了 usage conflict "now" vs "completion"
了 (le / ㄌㄜ˙) has two meanings.


  
  放在动词或形容词后，表示动作或变化已经完成：写完～。
  


When it follows a verb or adjective, it's a perfect or past tense.

In this case we can use 过 or 曾经 to clarify its tense.
For example, 

下过雨了 or 曾经下雨了
It has been rained.

昨天下雨了
It rained yesterday.


  
  助词，用在句子末尾或句中停顿的地方，表示变化，表示出现新的情况：刮风～。
  


(Particle) When it's used at the end of the sentence or as a pause in the sentence, it means something is changing, a new situation.

下雨了
It's raining.