50% means that they ran the simulation say 10 times and in 5 of those simulations there was rain in the area. How to translate that to real life - you should pr

50% means that they ran the simulation say 10 times and in 5 of those simulations there was rain in the area. How to translate that to real life - you should probably carry an umbrella if you don't like getting wet. Also interesting to note that weather presenters usually bump up the probability of rain by 10-20%, because people complain about false negatives (rained when it was predicted not to) rather than false positives (didn't rain when it was predicted to). If you're interested in a good intro to this topic I recommend the chapter on weather prediction from The Signal and the Noise by Nate Silver. Its a fascinating read.