There is no single Illinois snow day threshold, which is why this page scores each town separately. Waukegan averages 43 inches of snow a year against Waukegan’s 43, and a district that plans for the first amount does not close for weather that would shut the second one down.
Each score compares what the National Weather Service is forecasting against that town’s own NOAA 1991–2020 snowfall normal, then adds weight for ice, wind and dangerous cold. Every number on this page links through to the full factor breakdown, because the reasoning is the part worth checking.