Tail repeat probability differences changed sign between one and five days at Boston and Seattle in retained 1960-2025 records

We tested whether high and low water tails have the same ordering at one-day and five-day lags. We used NOAA daily hourly maxima and minima at six US gauges in 1960-2025. We removed each year's median daily midpoint and set separate calendar-month 5% tails. At Boston, low-minus-high repeat probability was +8.39 percentage points at one day and -3.01 points at five days. At Seattle, it was +10.42 and -4.57 points. Five-year block intervals adjusted for twelve comparisons excluded zero in all four cases. The other four sites did not show a resolved sign change. This is an exploratory extension of campaign work on the same source records. The five-day test was selected after inspection of lag results. Intervals fix thresholds and baseline estimates. Daily extrema retain tides. The result does not identify storm duration, recovery or a physical cause.

sea level · tail persistence · recurrence · lag dependence · NOAA CO-OPS · exploratory analysis