Snow day predictor

What the number means

You type in a ZIP code. The site gives you two numbers. The first is the chance that a typical public school district in that ZIP closes for the next school day. The second is the chance of at least a delay. A delay means school starts late or the schedule changes.

The number is an estimate, not a promise. It is based on one model that treats every district in a state the same way. Real districts look at road crews, bus routes, staff and local rules. They decide. You should check your district's own website, app or phone line for the final answer.

The model is published in full at how it works. You can read every rule and every number behind the result.

Where the forecast comes from

The forecast comes from the National Weather Service public API at api.weather.gov. For a ZIP code, the site looks up the ZIP's map centre from the US Census Bureau 2020 ZCTA gazetteer. That file lists 33,144 ZIP areas. The site then asks the NWS for the forecast grid that covers that point.

It reads the quantitative forecast. That means 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.

Some ZIP codes are PO boxes and have no map area. For those, the site uses the nearest numbered ZIP in the same area. The page tells you when that happens.

What window the model looks at

The target day is the next school day, Monday to 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.

Snow and ice are totalled from 3 pm on the day before the target through 9 am on the target day. Snow that falls between 4 am and 9 am counts as falling during the commute. The model also uses the lowest temperature and wind chill in the window, plus the highest wind speed.

What pushes the number up and down

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. A small amount shuts down schools in Georgia, Florida, Alabama, Mississippi, Louisiana and South Carolina. Their closing point is 0.5 inch. Texas is 0.75 inch. North Carolina, Tennessee and Arizona are 1 inch. Virginia, Kentucky and California are 1.5 inches. Maryland, Washington and Nevada are 2 inches. New Jersey, Missouri and Kansas are 3.5 inches. Pennsylvania, Ohio, Indiana, Illinois, Colorado, Utah, Idaho and Nebraska are 4 inches. Connecticut and Rhode Island are 5.5 inches. New York, Massachusetts, Michigan and Wyoming are 6 inches. New Hampshire, Vermont, Maine, Minnesota, North Dakota and Montana are 7 inches. Alaska is 10 inches. Any state not listed uses 4 inches.

The chance of closing rises along an S-curve centred on the closing point. It is about 15 percent at half the closing amount, 50 percent at the closing amount, and about 97 percent at double it. Zero snow gives zero. The chance of at least a delay uses the same curve, centred 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 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 check a ZIP in Ohio. The forecast shows 2 inches of snow in the window. Ohio's closing point is 4 inches. Two inches is half the closing point, so the S-curve gives about 15 percent. The chance of precipitation is 80 percent, so no discount applies. The snow falls between 4 am and 9 am, so the commute rule adds 15 percent of 15, which is about 2 points. The total is about 17 percent. That lands in the "Very unlikely" band. The delay chance uses a curve centred at 45 percent of 4 inches, which is 1.8 inches. At 2 inches, the delay chance is a bit above 50 percent.

Verdict labels

The site turns the number into a short label:

The district decides. This tool does not. Check your district's own channels before you plan your morning.