What led up to Thursday's tornadoes

Often times we watch the destruction of large tornadoes in other parts of the country and think "That will never happen here".  But on Thursday evening, the worst fear of many came true as the first EF-4 tornado touched down in Illinois since the devastating EF-4 tornado that hit Washington, IL back in 2013.

Multiple tornado warnings at one point were issued for Northern Illinois as a powerful storm system spawned what ended up being five tornadoes across the Stateline - two of those were satellite tornadoes - and nine throughout the state that day.

But, what were the conditions that led up to the massive EF-4 tornado that evening?  Unfortunately, all the right ingredients came together to create a very dangerous situation.  As early as Saturday, the warning was given that Thursday could potentially be a day that needed watching.

Surface map from the Chicago NWS

The map to the left is a surface analysis from Thursday evening around 6pm from the National Weather Service.  During the morning, cloud cover and non-severe thunderstorms developed ahead of a warm front that was forecast to pull north of the state line by the afternoon.  As cloud cover begin to clear to the south of Rockford, the air mass behind the warm front became increasingly unstable.  Surface dew points rose into the middle 60's - one of the ingredients needed for severe weather - and not typically felt this early during the month of April.

Instability was increasing causing the environment at the surface to become favorable for rotating updrafts within thunderstorms. 

Graphic from Thursday

In order for thunderstorms to have updrafts that rotate, a certain low level profile needs to be present.  This type of environment is typically found along some sort of boundary.  For example, a cold front or warm front.  Thunderstorms from earlier in the afternoon helped to reinforce the warm front right over Northern Illinois by leaving outflow boundaries (mini cool fronts). This caused what meteorologists refer to as 'backing winds'.  This means winds at the surface are from the south or southeast, but just a couple thousand feet above, they shift around to the southwest or west.  That turning of the wind throughout the atmosphere is what causes rotating updrafts.  And if strong enough, can cause tornadoes.  All of this at the same time surface low pressure continued to deepen (strengthen) in Northeast Iowa and Northwest Illinois. 

As the thunderstorm developed near Dixon and Franklin Grove, it took hold of the instability along the warm front and fed off of it as it quickly raced Northeast before ending in far southern Boone County.  Some chasers estimated the storm to be moving around 40-50 mph.  With a storm motion that fast, it doesn't allow much time to react.

Throughout the evening, several more tornadoes occurred - two of which were classified as satellite tornadoes.  In all, five tornadoes occurred in Northern Illinois that night.  Thankfully, the weather this week will remain much more quiet, outside of a few showers possible late Wednesday and Thursday, but severe weather is not expected. 

For the preliminary damage survey done by the Chicago National Weather Service, click here.