Tips for Weather Forecasting?

I am going to Prague this weekend and wanted to check the weather so I would know what kind of shoes to take. I checked couple of websites, only to find that their forecasts were not similar at all. So I went on to check several more websites only to find that none agreed with another. The forecasts varied from no rain to 70% chance of rain. Some websites expected some sun on one day, while others expected some sun on the other day. How is it possible that these weather forecasts can be so different? Since I have no idea which site is the most reliable, I have no freakin' idea what the weather is going to be like. Stupid useless sites....