How the Snow Day Calculator Works
School closings are decided by people, usually a superintendent checking roads before dawn, so no calculator can know the answer. Ours turns the forecast into a rough chance using a few simple rules, and shows all of them here.
1. The forecast
Your ZIP code is matched to its center point using the US Census Bureau's ZIP Code Tabulation Areas, or your browser shares your location. We then ask the National Weather Service's public forecast service (api.weather.gov) for that point and add up, for the school day you pick:
- snow from 6 pm the evening before to 6 am (overnight),
- snow from 6 am to 9 am (the morning commute),
- snow from 9 am to 3 pm (the school day),
- ice accumulation before 9 am,
- the lowest wind chill and the strongest gust before 9 am.
Your request goes from your browser straight to the Weather Service; we don't store your ZIP code or location.
2. Effective snow
Effective snow = overnight snow + morning snow + 0.5 × daytime snow + 20 × ice. Snow that falls after buses have run is less likely to close school (it's more likely to cause an early dismissal), and ice is weighted heavily because even a tenth of an inch makes roads treacherous. If gusts reach 35 mph with at least an inch of snow, effective snow is multiplied by 1.25 for drifting.
3. Your kind of area
Places that rarely see snow have fewer plows and less practice, so they close for less. Each area type has a closing amount where our estimate is 50%:
| Area | 50% at |
|---|---|
| Rarely gets snow (e.g. the South, the Pacific Northwest lowlands) | 1.5 in |
| Gets some snow (e.g. the Mid-Atlantic, the lower Midwest) | 3.5 in |
| Snowy every winter (e.g. New England, the Great Lakes, the Rockies) | 6 in |
The chance rises along an S-shaped curve: about 12% at half the closing amount, 50% at the closing amount, and about 88% at one and a half times it. When you use a ZIP code, we pre-select the area type from latitude (north of 40° snowy, 36–40° some snow, south of 36° rare). That's only a starting guess, so change it if it doesn't fit.
4. Adjustments
- Extreme cold: wind chill at or below −10°F adds a 5% chance of a cold day, −20°F adds 15%, and −30°F adds 35%.
- School type: Public K–12 ×1, Private K–12 ×0.9, College / university ×0.6.
- Snow days used: each one lowers the chance by 4%, up to 5 days, since districts get stingier once makeup days pile up.
- Delays: the delayed-start chance peaks when effective snow is around 60% of the closing amount, the classic “two-hour delay” storm.
Why it's just for fun
These weights are our own rules of thumb, not a model trained on closing records. Every district has its own policy, roads, and bus routes, and forecasts change. Treat the percentage as a friendly guess and always confirm with your school district.