A thunderstorm is considered to be severe if it produces winds of 58 miles per hour. That number was selected because at the speed of 58 miles per hour, or 50 knots, the power of the wind is generally considered the threshold where large tree limbs or tree trunks can break.
The presence of water in the soil changes some of its physical properties. When the ground is saturated, water has soaked deeply into the soil. This water acts like a lubricant, making it easier for tree roots to slip when force is applied.
During a storm, wind pushes against the limbs of a tree, and exerts a force on the tree’s roots. If the root is sitting in water logged soil, the root will move more easily when a force is applied, and the tree will fall over. Heavy rains can also act to wash away some of the topsoil around the base of a tree, further loosening the tree root’s grip on the earth.
For this reason, wind damage is often more extreme after a heavy rain event. The presence of strong winds during a weather event can also increase flash flooding dangers. In storm with significant winds, the displaced leaves, limbs and other debris can clog drainage and allow deeper water to collect.