The threat for severe weather was very, very low Thursday, but gusty winds came with non-severe showers and thunderstorms Thursday evening. A line of scattered showers and isolated thunderstorms developed to the northwest during the afternoon ahead of an advancing cold front, as well as colder air aloft. That line moved southeast, passing through southern Wisconsin and northern Illinois early Thursday evening. While lightning strikes were very minimal, some of the stronger cores and isolated thunderstorms did produce some rather strong wind gusts, downing a few trees and tree branches stretching from Freeport to Rockford.
These strong winds are known as ‘dry microbursts’ and form in an environment that has quite a bit of dry air. And while they didn’t look like much on radar, wind gusts to near 60 mph were noted across northern Illinois. Following the rain showers Thursday morning skies were quick to dry and clear, allowing temperatures to quickly warm into the low to mid 70s. Rockford’s high made it all the way up to 75 degrees. While the temperatures warmed into the 70s, drier air remained in place down near the surface with dew point temperatures in the 30s and 40s, producing a rather large temperature/dew point spread. There was also a slight increase in instability, given the increased sunshine and warmth during the afternoon.
As the rain showers moved into the unstable and dry air mass, a few of the showers grew to thunderstorms – especially over Wisconsin. As the rain fell from the clouds and into the drier surrounding air, some of that rain evaporated. Since evaporation is a cooling process the air cooled and became a little more dense, causing it to quickly sink to the ground. This process is known as a microburst, or downburst. And even though it occurred on a much smaller scale, the wind gusts produced from the showers were enough to cause some damage. Skies and conditions will remain much more quiet through Thursday evening.