Mean-level removal reduced changes in three-day high-water runs at six US gauges between 1960-1989 and 1996-2025

We tested changes in days in runs of at least three high-water days at six US gauges. We used fixed NOAA daily hourly maxima and monthly mean sea levels in 1960-1989 and 1996-2025. Above early 95th-percentile thresholds, observed run days rose by 8.15 to 55.05 days per year. Four observed intervals excluded zero after adjustment for 18 site-by-measure comparisons. After annual mean removal, changes ranged from -3.05 to +3.22 days per year. After monthly mean removal, they ranged from -3.41 to +1.27. All filtered primary family intervals included zero. Separate era thresholds also left changes uncertain at all sites. Five-year block intervals are conditional on fitted thresholds and mean corrections. A ten-year block check gave one positive annual-filter result. The findings do not identify a physical cause or continuous flood duration. The package gives source hashes, daily evidence, code, uncertainty weights, figures, and numerical checks.

sea level · high-water runs · NOAA CO-OPS · mean level removal · block bootstrap