Above Image: Michelle Wilcox (Polo, IL)
Isolated severe thunderstorms were quick to move north Tuesday evening across northwest Illinois and southwest Wisconsin, producing quite a bit of wind damage. Reports of downed trees and power lines, as well as damage to many outbuildings, were reported from Whiteside County all the way into southern Green County.
The greatest threat for tornadic thunderstorms remained west of the Mississippi River Tuesday evening, but the storms that developed over west-central Illinois were quick to race north into a highly unstable environment. While there were great amounts of instability (energy) over northern Illinois and southern Wisconsin, wind shear in the atmosphere was very weak. This meant any storm that developed would likely produce strong downburst winds, along with extremely heavy rainfall.
All thunderstorms have updrafts and downdrafts. The updraft is what transports the warm, moist air into the thunderstorm and the downdraft is the rain, hail and wind that falls out of the thunderstorm. In the development of a downburst, the large area of rain and hail within the updraft become too heavy for the storm to support, causing it to quickly race to the ground. Because it falls rather quickly throughout the thunderstorm it begins to ‘drag’ a lot of air downward with it, gaining speed as it falls to the ground. If the air under the storm has low relative humidity, or if there is dry air coming into the storm from aloft, the downward moving air begins to evaporate causing it to speed up even further through a process known as ‘evaporative cooling’, causing the downdraft to become even stronger.
When the downdraft reaches the ground it spreads out rapidly in all directions and is known as a downburst. Winds within downbursts can exceed 100mph. If the damage from the downburst is confined to an area that is less than two and a half miles, it is known as a ‘microburst’. There are two types of microbursts – wet and dry. A ‘dry’ microburst is the strong downward motion of air without heavy rainfall. A ‘wet’ microburst occurs with a very heavy rain.