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Forecasts of near-surface temperature by the Global Ensemble Forecast System (GEFS) at a particular location in Texas every six hours for April 1, 1999 (pink) and April 1, 2019 (blue). For each year, the models used 11 slightly different initial conditions to represent the uncertainty of the initial state and the subsequent range of forecast outcomes. The periodic oscillations every 24 hours reflect the day-night cycle (e.g., afternoons tend to be warmer than evenings). For reference, days 2 and 13 are indicated by thin vertical lines. Forecasts from the two years become harder to distinguish as time goes on, indicating a decrease in predictability. NOAA Climate.gov figure, adapted from original by Michael Tippett and Tim DelSole.