The Butterfly Effect is the reason. For small pieces of weather -- and to a global forecaster, small can mean thunderstorms and blizzards -- any prediction deteriorates rapidly.

• 原因是蝴蝶效应的存在。对于小范围地区的天气，任何预测都会迅速的恶化；而对一个全球性的预报，小可能一位这暴雨和暴风雪。