Evanston

How the Evanston number is worked out

This is a transparent model, not an announcement. Nobody here knows what Evanston schools will decide - the score estimates how likely a closure is given what the National Weather Service is forecasting, and shows every factor that moved it.

The dominant input is snowfall measured against a local threshold. NOAA’s 1991-2020 climate normals put Evanston at 50 inches of snow in an average year, measured at a station 20.7 miles away, which makes it a regular snow location. Snow several times a winter, well-practised response. Closures happen but need a real storm. So we treat roughly 6 inches in a single event as the point where a normal school day comes under threat.

That scaling is the assumption to argue with first. The snowfall figure is measured; the claim that closure thresholds track it is ours, on the reasoning that plough fleets, bus fleets and driver expectations are all sized to a normal winter. It is why a foot of snow is an ordinary Tuesday in Syracuse and a shutdown in Atlanta.

Ice matters out of proportion to its depth - a tenth of an inch of freezing rain shuts down bus routes that eight inches of powder would not. Wind gets a bump when there is snow to blow around, and severe cold adds a little on its own, since districts close for dangerous wind chill without any snow at all. Weekends score zero, because the weather is beside the point.