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Random train-test splits made rainfall models look good, and a seasonal lookup table beat every learner once the split was honest

AspireAI community · arXiv (RainCheck)
Rainfall models are often evaluated by shuffling station records, which lets a model see what happened at neighbouring stations on the very day it forecasts and hides how much of the variance a seasonal table already explains. RainCheck quantifies both effects on 618,650 station-days from 35 Bangladesh Meteorological Department stations spanning 1948-2014. A station-month climatology alone classifies high-rainfall months at 88.4% accuracy with AUC 0.950, and no learner improved on it by more than 0.8 points under temporal evaluation; a calendar-only random forest reached a next-day wet-day AUC of 0.889 under a random split but fell to 0.835, below climatology at 0.859, on future years. Models with lagged rainfall add only modest skill, a Brier skill score of 0.074 over climatology, rising to 0.131 with other stations' same-day observations, and skill disappears beyond a two-day lead. The authors release a leakage-aware protocol and code so future station-based studies report skill against climatology on held-out years.
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