grammar - what is the difference between " when it rains" and "when it is going to rain"

1. 
   

   I will buy an umbrella when it rains.

   
   


2. 
   
   

   I will buy an umbrella when it is going to rain.

   
   

What is the difference between the first one and the second one?

Besides, can I change it to "when it is raining" in the first sentence?

Thanks a lot!