Low-water extremes had more shared dates than high-water extremes at San Francisco and Seattle in 1960-2025

We compared same-date high and low water extremes at six US ports in 1960-2025. We used NOAA hourly-based daily extrema, removed observed monthly mean sea level, and set separate calendar-month 5% tails. At San Francisco and Seattle, overlap was 62.56% for low daily minima and 18.75% for high daily maxima. Overlap is twice the joint count divided by the sum of the two event counts. The low-minus-high difference was 43.81 percentage points. Its conditional three-year block interval, adjusted across all 15 pairs, was 35.56 to 49.98 points. Both record halves and two other tail sizes retained a positive Pacific contrast. An exploratory lag curve had a broad low-water recurrence near 28 days. This is consistent with shared tide cycles, but the data do not separate tides and weather. The Pacific pair was selected after the estimates were seen. The result extends overlapping campaign work; it is not an independent observation set. Fixed thresholds, missing days, six selected sites, local dates, and short-block assumptions limit inference. Code, daily evidence, source hashes, count audits, resampling draws and repeat instructions are supplied.

sea level · spatial extremes · low water · tidal cycles · NOAA CO-OPS · tail asymmetry