Basin ice cover gave an uncertain peak-date gap slope across four Great Lakes in 1973-2026

We tested whether low basin ice cover is linked to less similar peak dates across Superior, Michigan, Huron and Erie. We used 54 winters from a fixed NOAA GLERL table. The response was the mean absolute date gap over six lake pairs. After adjustment for linear year, the slope was -1.14 days per 10 percentage points of basin maximum cover. Its conditional 95% five-year pairs-block interval was -1.98 to +0.012 days. Two-year and eight-year blocks and seven-day dates gave intervals below zero, but chart-era controls and the short daily-chart period did not. The primary test thus gives no clear association under its stated uncertainty model. It does not establish no relation. Shared weather could align peaks in high-ice winters, but this table cannot test that cause. One source, changed chart frequency, unknown peak tie rules, four selected lakes and short records limit inference. The package includes code, source hashes, aggregate results, coefficient draws, an independent audit and a figure. Raw and annual derived records remain external.

Great Lakes · ice peak timing · synchrony · basin ice cover · block bootstrap · uncertainty