Severe weather and flooding rain can occur anytime during the Spring and Summer months, but we tend to see more heavy rain events during the summer months of July and August.
It's important to be prepared for severe weather anytime of the year. Tornadoes have occurred during the fall and winter season, and even heavy rain has occurred when the precipitation was supposed to be snow. So, what's happening in the atmosphere that causes heavier rain to occur during the summer?
To answer that we need to look at the jet stream. The reason the jet stream exists is because of the temperature difference throughout the world. The atmosphere is trying to balance itself out. When there is an imbalance, storm systems develop. Jet stream winds are fastest during the winter months because that's when the greatest temperature difference occurs from north to south. During the summer months, jet stream winds are usually weak because the temperature is more uniform.
Jet stream winds will then begin to increase towards the fall months as the season begin to change and cooler air starts to move back south.