What the map shows
Each dot on the map is one place. The colour and size of the dot show the modelled chance that a typical public school district there will close for the next school day. The chance comes from the same model used by the ZIP code predictor on the home page. That model is described in full on /how-it-works/.
The map covers 260 places across all 50 states, DC and Puerto Rico. Every state has at least three places. The map is rebuilt every hour, and the page shows when it was last updated. Under the map, a table lists every figure. You can search the table by place or state.
The map does not know about announced closures. It is a model of the weather. The district makes the decision. Check your district's own channels for the real answer.
How to read the map
Each dot is one place. The colour and size show the modelled chance, in six bands:
- Under 10%: very unlikely
- 10 to 29%: very unlikely
- 30 to 49%: unlikely, maybe a delay
- 50 to 69%: toss-up
- 70 to 84%: likely a snow day
- 85% and over: very likely a snow day
The table below the map gives the exact number for each place. You can search it by place or state. The chance is for a typical public district. Private schools, universities and individual districts decide for themselves and often differ.
Why use the map and the ZIP tool
The map shows the shape of a storm. You can see whether your region is inside it. A single ZIP code answers only for one spot. The ZIP code predictor on the home page gives the number for your town. Use the map to see the big picture, then use the ZIP tool for your own area.
The 260 places are chosen to cover every state. The nearest dot may be some distance away. That is normal. The ZIP code predictor on the home page answers for a specific town.
Outside winter, almost every dot reads under 10 per cent. That is the correct answer, not a fault. The model sees little or no snow in the forecast, so it gives a low chance.
How the model works
The model uses the National Weather Service forecast for each place. It reads snowfall amount, ice accumulation, chance of precipitation, temperature, wind chill and wind speed. It also reads the short text forecast. The forecast covers a window from 3 pm the day before the target school day through 9 am on the target day. Snow between 4 am and 9 am counts as falling during the commute.
Each state has a snow amount at which a typical public district has about a 50 percent chance of closing. This is called the closing point. For example, Georgia, Florida, Alabama, Mississippi, Louisiana and South Carolina use 0.5 inch. Texas uses 0.75 inch. North Carolina, Tennessee and Arizona use 1 inch. New York, Massachusetts, Michigan and Wyoming use 6 inches. Any state not listed uses 4 inches.
The chance of closing rises along an S-curve centred on the closing point. At half the closing amount, the chance is about 15 percent. At the closing amount, it is 50 percent. At double the closing amount, it is about 97 percent. Zero snow gives zero.
Other rules adjust the number:
- If the chance of precipitation is under 40 percent, the snow total is discounted by 40 percent.
- Ice: from 0.05 inch of ice, the closing chance is raised sharply. A quarter inch of ice alone gives roughly a 60 percent closing chance in any state.
- Cold: 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.
- Timing: 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 it by 15 percent of itself.
- Wind: 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. The chance of at least a delay uses the same curve centred at 45 percent of the closing point.
Worked example
Suppose a place in New York has a forecast of 6 inches of snow in the window. The closing point for New York is 6 inches. The S-curve gives a 50 percent chance of closing. If half an inch or more falls between 4 am and 9 am, the chance rises by 15 percent of itself. That adds 7.5 points, bringing the chance to 57.5 percent. If the wind is 30 mph or more with at least an inch of snow, that adds another 10 percent of itself, about 5.75 points. The final chance is about 63 percent. That falls in the toss-up band.
What the map cannot know
The map does not know about holidays. It does not know about announced closures. It does not know if your district has a different threshold. It is an estimate for a typical public district. The district decides. Check your district's own channels for the actual decision.
The forecast comes from the National Weather Service. Boundaries and city points come from Natural Earth, which is public domain. The model is the same one described on /how-it-works/.