It's difficult to predict the weather accurately more than a week in advance.

accurately 是形容词 accurate（准确的）加上副词后缀 -ly 构成的副词形式，表示"精确地、准确地"。accurate 来自拉丁语 accuratus，由前缀 ac-（表示"朝向"）和词根 cura（"关心、注意"）组成，字面意思是"小心谨慎地做"，引申为"精确的"。