No matter how sophisticated our Earth system models may be, the quality of the forecasts they produce crucially depends on the accuracy of the initial conditions.
Using the model state from a previous forecast as an estimate of the initial conditions is not good enough: all forecasts come with errors, which grow over time.
A vital corrective is provided by weather observations.
Millions of them are made every day. They come from many sources, including satellites, aircraft, ground stations, ships, drifters and balloons.
However, on their own they cannot provide the initial conditions, either: they are unevenly distributed in space and time, and they too come with errors.