Snow Day Calculator
Tomorrow's forecast, scored against the things that actually close schools — and told you why.
https://calculators.nirajiitr.com/weather/snow-day-calculator
Tomorrow's forecast, from Open-Meteo.
Enter a location to see tomorrow’s odds.
How it works
Five things drive a closure, and they are not equally important. Accumulation matters most, but a forecast of six inches landing after lunch is a very different problem from four inches landing at 3am, because the decision is made before dawn. Each factor scores out of its own ceiling, the five are added to give a number out of 100, and that total is then scaled by how readily your district actually closes — which is the single largest source of variation between two towns getting identical weather.
chance = (snow + ice + wind + timing + lying snow) × district factor- snow
- Forecast accumulation, out of 35 — steepest between 1 and 6 inches
- ice
- Out of 20, peaking just below freezing where meltwater refreezes
- wind
- Out of 15, for blowing snow closing roads that were just cleared
- timing
- Out of 20 — overnight is worst, afternoon barely counts
- lying snow
- Out of 10, for snow already on the ground
- district factor
- 0.55 if yours rarely closes, 1.45 if it often does
Worked example
A Chicago suburb: 5 inches forecast, most of it between midnight and 6am, an overnight low of 29°F, winds gusting to 28 mph, and 2 inches already down. The district closes sometimes.
- 1Snow: 5 inches sits in the steep part of the curve — 23.5 of 35
- 2Ice: a low of 29°F is in the refreeze band — 20 of 20
- 3Wind: 28 mph blowing snow — 6 of 15
- 4Timing: it lands overnight, the worst case — 20 of 20
- 5Lying snow: 2 inches already down — 3 of 10
- 6Total 72.5, scaled by the 1.0 factor for a district that closes sometimes
A 73% chance — high, driven as much by the overnight timing and the refreeze risk as by the five inches themselves.
Frequently asked questions
How accurate is a snow day calculator?
It is an estimate built from a weather forecast, so it inherits every bit of that forecast's uncertainty and adds its own. What it does well is weigh the factors in roughly the way a superintendent does — accumulation, ice, wind and above all timing. What it cannot know is your district's budget, how many drivers called in sick, or whether the neighbouring district has already called it, which in practice is one of the strongest predictors of all.
Why does the time of day matter so much?
Because the decision is usually made between 4am and 5:30am, before buses roll. Snow that falls overnight is already on the road, unploughed, when that call happens — so it counts for the most here. Snow that starts mid-afternoon tends to produce an early dismissal rather than a closure, because by then everyone is already at school. Identical totals at different hours give genuinely different answers.
Why is just below freezing worse than deep cold?
Between about 28°F and 35°F, snow partially melts during the day and refreezes overnight into black ice, which is far more dangerous for buses than snow is. At 10°F the snow stays dry and powdery — miserable, but it ploughs cleanly and does not glaze the road. So the ice score peaks near freezing rather than at the coldest temperatures.
Why does it ask how often my district closes?
Because it is the difference between a 30% answer and an 80% one on identical weather. Six inches in Buffalo is a normal Tuesday; two inches in Atlanta closes everything for two days. Districts that see snow constantly have the ploughs, the salt and the drivers to run through it, and the calculator would be badly wrong for both places if it did not ask.
Where does the forecast come from?
Open-Meteo, which aggregates national weather-service models and is free to use. The calculator reads tomorrow's hourly snowfall, temperature, wind and apparent temperature for your location, then buckets the snowfall by part of day to work out when the bulk of it lands. Forecasts are cached for about half an hour, so checking repeatedly within that window returns the same numbers.
Why does it never say 100%?
Because it genuinely cannot know. The model can tell you a blizzard is coming and that every factor points the same way, but the announcement comes from a person weighing things the model cannot see. Capping at 99% is a way of being honest that this is a probability rather than a result.