How the snow day calculator works
The calculator turns a weather forecast into two numbers: the chance that school is closed and the chance of a late start instead. This page gives the whole method, including every threshold, so you can judge how far to trust it.
What it is and is not
It is a set of rules written down by people, applied to the official forecast. It is not a model trained on thousands of past closures. The rules have had one check against real closings, in 17 US districts over two winters; what that found is on the accuracy page. Read 70% as "the forecast looks like the mornings when schools usually close."
Your superintendent decides, usually before dawn, with information no forecast holds: which roads are plowed, whether buses start, whether buildings have heat and power. Only the district's own announcement counts.
Where the forecast comes from
- Your ZIP code is matched to the town its post office serves, using the GeoNames postal code file. That holds for a ZIP that is only PO boxes too. The few ZIP codes the file does not place on a town are matched to the middle of their area, from the US Census Bureau's 2025 ZIP Code Tabulation Area gazetteer.
- Your browser then asks the US National Weather Service API for its grid forecast at that point: a square about 1.5 miles across, hour by hour, for about a week ahead. It covers the 50 states, Puerto Rico and Guam.
- A city is matched by name to the center of the town, using the Census Bureau's 2025 places gazetteer. Where several states have a town of that name, the calculator asks for the state.
- A public school is matched by name to its own building, using the National Center for Education Statistics' list of public school locations for 2023-24. Where several schools share a name, the calculator asks for the town. Private schools and colleges are not in the list.
- The forecast is fetched fresh each time you check. The result shows when the weather service issued it.
- If the weather service does not answer, which happens most in big storms, the calculator falls back on the Open-Meteo forecast for the same point and says so under the sign. That forecast carries no weather alerts, so the Alerts row stays empty.
Canada
- In a city, the first three characters of a Canadian postal code name an area a few kilometres across, and the forecast is for the middle of it. Where those three characters cover towns that are far apart, as in every rural area, the calculator asks for the full code and uses the town that code belongs to. Both come from the GeoNames postal code file.
- The forecast comes from Open-Meteo, which builds on Environment Canada's weather models. It includes the previous day, so snow that has just fallen is counted.
- Winter alerts come from Environment Canada's public alerts service: a blizzard warning counts like the US one, a freezing rain warning like an ice storm warning, a snowfall, winter storm or snow squall warning like a winter storm warning.
- Under the sign, the calculator names the school boards that serve the postal code and links to each one, in Ontario, Quebec, British Columbia, Alberta and Manitoba. The boards are found by placing the postal code's point on the provinces' published board boundaries, so a rural area that straddles two boards shows the one at its point. In much of Ontario, buses are cancelled by a regional transportation consortium rather than by a board; where we know its page, that link comes first. These links are where the decision is announced. The calculator does not predict bus cancellations separately.
- One thing is weaker than for the US. There is no ice-accumulation figure, so rain in an hour forecast as freezing rain is counted as ice, which can overstate it.
- Results read in centimetres, °C and km/h. The thresholds below are the same, converted.
The six things it reads
Each one adds to a score. A score of 1 means even odds of a closure.
Snow, and when it falls
An inch of snow counts differently depending on the hour. Snow that stops early has been plowed by the first bus run; snow falling overnight has not.
| When it falls | How much of each inch counts |
|---|---|
| 6 a.m. to 6 p.m. the day before | 25% |
| 6 p.m. to midnight | 60% |
| Midnight to 8 a.m. | 100% |
| 8 a.m. to 3 p.m. | 70% |
The bars on the sign show the snow hour by hour from 6 p.m. to 3 p.m., with the midnight to 8 a.m. hours in yellow and a mark at 7 a.m. for the first buses. The weighted inches are divided by your district's closing line: the overnight snowfall at which closing is a coin flip.
How your district handles snow
The same storm closes schools in one place and not another. The calculator starts from your state and lets you change the setting, because a state is a blunt unit: Buffalo and Manhattan share one. Each state page starts from its group. The groups are our judgement from how much snow each state normally gets. They are not measured closure data.
| Setting | Snow | Ice | Wind chill | Starting point for |
|---|---|---|---|---|
| Closes for a dusting | 1 in | 0.05 in | -5°F | AL, AZ, CA, FL, GA, HI, LA, MS, NV, SC, TX |
| Closes for a few inches | 3 in | 0.1 in | -15°F | AR, DC, DE, KY, MD, MO, NC, NM, OK, OR, TN, VA, WA, BC |
| Used to snow | 6 in | 0.15 in | -25°F | CO, CT, IA, ID, IL, IN, KS, MA, NE, NJ, NY, OH, PA, RI, UT, WV, ON, NS |
| Almost never closes | 9 in | 0.2 in | -35°F | AK, ME, MI, MN, MT, ND, NH, SD, VT, WI, WY, AB, MB, NB, NL, NT, NU, PE, QC, SK, YT |
Each figure is the amount that, alone, makes closing a coin flip. States not listed, and the US territories, start at the first row. The same table lists Canadian provinces by their two-letter codes.
Ice
Freezing rain from 6 p.m. the day before to 10 a.m. is added up and divided by the ice figure above. A tenth of an inch of glaze does more to roads than several inches of snow, which is why ice with almost no snow can give a high chance.
Cold
The lowest wind chill between 5 and 9 a.m. is used, when children wait for buses. It starts to count 20 degrees above the figure in the table and reaches a full point at that figure.
Wind
Wind matters when there is snow to move. With at least 2 inches of snow, gusts of 35 mph or more add 0.25 to the score and gusts of 45 mph or more add 0.4, for drifting over cleared roads. Without snow, wind adds nothing.
Early dismissal
When snow or ice is forecast for school hours, 8 a.m. to 3 p.m., a note under the sign gives a rough chance that school opens on time and sends students home early. It is the same scale applied to those hours alone. Districts rarely record early dismissals, so this number has not been tested against anything and is shown only at 10% or more.
Weather service alerts
If a winter alert is in force between midnight and noon on the school day, the strongest one adds to the score. Forecasters issue these by local standards, so an alert already reflects what counts as serious where you live. In the two winters we checked, schools closed on 51 of the 57 mornings with a warning in force, so a warning alone puts the chance well past even. An advisory alone does not.
A storm does not end when its warning does: roads stay icy and power stays out. So the strongest alert of each of the last 3 days still counts, at 0.3 of its weight the day after, 0.09 the day after that, and so on. An alert that only began in the evening counts for the morning it was issued for, not for the day it began on. For the US these earlier alerts come from the National Weather Service's own record of the past week; for Canada only alerts still in the forecast are known.
| Alert | Adds |
|---|---|
| Blizzard Warning | 1.5 |
| Ice Storm Warning | 1.5 |
| Winter Storm Warning | 1.3 |
| Lake Effect Snow Warning | 1.3 |
| Extreme Cold Warning | 1.2 |
| Wind Chill Warning | 1.2 |
| Winter Storm Watch | 0.3 |
| Winter Weather Advisory | 0.45 |
| Cold Weather Advisory | 0.45 |
| Wind Chill Advisory | 0.45 |
| Blizzard Warning | 1.5 |
| Freezing Rain Warning | 1.5 |
| Winter Storm Warning | 1.3 |
| Snowfall Warning | 1.3 |
| Snow Squall Warning | 1.3 |
| Extreme Cold Warning | 1.2 |
| Flash Freeze Warning | 1.2 |
| Arctic Outflow Warning | 1.2 |
| Winter Storm Watch | 0.3 |
| Snow Squall Watch | 0.3 |
| Blowing Snow Advisory | 0.45 |
| Freezing Drizzle Advisory | 0.45 |
From score to percentages
The score goes through an S-shaped curve: about 6% at 0.3, 50% at 1, about 98% at 2. The chance is never shown above 99%, because no forecast is certain. With nothing in the forecast at all, it shows 0%.
A late start needs less than a closure. The chance of any disruption uses the same curve with the score multiplied by 1.7; the chance of a late start is that figure minus the chance of a closure.
| Example, nothing else in the forecast | Closed | Late start |
|---|---|---|
| 6 in overnight, district used to snow | 50% | 44% |
| 6 in ending at midnight, same district | 17% | 35% |
| 3 in overnight, same district | 12% | 23% |
| 2 in overnight, district that closes for a dusting | 98% | 1% |
| 2 in overnight, district that almost never closes | 4% | 4% |
Which day it answers for
- Before 9 a.m. in the school's time zone, the first answer is for this morning. After that, it is for the next school day.
- Saturdays, Sundays, the federal holidays nearly all schools observe, the day after Thanksgiving and December 24 to January 1 are skipped, and the result says which days were skipped.
- Spring breaks, teacher work days and local holidays differ by district and are not known to the calculator.
- You can look two more school days ahead. A confidence line under the sign says how far to trust it: high when the morning is under a day away, medium up to about a day and a half, low beyond that. It reflects the forecast's lead time only, not a measured accuracy.
What it cannot know
- Snow already on the ground. The forecast only looks forward, so a storm that ended before the forecast was issued is not counted.
- Where in town you are. The forecast is for the town your ZIP code belongs to. A school up the valley or on the far side of a large district can get different weather. "Near me" uses the spot you are standing on instead.
- Your district's habits, its remaining snow days, and whether it switches to remote learning instead of closing.
- Road crews, bus fleets, power outages and building problems.
- Forecast error. A storm track that shifts 30 miles overnight changes everything, and the calculator is only as good as the forecast it reads.