The earth's tilt. The tilt of the Earth on it's axis is what makes the sun heat up different parts of the Earth at different times, thus making wind, thus making different weather patterns. Depending on how much sun (heat) the area is getting, it changes the weather.
The median for group 2 is almost double the median for group 1. Group 2's median would be 80, and 1's would be a little over 40, which is about half. The median is the middle line of the box on the graph.
You didn’t list the following! However, here are some examples of severe weather events that are not likely to result in flooding: wildfires, droughts, tornados, dust storms, heat waves, etc