Forecasting is done, and it's done by creating ensembles of weather simulations.
Weather is chaotic, which is why forecast accuracy rapidly drops off the further out in time you make predictions. But still, they can be accurate enough far enough out into the future to be extremely useful.
It does seem to me like a lot of that accuracy comes from real-time metric collection from all of the stations around the world (And there are many more stations, too).
For example, in London iPhone gives you almost real-time "current weather" state. Stuff like "it's raining now and it's going to stop raining in the next 5 minutes". You can only do that via collecting the radar data in real-time and correlating it with GPS. Not so much prediction.
Weather is chaotic, which is why forecast accuracy rapidly drops off the further out in time you make predictions. But still, they can be accurate enough far enough out into the future to be extremely useful.