What the number means
The number you see is an estimate of the chance that a typical public school district in your area closes for the next school day. It is not a promise. It is a percentage built from the forecast for your ZIP code or postal code.
You also get a separate chance of at least a delay. A delay means school starts late. The delay number is often higher than the closing number because districts call delays more easily than full closures.
The tool looks at the next school day, Monday through Friday. Before 6 am on a weekday, the target is that same day. After 6 am, the target moves to the next weekday. The site does not know about holidays.
Districts decide. This calculator does not. Check your district's own website, phone line, or social media for the official call.
Where the forecast comes from
For US ZIP codes, the forecast comes from the National Weather Service public API at api.weather.gov. The site looks up your ZIP code's map center using the US Census Bureau 2020 ZCTA gazetteer, which covers 33,144 ZIP areas. Then it asks the NWS for the forecast grid covering that point.
For Canadian postal codes, the forecast comes from Environment Canada's GeoMet API at api.weather.gc.ca. That API publishes the same city page forecasts shown on weather.gc.ca for 844 locations. Only the first three characters of your postal code are used. Those characters are called the forward sortation area, or FSA. There are 1,643 FSAs, located with Statistics Canada's 2021 FSA boundary file.
The site reads the quantitative forecast: snowfall amount, ice accumulation, probability of precipitation, temperature, wind chill, and wind speed. Each value comes with the hours it applies to. The site also reads the short text forecast. Results refresh at most every 30 minutes per ZIP code.
PO-box ZIP codes have no map area. For those, the site uses the nearest numbered ZIP in the same area, and the page says so.
What pushes the number up or down
The model totals snow and ice from 3 pm on the day before the target through 9 am on the target day. Snow between 4 am and 9 am counts as falling during the commute. The lowest temperature and wind chill in that window are used, along with the highest wind speed.
Each state has a snow amount at which a typical public district has about a 50 percent chance of closing. The model calls this the closing point. For example, Georgia's closing point is 0.5 inch. New York's is 6 inches. Alaska's is 10 inches. Any state not listed uses 4 inches.
The chance of closing rises along an S-curve centered on the closing point. At half the closing amount, the chance is about 15 percent. At the closing point, it is 50 percent. At double the closing amount, it is about 97 percent. Zero snow gives zero.
The chance of at least a delay uses the same curve, centered at 45 percent of the closing point.
Several things move the number:
- If the chance of precipitation in the window is under 40 percent, the snow total is discounted by 40 percent.
- Ice raises the closing chance sharply from 0.05 inch. A quarter inch of ice alone gives roughly a 60 percent closing chance in any state.
- A wind chill of minus 20 F or lower raises the chance. Minus 35 F or lower raises it strongly and puts the delay chance at 70 percent or more.
- Half an inch or more of snow falling between 4 am and 9 am raises the closing chance by 15 percent of itself.
- Snow that ends before midnight, with none in the commute, lowers the chance by 15 percent of itself.
- Wind of 30 mph or more with at least an inch of snow raises the chance by 10 percent of itself.
The result is capped at 98 percent. It is an estimate for a typical public district. Private schools, universities, and individual districts decide for themselves and often differ.
Worked example
Suppose you live in New York, where the closing point is 6 inches. The forecast shows 3 inches of snow in the window. That is half the closing point, so the base chance is about 15 percent.
Then the forecast adds 1 inch of snow between 4 am and 9 am. That is at least half an inch during the commute, so the chance rises by 15 percent of itself. Fifteen percent plus 15 percent of 15 is about 17 percent.
Now suppose the wind is 35 mph. That is 30 mph or more with at least an inch of snow, so the chance rises by another 10 percent of itself. Seventeen percent plus 10 percent of 17 is about 19 percent.
The verdict label for 19 percent is "Unlikely, maybe a delay." The delay chance uses the same curve centered at 45 percent of 6 inches, which is 2.7 inches. With 3 inches of snow, the delay chance is above 50 percent.
Canada: buses and schools
In most of Canada, the usual weather event is a school bus cancellation rather than a school closure. The number shown for Canada is the chance of buses cancelled or school closed.
Snow amounts come from Environment Canada's forecast periods. The night before the target day counts in full. Thirty-five percent of the target day's total counts as falling before 9 am. Amounts are read from the forecast's accumulation field or from the text. For example, "Amount 5 to 10 cm" is read as 7.5 cm. "Flurries" is read as 1 cm. "Periods of snow" without an amount is read as 3 cm. "Blizzard" or "heavy snow" is read as 15 cm.
Freezing rain in the text counts as ice: 0.1 inch (2.5 mm), or 0.25 inch (6 mm) if described as significant, heavy, or prolonged. Ice pellets or freezing drizzle count as 0.05 inch. Wind chill is taken from the forecast's wind chill field or from the text, such as "Wind chill minus 38."
Provincial closing points are the snow amount at which boards have about a 50 percent chance of cancelling buses or closing. British Columbia is 5 cm. Nova Scotia and Prince Edward Island are 10 cm. Ontario, New Brunswick, and Newfoundland and Labrador are 15 cm. Alberta, Saskatchewan, Manitoba, and Quebec are 20 cm. Yukon, Northwest Territories, and Nunavut are 25 cm. These are our judgements, not measured statistics. The same S-curve, ice, cold, timing, and wind rules apply as in the US.
Temperatures are shown in Celsius, snow in centimetres, and wind in km/h.
For the full model, see how it works.