Lightning continually generates more than a million Megawatts of power. Vapor condenses, falls and seeps into the ground. Vapor condenses, falls, and runs off. A flood forecast is a prediction of the occurrence of a condition. A flood warning indicates a dangerous condition is occurring or is imminent.
River floods occur when the runoff exceeds the channel capacity.
Let's assume we had 1,000 years of stream gauging records on the Thames River at London Bridge. Other things being equal, we would assume that we would get ten floods of the 100-year variety in London during that period of time. We should also reasonably expect a couple of 500-year floods during that time. The ten 100-year floods would be scattered in time, rather than occurring at the rate of one per century. It is possible to have several hundred years pass between 100-year floods.