Thursday, July 28, 2016
Isolated Severe Weather and Funnel Clouds Over Northern Illinois.
The biggest story, however, were the reports of multiple funnel clouds over Northwest Illinois early this afternoon. Thankfully there were no confirmed reports of tornadoes or damage, but it was enough to probably rattle some nerves. There was even enough of a strong storm to prompt a Tornado Warning for Northwest Stephenson and Northeast Jo Daviess counties a little before 3pm.
Low pressure over Northwest Illinois and a stationary boundary with a cold front near the Wisconsin/Illinois border was the focal point for thunderstorms this afternoon. As the low got closer to Illinois late this morning, it caused the air ahead of it to rise. Because we had sunshine to start the day, there was enough instability to allow the rising air to turn into towering cumulus clouds and eventually thunderstorms. And since the storms were forming so close to a frontal boundary with shifting winds from south to north (south winds south of the boundary and northeast winds north of the boundary) it caused the air to spin - aided by the air already spinning around low pressure.
The thunderstorms that moved through were not supercellular (which is where tornadoes come from). But the spinning air within these thunderstorms was enough to generate rotation, which then formed into funnel clouds. Funnel clouds like what occurred today rarely touch down. They usually come about half way down, spin like a top for a few minutes and then lift back into the cloud. There just wasn't enough large scale spin in the atmosphere to sustain the funnels. Scary to see, but most of the time harmless.
With an almost same atmosphere tomorrow, we could see a repeat of thunderstorms but less in coverage. And with very little wind movement aloft, any storms that do develop will be heavy rain producers.