The science behind the funnel clouds reported Monday afternoon

Cindy Clark
Rochelle, IL

The atmosphere Monday afternoon was not supportive of severe weather, but many of you did report funnel clouds late in the day.  And while the chance for those funnels actually reaching the ground was low, I'm sure it had many of you wondering.

There was a reason why those storms produced the funnels you saw.  And to help understand why, we have to look at what was going on in the atmosphere at the time the storms developed. 

In order for severe weather to occur, the right ingredients need to be in place.  It's like following a recipe in a cookbook.  Don't have one of the ingredients, and your recipe doesn't work.  The same holds true with weather.  Moisture, lift and instability are the key ingredients for severe weather.  Miss out on one, or more, and the threat goes down.  One big factor for getting storms to spin (which is one element needed to produce funnel clouds) is directional wind shear, or winds coming from different directions in the lowest level of the atmosphere.  Our atmosphere Monday didn't have that.  From above, down to the surface, the wind was pretty much coming from the same direction.  So with that ruled out, what else could have caused those funnels?


5 miles Southeast Ashton



Sandy Ebert
5 miles Southeast Ashton

As the Chicago National Weather Service put it: "Boundaries"



Chicago NWS Radar
Late Afternoon

A lake breeze developed from Lake Michigan late in the day and actually moved pretty far inland.  Along with the lake breeze, outflow boundaries from thunderstorms were very common across Northern Illinois.  What is an outflow boundary?  Think of it as a mini cold front that the thunderstorm produces.  Often times, outflow boundaries from thunderstorms can help ignite new storms in an environment that is unstable because it acts as a lifting source for air to rise.  When these outflow boundaries intersect, or intersect thunderstorms at right angles, they can sometimes enhance the low level spin in a thunderstorm.  While brief, this interaction did allow for a strengthening in some of the storms.  If the storms would have followed along those boundaries for a greater amount of time, there could have been a greater tornado threat.  Thankfully that didn't happen!