Four signals, one number, and the reasons behind it.
Every brief shows its work. This page is the long version.
What goes into the number
Each signal is scored on its own, then combined into a chance for each outcome: open, delayed start, closed, or remote day. Your brief lists the factors that moved the number, biggest first.
- The forecast
How much snow, and how sure the models are. In the US that's NOAA's National Blend of Models, which publishes a range rather than one figure, alongside hourly National Weather Service forecasts. In Canada it's Environment and Climate Change Canada. Two to six inches is a different morning from four either way, so the range counts.
- Temperature and ice
Air temperature, overnight and morning wind chill, and forecast ice. They decide whether snow sticks or turns to slush, and ice can close schools with no snow at all.
- Official alerts
Winter warnings from the National Weather Service or Environment and Climate Change Canada. When one is in effect, your brief names it and who issued it.
- Local tolerance
How much snow your state or province is used to handling. A place that averages sixty inches a season shrugs off a storm that shuts one averaging four, so the thresholds move with it. It's why the same forecast gives different answers in Buffalo and Nashville.
The hours that decide it
4–7 AM
Schools don't decide on a day's snowfall total. They decide on what the roads look like when the buses leave, which for most of the US and Canada is somewhere in these three hours. Every forecast is weighed against them.
Snow that finishes by midnight has hours to be plowed. The same amount starting at 5 AM lands right in the window. That's why two forecasts with the same total can get very different numbers.
What you can tell it
Three optional settings on the brief cover what a forecast can't see. Each one shows up as its own factor, so you can see what it changed.
- School type
Public schools, private schools, and colleges close on different thresholds. A campus where students live is not a district busing them forty miles.
- Area type
Rural routes with unplowed back roads close sooner than city districts where most students walk. The forecast can't tell which you are.
- Local pattern
If your school closes at the first flake, or famously never does, you know it and we don't yet. This fills that in until a few seasons of results can.
When a signal is missing
A forecast of no snow is a reading, and it lowers the chance of a closure. A signal that never arrived is a gap, and we don't treat a gap as a zero. It stays out of the number, and when enough is missing to matter, the brief says so in plain words: We're missing tonight's ice-accumulation signal and hourly snowfall signal, so confidence is lower.
Where the method runs out
- It's our first winter
The weights are a starting point, not a measured result, and local tolerance is set per state or province for now rather than per town. Both will change as readers' answers and storm recaps show where the number runs high or low, and every prediction records which version produced it.
- Canadian data is coarser
Environment and Climate Change Canada publishes snowfall per forecast period, not hour by hour, so timing north of the border is less precise.
- We don't know your superintendent
A person makes the call, weighing buses, staffing, and a 4:30 AM report on the roads. We see the same conditions they do. How they weigh them is theirs.
- A gridpoint isn't your street
Forecasts cover a grid a few kilometers across. Lake-effect bands and mountain valleys change faster than that.
Sources and checking
Forecasts come from the National Weather Service (NOAA) and Environment and Climate Change Canada. Both are public, and your brief names the one it used.
The 8 PM and 4 AM calls keep the forecast they were made from, so nothing gets recomputed later. The accuracy page explains how we'll check the number against what happens.