What this is. The National Weather Service's snowfall analysis — observed snowfall on a 2.5 km grid, built from surface reports, radar-derived precipitation and model guidance. It is the best gridded record of what fell, not a gauge reading: a 2.5 km cell averages over terrain, so a valley town and the ridge above it can share a number that suits neither exactly. Averages here cover the 18 complete winters since 2008-09 — the length of the archive itself, not a 30-year normal.