Autumn rain gave no clear added winter rain prediction gain beyond RONI in four western US states in 1981-2025

We tested whether September-November precipitation improves January-March precipitation estimates beyond an autumn ocean index in California, Nevada, Arizona and New Mexico. Each linear fit used earlier winters only. The test covers 45 winters in 1981-2025. Adding autumn rain increased regional squared error by 1.40%. The conditional 95% five-year block interval for error reduction was -4.12% to +0.72%. State gains ranged from -4.38% in Nevada to +2.99% in New Mexico. All four adjusted state intervals included zero. ONI, a 30-year training window and year terms also gave negative regional point gains. The test gives no clear support for this form of autumn rain memory after ENSO adjustment. It does not test physical soil moisture feedback or operational forecasts. Revised data, short records, state means, simple models and conditional uncertainty limit the result. Code, numerical seasonal inputs, predictions, source hashes, uncertainty draws, an audit and a figure are included.

seasonal precipitation · ENSO · RONI · western United States · chronological prediction · null result