Storm Prediction Center Catagorical Severe Weather Outlook - Issued 11:26 AM June 12
So what did we get? A few isolated thunderstorms – none severe. Severe weather did occur across northern Illinois into northern and central Indiana, but for our area not one report of severe weather. Storms originally developed across eastern Iowa and northern Illinois during the middle to late afternoon as expected, but never developed into the severe line of storms that we forecasted to impact central and southeast Illinois.
Severe Weather Reports - June 12
Dew Point Temperature analysis at 1 PM Wednesday (Graphic courtesy of the Atmospheric Sciences Department at the University of Illinois at Urbana-Champaign)
Warm air rises. Once a bubble of warm air begins to rise, if it remains warmer than the air it is rising through it will accelerate upward. This is known as an updraft. If the bubble becomes cooler than the air that it is rising through it will slow down and may eventually begin to descend. Once the air starts to descend it is called a downdraft. Thunderstorms are made up of updrafts and downdrafts. The stronger this up and down motion in a storm is the better are the chances for severe weather.
There was one “little” fly- in-the-ointment however. There was a layer in place on Wednesday about 5,000 feet above the ground where temperatures were actually a few degrees warmer than our rising bubble of air from the surface. This meant that there was a zone that the updraft would pass through that would cause the bubble to actually slow down and possibly even begin to descend before it could reach the big differences in temperature further up in the atmosphere. This is what happened much of the day Wednesday and is why we didn’t have thunderstorms earlier in the day. This was not unexpected. It just meant that the hot and humid air near the surface would need to wait for a strong nudge to push them up through this warm layer. That brings us to lift.
To get a strong updraft started, we must first have something to start moving our bubble upward. This mechanism can be a difference in air characteristics such as a front (which is what triggered the storms to our north and east), or the spreading out of downdrafts from previous storms (known as a “gust front”). This latter process is what we expected on Wednesday afternoon and evening to start the hot air moving upward over our area.
However, on Wednesday the quartet never quite got their timing right. We never got downdrafts from several storms to interact in such a way that they combined into a strong gust front that could push south into central and southeast Illinois. We did have a few individual downdrafts that pushed into the Interstate 74 corridor, but those downdrafts were not strong and deep enough to push the hot air upward through the warm layer several thousand feet above the surface and to tap the instability above.
The result – a few isolated thunderstorms and most of us remaining dry.
Radar, Mean Sea Level Pressure Pattern, and Winds from 7 PM Wednesday Evening