An expert system doesn't actually make decisions; the programmers made the decisions and then hard coded them in. Something along the lines of 'if (wind_pressure > 0 && rain_density < 0.5) do_something() else etc'. It's probably a bit more dynamic than that, but however you do it, it is always the programmers that make the decisions and then write them in, to be replayed by the software later with different variables.