Snow Day Calculator
Predict the probability of a school closure tomorrow from forecast snowfall, overnight low, wind speed, and where you live. Treat the result as a guide, not a forecast.
Snow day chance
0.52%
- Verdict
- Coin flip
- Base probability
- 0.05%
- Forecast snowfall
- +30%
- Temperature
- +12%
- Wind speed
- +5%
- Region
- +0%
- Time of season
- +0%
Heuristic estimate. Each factor adds or subtracts percentage points from a 5% base, then the total is clamped to 0–99%. There is no official formula — districts decide closures using their own rules, so treat this as a rough guide built from publicly documented weather signals.
How to use this calculator
Pull the latest forecast for your area from the National Weather Service, BBC Weather, or any local TV station. Enter the predicted snowfall in inches, the overnight low in °F, and the expected wind speed in mph. Pick the region that best matches where you live — rural areas with long bus routes close more readily than tightly-plowed urban districts. Pick the season phase: districts are quicker to close in early November when crews are still rusty and slower in March when they have a season of practice behind them. The calculator returns a probability and a plain-English verdict, plus a breakdown showing exactly how each factor contributed.
How the calculation works
There is no official "snow day formula". Districts decide closures using a mix of local rules, plow status, road temperatures, bus operator availability, and the superintendent's read of the situation. The calculator combines the same publicly documented weather signals that those decisions weigh: snowfall, temperature, wind, district type, and time of season. Each signal contributes a number of percentage points to a 5% base. Snowfall adds 5 points per inch up to a 12-inch cap. Temperature adds 18 points when the overnight low is between 5°F and 17°F (cold enough that snow sticks but not so cold that schools fear frostbite policies more than closure), drops to 5 points in the 26–32°F band, and subtracts 15 points above 35°F (rain, not snow). Wind contributes 0.4 points per mph up to 40 mph, reflecting drifting and visibility risk. Region adds 10 for rural, subtracts 8 for urban. Season adds 8 for early, 4 for late, 0 for peak. The sum is clamped to 0–99%.
Worked example
A 6-inch overnight snowfall with a 22°F low, 12 mph winds, in a suburban district in mid-January. Base: 5%. Snowfall: 6 × 5 = +30%. Temperature (18–25°F band): +12%. Wind: round(12 × 0.4) = +5%. Region (suburban): 0. Season (peak): 0. Total: 5 + 30 + 12 + 5 + 0 + 0 = 52% — a coin flip. Compare against the same storm in a rural district in early November: add 10 for rural and 8 for early season, total 70% — likely closure. Same weather, different community calendars.
Frequently asked questions
Is this an official formula?
No. There is no national or international standard for predicting school closures. Every district decides on its own, weighing forecast severity against plowing capacity, road conditions, bus operator availability, and a handful of other local factors. This calculator combines the same publicly documented weather signals that those decisions tend to weigh, with weights tuned to match the qualitative behaviour described in the NWS Winter Storm Severity Index. It is a guide, not a forecast.
Why does the calculator use Fahrenheit?
Snow day calculators are most commonly used in the US, where weather forecasts are in Fahrenheit. The temperature bands (5°F, 18°F, 26°F, 32°F, 35°F) correspond to physically meaningful thresholds: 32°F is the freezing point of water (above it, snow turns to rain and roads stay clear), 18°F is roughly where road salt stops working effectively, and 5°F is the rough threshold where extreme cold becomes the dominant safety concern rather than the snow itself. If you forecast in Celsius, the rough equivalents are −15°C, −8°C, −3°C, 0°C, and 2°C respectively.
Why does rural add more probability than urban?
Rural districts have long, winding bus routes that take a long time to plow and salt. A single un-plowed mile can isolate dozens of pickup points. Urban districts have short routes, dense plow coverage, and most students within walking distance — so the same 6-inch snowfall causes far less disruption. Suburban districts sit in the middle: enough plow capacity to handle moderate storms, but enough route length to be vulnerable to heavy ones.
Why does early season add more than peak winter?
Plow crews, salt stockpiles, and bus drivers all hit their stride after a few weeks of practice. The first snowstorm of November often catches districts before they have done their seasonal equipment check, before drivers have re-learned route timings on icy roads, and before students have winter routines. The same 6-inch storm in late January, when crews have had a dozen rehearsals, is much less likely to cause a closure. Late season adds a smaller bump because spring snowstorms often arrive on top of saturated ground, but crews are still warmed up.
Why is the probability capped at 99%?
A 100% prediction is overconfident — there is always a non-zero chance the storm tracks elsewhere, the forecast is wrong by 6 inches, or the district simply makes an unusual call. Capping at 99% reflects that no weather-only model can be certain about a human decision. The cap kicks in for extreme inputs like a 12-inch forecast at 10°F with 30 mph winds in a rural early-season district — at that point the verdict is "very likely closure" and the precise probability matters less than the action.
Does the calculator work outside the US?
The factors apply anywhere it snows, but the calibration is tuned to North American school district behaviour. UK schools tend to close at lower snowfall thresholds than US schools because UK plow coverage is much sparser; the same 4-inch snowfall that barely registers in Massachusetts often closes schools across southern England. If you are using the calculator for a non-US district, treat the output as a relative guide — higher numbers mean higher closure chance — rather than a literal probability.