accurate, including this one. The biggest source of error is the forecast itself. A storm that shifts 30 miles can change everything. Results are most useful the evening before, when the forecast for the next morning is quite reliable. More than two days ahead, treat them as a rough idea.
Some sites claim very high accuracy. We do not, because we have not measured it yet. When we have a full winter of results to compare with real closures, we will publish them here.
What we do not know
- Your district's own rules and how many snow days it has left.
- Road conditions, power cuts and building problems.
- Decisions already announced. Always check your district first.
How the estimate is made
No magic and no secret formula. These are the four steps behind every result.
1. Your location
A ZIP code is turned into a map point using the US Census Bureau's public ZIP code (ZCTA) list. A city name uses the Census list of places. Nothing is sent anywhere for this step.
2. The forecast
We ask the National Weather Service (NWS) for the detailed forecast at that point (its 2.5 km forecast grid). From it we take, for the school day you pick:
- snow expected from 6 pm the evening before until 8 am,
- snow between 4 and 8 am, when buses and roads have to be ready,
- snow during the school day (8 am to 3 pm),
- ice from freezing rain,
- the lowest wind chill before 8 am and the strongest wind gusts,
- active NWS winter alerts for your forecast zone (for example a Winter Storm Warning).
The forecast is refreshed at most every 30 minutes, and alerts every 5 minutes.
3. How used to snow is your area?
The same storm means very different things in different places. We group states into four levels. Places above 1,800 m (about 5,900 ft) move up one level, because mountain towns handle snow better.
| Level | Snow that gives about a 50% closure chance | Ice |
|---|---|---|
| Snow is rare here | 1 in | 0.03 in |
| Some snow most winters | 2.5 in | 0.06 in |
| Snow every winter | 5 in | 0.1 in |
| Snow belt: schools are well prepared | 7.5 in | 0.15 in |
This is a simple, state-level rule. A snowy corner of a mild state (or the opposite) can be off.
4. The score
Each factor gives its own chance on a smooth curve: a little snow adds a little, and the chance climbs fast near your area's typical closing amount. Snow during the morning commute counts extra; snow during the school day counts half. Strong wind adds to snow, because it drifts back onto cleared roads. Very low wind chill closes schools on its own in cold states. NWS warnings add weight because forecasters only issue them for serious weather.
The chances are then combined, so two medium risks (say, some snow and some ice) add up to more than either alone. Last, we adjust for the school type:
- Public school, rural: × 1.15
- Public school, suburban: no change
- Public school, city: × 0.9
- Private school: × 0.9
- College or university: × 0.55
The delay chance uses the same factors with lower limits (a dusting at 6 am often means a 2-hour delay, not a closure). We show only the part of it that is not already a closure.
We cap every result at 95%. Forecasts change, and districts also weigh things we cannot see, such as staff, buses and how many snow days they have left.
Ready to check your area? Enter your ZIP code in the snow day calculator.