Q.Petunias
How much rain can petunias survive in?

Like most plants, how well petunias will survive heavy rain depends on the soil type you have. If you have well-drained soil, they can survive a lot of rain. If you have poorly drained soil or soil that stays wet most of the time, the roots will drown with too much rain. Disease can also be a problem.
Planting in a raised bed with an open bottom is one option for dealing with this.
https://www.gardeningknowhow.com/plant-problems/environmental/rain-knocking-down-plants.htm
https://www.gardeningknowhow.com/plant-problems/environmental/excessive-rain-on-plants.htm