Revised 27 September 2026. Re-run with the same scripts and seeds on the storm table rebuilt 19 September 2026 from hurdat2-nepac-1949-2025-091426.txt (the June run used an earlier download). The June values were Pettitt K = 667, CUSUM 268, Mann–Kendall z = −2.68, ΔAIC = −7.68 and a step of −17.5 days; they are now K = 641, CUSUM 260, z = −2.48, ΔAIC = −7.36 and −17.0 days. The linear trend this page compares against is now −2.30 days per decade (was −2.45). Both verdicts are unchanged: 5 of 5 methods on the full record, 1 of 5 (null) on the satellite era.
Pettitt + CUSUM + MK + AIC + permutation flagged a 17-day jump at 1969. Re-run on the satellite-era subset only (1978–2025), 4 of 5 methods turn null. The 1969 jump was the pre-satellite catalog catching up with reality, not the atmosphere shifting.
Five methods, one year.
The first day-of-year that any East Pacific tropical cyclone reaches within 1,500 km of Saladita has been tracked yearly since 1949. The mean across the full record is day 156 — early June. Linear regression over 1949–2025 shows that mean drifting earlier by about 2.3 days per decade. That fit was the existing finding; refit on 1978–2025 alone it is flat (+0.55 days a decade, p=0.72).
Run five different change-point tests and a different shape emerges. The data is fit better by a step than by a slope.
All four p-values reach conventional significance. The independent ΔAIC test prefers a model with one change-point over a continuous linear trend. CUSUM alone clears a Bonferroni correction for 11 cumulative chorus tests run to date.
17 days earlier.
Mean first arrival
Mean first arrival
The two regimes are well-separated. Year-to-year variability is similar before and after; the shift is in the mean. Within each regime, no additional trend reaches significance.
1969 sits at the dawn of routine satellite hurricane surveillance. Operational geostationary coverage of the East Pacific basin was building through 1966–1980; pre-1969, ship and aircraft reconnaissance were the primary detection methods. Short-lived early-season tropical depressions that did not strengthen into named storms — and those that formed outside shipping lanes — would be systematically missed.
A jump in detection capability mimics a jump in real phenology. The first recorded arrival shifts earlier as the catalog catches more early-season weak storms, even if real storm timing did not change. The 17-day discrete shift cannot be confidently attributed to climate without further controls.
What the data shows: a real signal in the catalog. What we don't yet know: how much of it is the atmosphere and how much is the satellites watching it.
A regime jump or a slope.
The previously published linear-trend finding — about 2.3 days per decade earlier over 77 years — implies a total shift of around 17.5 days end-to-end. The piecewise model shows a 17.0-day shift at one moment in 1969. The total magnitudes match almost exactly. The data is compatible with both shapes; AIC prefers the piecewise model by 7.36 units, which is strong but not decisive.
On the full record the two shapes cannot be told apart. The satellite-era retest below separates them: neither survives once the pre-satellite years are removed.
Same tests, satellite years only.
The same five-method ensemble, run only on 1978–2025 (n=48 years), where geostationary catalog completeness is uniform.
Four of the five methods turn fully non-significant once the pre-satellite years are removed. AIC still marginally prefers a piecewise fit — but AIC will almost always prefer the extra parameter on noisy data, and the best piecewise break within the satellite era lands at 2022, three seasons from the end of the record, with a magnitude of −3 days, well within sampling noise (Pettitt’s own best candidate, 1999, has permutation p = 0.92). The hypothesis-testing methods all converge on null.
Conclusion: the satellite-era record shows neither the 1969 jump nor a linear shift (1978–2025: +0.55 days a decade, p=0.72). The 77-year linear trend comes from the same pre-satellite years as the jump.
The 1969 jump in the data was the satellites learning to see early-season storms — not the atmosphere learning to send them sooner.Synthesis · June 2026
A worked example.
The chorus methodology agrees a pattern is real only when multiple statistical methods cross-check each other. Iteration 3 of the pattern hunt produced 5/5 method agreement on a striking 17-day discrete shift. Iteration 4 — a follow-up designed specifically to disambiguate climate from catalog — turned the ensemble verdict to null.
The page stays up. False positives, caught and documented, are part of how science is supposed to look.
Methodology & sources
Data. HURDAT2 East Pacific best-track database (NHC), 1949–2025. First-arrival DOY = earliest day of year on which any tracked tropical cyclone enters 1,500 km of Saladita (17.8375°N, 101.7606°W). See _storm_climatology.js.
Methods. Pettitt 1979 non-parametric change-point statistic with normal-approximation p-value, plus a 5,000-shuffle permutation null. CUSUM (cumulative sum of deviations from the grand mean) with bootstrap null. Mann–Kendall trend test, normal-approximation z. AIC comparison of two-parameter linear regression vs. three-parameter piecewise-constant model (best break selected by minimum SSE). The first four are agreement votes; the fifth is an independent model-selection criterion.
Ensemble verdict. "Real" if ≥3 of 4 voting methods clear p < 0.05 AND ΔAIC favors the structural alternative. All 5 conditions satisfied here.
Reproducibility. python3 scripts/loop_iter3_regime_change.py and scripts/loop_iter4_regime_change_satellite_era.py, re-run 27 September 2026 on hurdat2-nepac-1949-2025-091426.txt. Artifact: _findings_epac_regime_change.js. This finding was generated by chorus-stack iteration 3 of the self-paced pattern hunt; iteration 1 and 2 returned null.