On Thursday, April 9, 2015, several tornadoes tracked through Northern Illinois, including one extremely powerful EF4 tornado that hit the town of Fairdale, Ill. The Valparaiso University Storm Intercept Team (VUSIT) trekked into Illinois on a storm chase that morning, and over the course of the day witnessed three tornado-warned storms and two tornadoes, including the early stages of the Fairdale EF4.
According to the National Weather Service in Romeoville, Ill., 11 tornadoes struck Illinois that day, with the most powerful tornado of the day being the Fairdale EF4, which was on the ground for 41 minutes and 30.2 miles with estimated peak winds of 200 miles per hour. “EF4” refers to the Enhanced Fujita Scale, which meteorologists use to rate the strength of tornadoes. The scale is based on the type of damage that a tornado does, which is used to estimate how strong its winds likely were and give it an EF rating. The scale goes from EF0 to EF5, with the 200 mile per hour estimated winds from the Fairdale EF4 putting it right on the threshold of the next highest classification, EF5. A tornado of this strength is a very rare event in any region, with the last EF4 or stronger tornado in the Romeoville, Ill. forecast office’s area of responsibility being the Plainfield EF5 tornado in 1990, according to the National Weather Service.
VUSIT started off the day with a meeting at 7:30 a.m. to determine if they wanted to chase and where. According to VUSIT director Russell Danielson, a senior meteorology major, much of the discussion focused around whether there would be enough clearing in Illinois during the day to generate the unstable atmosphere needed for severe storms and tornadoes. After a second meeting later in the morning, VUSIT decided to chase and left for their initial target location of Peru, Ill. Once they reached Peru, a storm to the south near Peoria strengthened and produced a brief rope tornado. VUSIT tried to get south to intercept the storm, but got there just in time for the thunderstorm to begin to decay rapidly.
According to Danielson, after this storm, VUSIT decided to move progressively closer to the warm front. There, after much waiting, things rapidly began to get interesting.
Danielson said “We headed west, on a paved road in the middle of nowhere, and we just waited for storms to develop. Most of the storms were not really becoming strong. So, we decided to go farther northwest toward the warm front, towards a storm that had already formed, and then as we headed west, this storm started producing tornadoes in extreme eastern Iowa. We intercepted it near Fulton, Ill. where it did produce a tornado.”
As this storm began to weaken, VUSIT focused on a newly developing storm to its south and east, which looked to be rapidly strengthening and would go on to produce the Fairdale EF4.
Danielson said, “We headed east with the storm complex, and that is when we saw the beginning of the EF4 storm that later absolutely damaged the town of Fairdale.”
After witnessing the early stages of the tornado from the west, VUSIT began following a safe distance behind the storm to try and get a better view. Their route along Illinois 64 took them through a segment of the tornado’s path where it had been less than half an hour earlier.
Danielson said, “Something that I will probably never forget is coming up upon the damage that we saw on Illinois 64, with the town almost completely wiped off the earth there. The restaurant with the semi [truck] blown over it and the roof caved in, those images—stop signs twisted, I will definitely remember that, and it’s definitely a sobering experience. You really think, you really have to be in awe of these storms, and really have to take shelter and be prepared for them.”
Cameron Nixon, a sophomore meteorology major and member of VUSIT, also thought that seeing the damage was a sobering experience. This impression seemed to be shared by all of VUSIT, and served as a reminder of the ultimate purpose of meteorology and forecasting: to protect the public from dangerous weather through warnings and forecasts.
Nixon said, “The most memorable moment for me was driving into that little street corner where we just realized everything was completely gone.” He thought that this reinforced the idea that chasing must not be all about simple thrill seeking, since “on any chase, we could see this happen.”
Zach Bruick, a sophomore meteorology and geography major and member of VUSIT, said that this chase really impressed on him the public service aspect of meteorology.
Bruick said, “The lesson learned of the day is that us as students have the ability to forecast and help in warning the public, which can help save lives. Meteorology, as I always call it, is a science that serves the public, and working to continually increase the public’s awareness about severe weather is critical, and working to improve our forecasting skill is critical. I think that we were able to see why both are important on this day, and we were able to accomplish our goals that we set forth.”
Why were conditions so favorable for tornadoes on Thursday, April 9? Severe storms that produce tornadoes need four main ingredients—moisture, instability, lift and vertical wind shear—and the atmosphere over Northern Illinois on the evening of April 9 had all four of these in abundance. The first of these parameters, moisture, was provided by a warm air mass wafting northward ahead of a low pressure center moving through Iowa during the day. Instability—the tendency for small patch of air, called a ‘parcel’, to accelerate upward when given a push in that direction—is crucial for fueling the powerful updrafts needed for strong and severe thunderstorms, and was also present in the warm, moist air mass over Illinois that day.
Lift—the push needed to get unstable air started on its upward acceleration—was provided in Northern Illinois by a warm front draped across the region and an approaching cold front stretching south from the low pressure center in Iowa. Finally, vertical wind shear—the turning of winds with height which encourages the updrafts of powerful thunderstorms to rotate—was quite abundant that evening near the warm front in Northern Illinois. With all of these ingredients in place, storms that formed in the region just south of the warm front quickly intensified and began to rotate, organizing themselves into a powerful variety of thunderstorm known as a supercell. Supercells are thunderstorms with strong, persistent rotating updrafts, and produce the vast majority of strong and violent tornadoes. This characteristic was dramatically demonstrated on April 9, since according to the National Weather Service in Romeoville, Ill., six of the tornadoes in Northern Illinois—including the Fairdale EF4—were produced from one extremely powerful supercell as it tracked across the region.
Contact Matt Wilson at firstname.lastname@example.org.