Prospective comparison of El Niño rainfall forecasts across Tanzania

This repository provides the frozen pre-season forecast archive and reproducibility materials supporting a prospective comparison of October–December (OND) 2026 rainfall forecasts across Tanzania's 31 first-order administrative regions. The study compares four approaches: a Relative Oceanic Niño Index (RONI)-based statistical forecast, an Indian Ocean Dipole Mode Index (DMI)-based statistical scenario, a combined RONI–DMI statistical scenario, and a bias-corrected ECMWF dynamical seasonal forecast. Historical rainfall is based on CHIRPS v3 for 1981–2025, with 1991–2020 used as the climatological reference period. Regional historical distributions are used to calculate tercile thresholds and historical rainfall percentiles. All forecast values, climate-index values, hypotheses and verification criteria contained in this archive were frozen on 8 September 2026, before observations for the target OND 2026 season became available. No observed OND 2026 rainfall was used to fit, select, recalibrate or modify the archived forecasts. The repository includes regional historical rainfall data, historical RONI and DMI predictors, processed regional ECMWF forecast values, regional historical benchmarks, the frozen 31-region forecast archive, data underlying the pre-season figures, R code, prospective hypotheses and the pre-specified verification protocol. The DMI-based products are labelled statistical scenarios because the historical regression uses NOAA/HadISST DMI whereas the frozen 2026 DMI value is ECMWF-derived. Version 1.0 represents the frozen pre-season archive. Post-season observations and verification results may be deposited as a subsequent version without altering the forecasts archived here.