Forecasting

Physics plus enormous computation.

How it is done

Observations feed a physical model of the atmosphere divided into a grid, stepped forward in time. Models run many times with slightly different starts to see how the outcomes spread.

A 70 percent chance of rain means it rained on 70 percent of past days like this one.
Forecast skill has improved by about a day per decade for fifty years.

Limits

Small errors grow, which caps useful detail at roughly a week or two. Beyond that you can forecast the statistics but not the day.