Snow day calculator

How to use the snow day calculator

Type your ZIP code and press Go. The ring shows the chance school is closed on the morning you picked, and the boxes split that morning into closed, delayed and open on time. The tabs move to the next two mornings. The chart replays the storm hour by hour. Use the sliders to ask what would happen if the storm came later, or brought more snow, or some ice. The forecast comes from the National Weather Service and is refreshed about every 30 minutes.

The Imperial | Metric switch beside the ZIP code shows snow in inches or centimeters, ice in inches or millimeters, and temperatures in °F or °C. The page opens in the system your browser's region uses and remembers your choice. The calculator itself works in inches and °F, as the National Weather Service forecast does: 1 inch is exactly 2.54 cm, and °C = (°F − 32) × 5 ÷ 9. The forecast covers US ZIP codes only.

How the number is made

The calculator turns the forecast into one score, and the score into three chances that add to 100 percent: closed, delayed and open. The score starts low and rises with each risk: snow overnight compared with what is routine where you live, snow in the early-morning decision window, snow still falling when buses roll, ice, dangerous wind chill, and National Weather Service winter alerts. Snow that ended early in the evening lowers it, and so do snow days already used. An ordered scale then turns the score into chances, so more risk can only raise the chance of a closing.

Here is a worked example for a lake-effect night, first morning, with a Buffalo-area ZIP code. Routine snow there is set at 6 inches (about 15 cm).

A score of 4.37 gives 99 percent for a delay or a closing, and 91 percent for a closing alone. That leaves 7 percent for a delay and 1 percent for an on-time open.

Why the hours matter

A foot of snow that ends by 9 PM gives road crews the night. A few inches that start at 4 AM and keep falling through the morning can be worse, because the roads are untreated when buses roll. That is why the calculator reads the hours, not just the total: snow from 9 PM to 7 AM, snow in the 3 to 6 AM window, and how hard it is snowing from 6 to 9 AM. Districts decide at different times, so that window is our assumption, and you can change it in Fine-tune.

Ice and cold

Ice moves the score far more than the same amount of snow, because even a thin glaze makes roads and sidewalks slick. Cold counts when it is dangerous. The calculator uses the National Weather Service wind chill formula and adds risk when the coldest wind chill between midnight and 9 AM falls below about −10°F (−23°C), rising toward −25°F (−32°C).

For example, 0°F (−18°C) with a 15 mph (24 km/h) wind feels like −19°F (−28°C). In metric the page shows the result in °C. Those cutoffs are starter settings, and districts differ.

Lake-effect snow

Near the Great Lakes, snow can fall in narrow bands that sit over one area for hours, so two places a few miles apart can see very different totals. A ZIP code forecast can miss a band, so on a lake-effect night the calculator shows a wider range and says so.

How accurate is it?

We do not print an accuracy percentage, because we have not earned one. The weights are starter settings. After each storm you can tell us, anonymously and without an account, what your school did. We use those reports only to compare what we predicted with what happened, and we will publish the season scorecard here. Until it has enough reports, treat the number as an estimate.

Where the numbers come from

• Forecast. National Weather Service gridpoint forecast data (api.weather.gov): snowfall and ice in inches, temperature, wind and gusts, hour by hour, for the forecast grid cell at the center of your ZIP code.
• Alerts. National Weather Service watches, warnings and advisories in effect at that point, under their current names (Extreme Cold Warning and Cold Weather Advisory replaced the wind chill alerts).
• Wind chill. The National Weather Service formula shown above.
• Routine snow. NOAA 1991–2020 Climate Normals at the nearest weather stations: average snowfall a year and days with an inch or more on the ground. About 60 days of snow cover a year or more counts as a heavy-snow area; under 8 inches a year and under 5 days counts as rare-snow; the rest is moderate. That is why 3 inches (7.6 cm) counts for more in Atlanta than in Buffalo. The cut-offs are starter settings.
• ZIP codes. US Census Bureau ZIP Code Tabulation Areas (2025). A PO box ZIP uses its 3-digit area.
• Checking ourselves. Anonymous reports of what schools did, and afterward the snowfall measured at nearby stations (NOAA GHCN-Daily). Stations can report up to a month late, so the scorecard catches up a few weeks after each storm.
• Winter Storm Severity Index. The Weather Prediction Center publishes it for impacts. It is not a forecast of school closings, so we do not use it as one.
• No code applies. This is weather, not construction.

Tips and common mistakes

• Check again the evening before and early in the morning. Forecasts change, and tomorrow's is more reliable than the third morning's.
• A low number is not a promise. Districts also close for bus, heating or power problems.
• Look at the hours, not only the ring. Snow still falling at 6 AM matters most.
• Follow your school's own announcement. This page is not an official source.

Questions

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Written by a 20+ year homeowner and DIY'er, former repair shop owner, now building AerialQuote in Buffalo, NY.