First, you have to tell the examiner where is your town's location. Name of town is compulsory. Keep that in mind.
Next, you write your essay. Imagine that YOU really are in that scenario. When did the flash flood happen? At night? In the morning? When the sun shines upon your face, then out of the blue, rain starts to pour down and BOOM, a flash flood had occur in your town.
After that, describe the reaction from the residents there and also yours. Did your heart pounded as fast as the Jackrabbit's? Idioms and similes can actually attract the examiner so have fun with it. you may mention that your feet ran cold, you ran as fast as lightning to save your little sister, telling her to calm down when everyone was running for their lives to save their belongings.
Additionally, make sure you write an ending. Did someone rescue you? Were you told to go to a higher ground? Make your ending as intriguing as possible. Use good grammar and bombastic vocabulary. You can also state that you were relieved when finally everything is in order. You phoned your parents who were working that it was a minor flash flood and perhaps finish this essay about your feelings and as you and your sister look at the copper sky, grateful that no one was hurt.
Hope this helps.