A devastating EF-4 tornado carved a scar through 30 miles of farmland in
north-central Illinois on a muggy afternoon in April 2015. Two people died and
nearly a dozen more were injured as the storm levelled neighborhoods in
Rochelle and Fairdale, about an hour's drive west of Chicago.
Estimated winds reached 200 mph as the tornado swept homes and businesses
clean off their foundations. Falling just 1 mph shy of scale-topping EF-5
status, it stands in the records at the strongest tornado to hit Illinois that
decade.
Here's an in-depth look at how a modest rain shower grew into a killer
supercell over the course of just two hours.
We saw a classic springtime severe weather event unfold across the central U.S. during the second week of April 2015.
A strong trough diving over the Rockies spawned a low-pressure system over eastern Colorado on Wednesday, April 8, leading to a multi-day severe weather outbreak across the Plains and Midwest states.
A mile or two above the surface, southwesterly winds were racing along at highway speeds as air whipped around the strengthening low-pressure system. Even higher in the atmosphere—about five miles up—winds were blowing out of the west at nearly 100 mph.
Wind shear is a critical factor in pushing ordinary thunderstorms beyond severe limits. This sharp change in wind speed and direction with height creates a horizontal rolling motion in the atmosphere. A thunderstorm's strong updraft can force this horizontal rotation to tilt vertically, allowing the updraft to 'absorb' the rotation (so to speak).
A thunderstorm with a rotating updraft is called a supercell. Rotation makes a supercell's updraft stronger and more resilient than an ordinary thunderstorm, creating a storm that can last longer, travel farther, produce bigger hail, and spawn tornadoes.
Particularly intense supercells are capable of supporting the strongest tornadoes ever observed. These high-end twisters can grow more than a mile wide, packing winds close to 200 mph as they carve a path dozens and even hundreds of miles long.
The supercell that tore through north-central Illinois on April 9, 2015, was one of those storms.
Unstable air rapidly rising into the atmosphere about 70 miles southwest of Fairdale generated a vigorous batch of billowing cumulus clouds.
The weight of the water overpowered the strength of the updraft, falling to the ground as the first detectable batch of light rain just south of the town of Annawan.
5:48 p.m. CDT | Elapsed Time: 14 minutes
It took less than fifteen minutes for the shower to grow into a downpour as the thunderstorm kicks into gear. We can already see the storm taking on the vague outline of a supercell's classic shape.
Wind shear is tilting the storm's updraft toward the northeast, forcing the rain-cooled air in the downdraft to flow around the western side of the storm—a feature known as a rear-flank downdraft. This setup allows the storm to vent its rain-cooled air without interrupting the warm, unstable air feeding into the downdraft.
5:59 p.m. CDT | Elapsed Time: 25 minutes
By the top of the hour, the storm is rapidly maturing into a full-blown supercell.
The warm, unstable air streaming into the updraft leaves a precipitation-free vault in the base of the storm known as an inflow notch. We can see the rear-flank downdraft wrapping around the western side of the storm. These two features help generate the classic hook echo that make supercells so ominously recognizable on radar imagery.
However, supercells really thrive when they're all alone. Another thunderstorm that bubbled up a few miles to the west is interfering with its development. The storm will struggle to reach its full potential until the neighboring storm dissipates or moves away.
6:08 p.m. CDT | Elapsed Time: 32 minutes
Eight minutes after the hour, the supercell's appearance is already improving as the neighboring storm starts to pull away. We can see the beginnings of a hook echo showing up on precipitation and velocity (wind) imagery.
The storm is starting to develop a "v-notch" as strong winds aloft deflect around the updraft like water around the bow of a boat, leaving something of a rain-shadow effect immediately downwind from the updraft.
6:11 p.m. CDT | Elapsed Time: 34 minutes
Forecasters issue a severe thunderstorm warning for large hail and damaging winds.
6:27 p.m. CDT | Elapsed Time: 52 minutes
The leading edge of the supercell approaches Rochelle, Illinois, and its population of about 10,000 residents. A well-defined hook echo is present on radar now, a sign that rotation within the storm is rapidly tightening up as the supercell takes advantage of the favorable environment.
6:35 p.m. CDT | Elapsed Time: 1 hour, 1 minute
Forecasters at the National Weather Service in Joliet issue a tornado warning based on the strong rotation indicated by radar imagery.
A 3-D rendering of the supercell shows the powerful mesocyclone stretching from the base of the storm up beyond the cruising altitude of passenger jets—indicative of a very healthy and very hazardous supercell.
6:47 p.m. CDT | Elapsed Time: 1 hour, 12 minutes
One of the great benefits of modern weather radar is that we don't always have to wait for visual confirmation to know that a storm is producing a tornado.
Tornado debris reflects a significant amount of radiation back to the radar site. This return often shows up as a dark circle—called a debris ball—that coincides with the location of strong rotation within the storm.
A (then-recent) radar upgrade called dual polarization allows us to see the size and shape of the objects reflecting the radar beam, which is useful for telling rain, hail, and debris apart. A big mix of different sizes and shapes will show up with a low correlation coefficient. The data left no question that this is a significant amount of debris lofting into the atmosphere.
6:57 p.m. CDT | Elapsed Time: 1 hour, 22 minutes
This large, wedge tornado is now at its peak intensity as it spares Rochelle by tracking just west and north of the town center.
The most intense damage occurs along S Richard Road and E Kuehl Court, just northwest of Rochelle. Several homes on these roads were completely levelled, with wind-swept debris tossed a significant distance downwind from each residence. Surveyors used this damage to rate the tornado a high-end EF-4, just one tick below EF-5 strength.
Just north of Rochelle, though, a restaurant called Grubsteakers takes a direct hit from the tornado. Thanks to advanced warning—the tornado warning has been in effect for 23 minutes at this point—all 12 people in the building escaped injury by huddling in the restaurant's cellar.
Grubsteakers was a total loss, but it didn't take the full brunt of this powerful tornado.
Surveyors found EF-4 damage just a short walk north of Grubsteakers, where the tornado destroyed a home, scrubbed a barn down to its foundation, and debarked sturdy trees on the property.
7:00 p.m. CDT | Elapsed Time: 1 hour, 25 minutes
An exceptionally clear view of the tornado from the south allows multiple storm chasers and residents to capture crisp images of the storm as it passes through the area.
After hitting properties north of Rochelle, the tornado spent the next several minutes passing over farmland. Multiple homes and structures suffered damage, with surveyors finding EF-2 to EF-3 damage in a few spots.
Satellite imagery taken a few days after the storm revealed a remarkable pattern of cycloidal marks carved into the topsoil along the tornado's visible scar. Large tornadoes often contain multiple vortices swirling around the larger overall circulation. The vortices, as well as any debris caught in them, can gouge the ground in swirling patterns that are easily visible from the air after the storm.
7:13 p.m. CDT | Elapsed Time: 1 hour, 38 minutes
Fairdale takes a direct hit.
The tornado carved through the northwest corner of this 0.08-square-mile community, destroying more than a dozen homes and damaging just about every building in town.
Two people died and 22 more were injured in the few seconds it took for the tornado's intense winds to rip through Fairdale. One resident in the path of the storm filmed the tornado from a window as it approached the community; his wife died and he was injured when the storm destroyed their home.
Ground surveys and satellite imagery reveal the extent of the damage. Extensive ground scouring, debarked trees, and near-complete destruction of all structures and vehicles left the northwestern corner of Fairdale unrecognizable.
7:27 p.m. CDT | Elapsed Time: 1 hour, 52 minutes
Past its prime, the large supercell that produced the Fairdale tornado is in the waning stages of its life.
The supercell is beginning to merge into the squall line chugging through northern Illinois, which brought rough weather to the city of Rockford just a few minutes earlier. Rain-cooled air and the nearby thunderstorms are disrupting the supercell's structure, forcing it to break down.
7:50 p.m. CDT | Elapsed Time: 2 hours, 16 minutes
Just over two hours after a tiny shower in central Illinois grew into a supercell that dropped a violent tornado, the storm has fully absorbed into the squall line and is no longer its own, independent storm.
Animation
One of the most striking aspects of the storm was how the violent tornado itself developed.
If we animate the radar at the storm's peak strength, we can see that a tiny shower ingested into the inflow that fed the storm its supply of unstable air. There have been plenty of similar cases over the years where a nearby shower or thunderstorm brushes past or gets ingested into the hook of a supercell thunderstorm, enhancing the spinning and stretching motions needed to spawn a powerful tornado.
This tragic event provided a remarkable opportunity to watch the evolution of a destructive thunderstorm from infancy to dissipation. It's sometimes easy to forget that even the mightiest storm begins as a single puffy cloud.
[Top image courtesy of the NWS. All radar images courtesy of Gibson Ridge and compiled/annotated using Canva.]
