Ensemble Weather Forecasting — Why Forecasts Lose Skill

Real weather models don't run once — they run dozens of times from slightly different starting conditions, because the atmosphere is chaotic: any two runs that begin almost identically will drift apart at a roughly exponential rate until they saturate. This simulator builds a small numerical weather model from scratch — a pressure field advecting itself across a periodic grid via its own geostrophic wind — and launches a handful of ensemble members from near-identical initial states next to one deterministic "control" run. Watch the terrain-like pressure surfaces (the ensemble "spaghetti plot") peel apart from the control as simulated forecast days pass, while live readouts track the ensemble spread and estimate the model's predictability horizon — the same diagnostic operational centers like ECMWF use to decide how far out a forecast can be trusted.