if it's fine tomorrow,we'll go out..和if it were fine tomorrow,we would go out..有什么区别,各在什么情况下用

if it's fine tomorrow,we'll go out..和if it were fine tomorrow,we would go out..有什么区别,各在什
么情况下用

The difference is the plan to go out is no longer a plan.Therefore if it was fine tomorrow,we were not going to go out,as we have changed our mind.
In the first sentence,the plan is still valid.