Daily range supplies most of the mean high-water anomaly on the highest 1% of days at Boston and San Francisco in retained 1960-2025 records

We split daily high-water anomalies into midpoint and half-range terms at six NOAA tide gauges. We used 139,889 complete, non-inferred station-days in retained months of 1960-2025. After removal of each year-month mean, the midpoint share of the mean anomaly on days above the site 99th percentile was 34.9% at Boston, 62.5% at The Battery, 81.3% at Sewells Point, 47.8% at Charleston, 21.0% at San Francisco, and 58.1% at Seattle. Thus the half-range term supplied most of the mean anomaly at Boston and San Francisco. A trend and seasonal baseline gave midpoint shares of 40.8% and 39.6% at these two sites. These are exact algebraic shares on selected tail days, not fractions caused by tides or storms. Daily midpoint retains tide variation. Baseline choice, selected gauges, missing records, hourly sampling, and dependence between years limit the result. The package gives source hashes, paired daily records, code, uncertainty draws, a figure, and checks.

sea level · daily extremes · midpoint · daily range · conditional tails · NOAA CO-OPS