The period fell

A follow-up to Forerunners. For two days I watched the swell period at buoy 46026 fall, the way dispersion says it should. Here is the frequency carrying the most swell energy, read from the buoy’s raw spectra:

UTC	peak	period
Oct 12, ~11–14h	0.053 Hz	~19 s
Oct 12 18h – Oct 13 01h	0.058 Hz	~17 s
Oct 13, ~02–10h	0.063 Hz	~16 s
Oct 13, ~11–20h	0.068 Hz	~15 s

The steps are even, which is what a single distant source should produce. The arriving frequency rises in a straight line, f = g(t − t₀)/(4πD). So the slope gives the distance, and where the line meets zero gives the time the waves were made.

Fitting all 61 readings since midday on the 12th gives about 6,000 km (roughly 5,200–7,150), with the waves made around 18:00 UTC on October 7. Starting the fit earlier gives nonsense, because the week’s older long-period swell is still in the water. The first number I published came from a single bin step and was partly lucky. This one rests on the whole ridge.

The check: forecasts had Typhoon Choi-Wan curving northeast past northern Japan on the 7th and turning extratropical by the 8th, on its way toward the western Aleutians. A point 6,000 km from the buoy lies around 46–47°N, 160–163°E, which is on that path at about that time. I haven’t found the storm’s final best-track positions, so that’s agreement with the reported track, not proof.

What I got wrong: I predicted the swell would reach 4 m. It never passed about 2. I’d trusted the more dramatic forecast. The physics was the reliable part. My guess about size wasn’t.

Sources: NDBC 46026 spectral data; Weathernews Choi-Wan forecast, Oct 5; stormsurf.com, Oct 10.