How the snow day calculator works

1. Tonight's forecast for your ZIP

Every hour we download the National Weather Service's National Digital Forecast Database: the gridded forecast its offices publish for the whole country at 2.5 km resolution. For each of about 33,600 U.S. ZIP codes we read the forecast at the ZIP's center point:

• Snow and ice expected between 6 PM and 8 AM (the closure window), plus anything falling during the school day.
• The lowest wind chill (apparent temperature) between 5 and 8 AM, when buses run.
• Active NWS winter warnings, watches and advisories for your county.

The NWS publishes snow and ice as 6-hour totals. The "snow by hour" chart spreads each total evenly over its six hours, so treat it as the shape of the storm, not an hour-by-hour promise.

2. Your local snow normal

Three inches of snow closes schools in Atlanta and barely slows Buffalo. So each forecast is compared with what is normal where you live: the NOAA 1991–2020 climate normals from the closest weather station at a similar elevation, especially the number of days a year with 3 or more inches of snow. From that we set a local "closing line":

The closing line is our model's estimate, not a district policy. Each district sets its own rules.

Ice gets its own, much lower line (about 0.12–0.26 in. of ice accretion), because ice closes roads almost everywhere. Wind chill is compared with a cold line set from your area's normal winter low: about −33°F in Minneapolis, −22°F in Chicago, −24°F in Buffalo, −20°F in New York or Atlanta.

3. From weather to a chance

Each hazard is measured against its local line (1.0 means "at the line"). The biggest hazard counts in full, and the others add 35% of theirs, so a storm with snow and ice scores higher than either alone. That combined number goes through a logistic curve, the standard way to turn a score into a probability:

• At the closing line with no NWS alert, the chance of a closure is about 60%.
• A Winter Storm Warning, Ice Storm Warning or Blizzard Warning raises it; an advisory raises it a little.
• Snow that falls during the school day counts at 40%: it leads to early dismissals more than closures.
• The chance of a delay uses lower lines (about half the snow) and is never below the closure chance.
• Weekends, Thanksgiving and the day after, Christmas Day and New Year's Day show "no school". Winter break, MLK Day and Presidents' Day are scored with a note, because calendars differ.

We cap every chance at 95%. Districts sometimes stay open in bad weather and sometimes close for reasons a forecast can't see (power outages, staff, road crews).

What weighs the most

In a typical storm night, snow amount drives most of the result, then ice, then wind chill, with timing adjusting snow and ice and school type nudging the end result. We don't publish fixed percentage weights, because the factors aren't simply added up: any one hazard at its local line can close schools on its own.

Adjusting for your school

The forecast assumes a typical public school district. Under "Adjust for your school" on the result card you can change that, and the chance moves on the same curve:

• College: lower, since colleges close much less often than K–12 schools (a 60% night becomes about 40%).
• Private K–12: slightly higher (60% becomes about 66%).
• District closes easily / rarely closes: up or down (60% becomes about 73% or 45%).
• Snow days used: each one already used lowers the chance a little, up to five (60% becomes about 48%), because districts must make the days up.

These are rough settings, not measured from data yet, and a 0% night stays 0%. Your choices stay in your browser. To test a storm of your own, use the manual snow day calculator.

4. Updated every hour, all night

From mid-October to mid-April the forecast is refreshed every hour from 2 PM to 9 AM Eastern, and every three hours the rest of the day. Before 9 AM local time the calculator shows today; after that, tomorrow. Beyond three days the NWS doesn't forecast snow amounts, so days 4 and 5 are an outlook from the chance of precipitation and temperature, capped at 50% and marked "low confidence".

Known limits

• Your district decides. We have no inside information and never report official closings.
• ZIP codes are matched to districts by area (NCES boundaries). Where several districts serve a ZIP, pick yours from the list.
• Forecasts change. A storm that shifts 30 miles can change the answer completely.
• The model is new. We will check it against parent reports this winter and publish the results on the accuracy page.

Sources

1. National Weather Service, National Digital Forecast Database (NDFD) and active alerts API.
2. NOAA National Centers for Environmental Information, U.S. Climate Normals 1991–2020 (annual/seasonal).
3. National Center for Education Statistics: EDGE School District Geographic Relationship Files (GRF25) and Common Core of Data LEA directory, 2024–25.
4. U.S. Census Bureau: 2024 ZCTA, county and place gazetteers; Vintage 2024 population estimates.
5. Terrain elevation: AWS Open Data Terrain Tiles (for matching weather stations at a similar height).

Independent; not affiliated with the National Weather Service, NOAA or any school district.

Changelog

• October 2026: first version of the model; optional school adjustments and the manual calculator added.