We can get some very severe downpours here in PA with 3" overnight not unusual. The only effect I've ever noticed from rain is more water in the pool. If it was a particularly 'dirty' storm, i.e., if the winds driving it carried a lot of dust, I may have dust and debris in the water. I have not noticed a change in the values for FC, TA, or pH and don't do anything special or different after even a week of heavy rains.