I will take/always take an umbrella in case it rains.
I will take an umbrella in case it is raining means you can't see outside world because you do not have a window.
'I always take an umbrella with me if it rains/is raining'
To me, 'it rains' may refer to an ongoing process, even though the verb is non-continuous.
A: It is raining outside.
B: If it is raining, then I will take an umbrella.
Student or Learner