The first violent tornado of the year touched down in north-central Illinois last Thursday, packing winds of up to 200 MPH as it razed the small town of Fairdale. The storm killed two people and injured several more. Here's an in-depth look at the tornadic storm from beginning to end.

Here's What Happened

A violent, high-end EF-4 tornado tore a 30-mile path of damage through north-central Illinois on Thursday, killing two people and sending several more to the hospital. The storm decimated the small rural community of Fairdale, about 14 miles southeast of Rockford, Illinois, and 68 miles northwest of Chicago's downtown core. Survey crews studied the damage and determined that the tornado produced maximum winds of about 200 MPH near Fairdale, which is 1 MPH short of a top-of-the-scale EF-5.

The storm was caught on video by dozens of people, many of whom were entirely too close to the storm to be completely safe from its effects.

The supercell that spawned the tornado formed as part of a larger severe weather outbreak that affected 18 states from Oklahoma to Pennsylvania. The storms also produced numerous reports of golf-ball- and baseball-size hail and wind gusts of up to 91 MPH.

The Setup

We saw all of the ingredients for a classic severe weather event on April 9. A center of low pressure over Iowa dragged warm, moist air north from the Gulf of Mexico, allowing temperatures to climb into the 70s and 80s and muggy dew points in the mid-60s to stretch all the way up to the border between Illinois and Wisconsin. The supercell that produced the Rochelle/Fairdale tornado formed within the confines of the area circled in red on the 00z (7:00 PM CDT) surface analysis above.

The area had the instability at the surface, as well as the approaching weather system to force air to rise, so the next ingredient that turned regular thunderstorms into supercells was wind shear. Here's a look at the surface winds around 6:00 PM CDT on Thursday as forecast by the NAM model. The arrows point the direction in which the wind is blowing, with the shading showing wind speeds in MPH (the legend is on the left).

Across north-central Illinois, winds at the surface were generally between 10 and 20 MPH and blowing from slightly east of due south. You can see the abrupt change in wind direction behind the front, situated along the western Illinois border.

For the same time, here are the winds predicted by the NAM for the same time up around 300 millibars superimposed over the winds at the surface. Arrows that cross each other almost perpendicular to one another show intense directional wind shear in the atmosphere, or winds veering (turning clockwise) with height:

This is crucial in the development of a supercell. Strong wind shear—wind changing speed and direction with height—creates tubes of horizontal rotation in the atmosphere, which looks sort of like an invisible, spinning roll of paper towels turned on its side. When a thunderstorm forms in this type of an environment, its updraft can press up against these horizontal tubes of rotation and bend them like an arch until you have two vertically-oriented, rotating tubes of air.

Here's what the process looks like, where the red arrow in the middle is the thunderstorm's updraft:

The tube of air that's rotating counterclockwise will often get absorbed into the thunderstorm's updraft, beginning the storm's transition into a supercell. The rotation of an updraft in a supercell makes the column of air stronger and more resistant to being toppled over or broken up by very strong winds aloft. This causes the updraft to tilt on a diagonal, allowing the storm to ingest unstable air at its base and spew it downstream, allowing the storm to truck along the landscape like an efficient, terrifying engine, lasting for hours.

Some of the strongest supercells are capable of supporting monstrous tornadoes that can grow more than a mile wide, pack winds of more than 200 MPH, and stay on the ground for dozens of miles.

One such supercell in north-central Illinois took advantage of the favorable environment in which it formed and produced a violent, deadly tornado.

5:34:58 PM CDT

A small shower showed up on radar just south of the town of Annawan, Illinois, exactly 70 miles southwest of Fairdale. Just as the mightiest tree begins from the smallest seed, so too do supercells.

5:38:34 PM CDT

The shower quickly grows into a thunderstorm, which is actively transitions into a supercell after about ten minutes.

Four minutes have elapsed since the storm developed.

5:59:38 PM CDT

By six o'clock, the storm begins to display the classic signs of supercellular structure on radar imagery. You can see the inflow notch—an area of low (or no) precipitation where the supercell's powerful updraft is located—along with the rear flank downdraft, or the descending cool, stable air that wraps around the back side of the supercell.

21 minutes have elapsed since the storm developed.

6:08:28 PM CDT

As the supercell begins to fall out of the influence of the thunderstorm that formed alongside it back near Annawan, the storm continues to take on a more organized structure on radar imagery. The first signs of a hook echo begin to show up around this time.

The National Weather Service issues a severe thunderstorm warning at 6:11 PM CDT, alerting residents in the path of the storm for the risk of golf-ball-size hail and wind gusts up to 70 MPH. The warning notes that the area remains under a tornado watch.

34 minutes have elapsed since the storm developed.

6:27:58 PM CDT

The northeast edge of the supercell begins to approach Rochelle, Illinois, population of nearly 10,000. The storm's hook echo becomes more pointed on radar imagery, indicating that the storm is rotating and continuing to organize as it taps into the favorable atmosphere around it.

53 minutes have elapsed since the storm developed.

6:35:20 PM CDT

The National Weather Service issues a tornado warning for the supercell as Doppler radar indicates that the storm contains strong rotation near the surface. A three-dimensional rendering of the radar data shows a very well organized mesocyclone—the storm's rotating updraft—that extends the entire depth of the supercell.

61 minutes have elapsed since the storm developed.

6:45:03 PM CDT

By 6:45 PM, we start to get the first hint that there is indeed a tornado on the ground, heading in the general direction of Rochelle. Radar imagery shows a debris ball showing up in the area of the strongest circulation, meaning that the radar beam is reflecting off of debris swirling around in the atmosphere.

71 minutes have elapsed since the storm developed.

6:59:52 PM CDT

The tornado passes the downtown core of Rochelle to the west and north. Not all buildings are spared—a restaurant named Grubsteakers lies at the intersection of State Routes 64 and 251. The restaurant takes a direct hit from the tornado, but the establishment's owner hustled everyone into the building's cellar just before it leveled the building. 12 people were rescued from the cellar, all uninjured.

85 minutes have elapsed since the storm developed.

7:04-7:05 PM CDT

The Weather Channel streams live footage of the tornado from the vehicle of storm chaser Matt Salo.

[There was a video here]

90 minutes have elapsed since the supercell first developed from a tiny shower nearly 70 miles away.

7:06:51 PM CDT

The tornado begins to reach its peak intensity, producing winds estimated between 180 and 200 MPH, which makes it an EF-4 on the Enhanced Fujita Scale. The tornado is rapidly closing in on the small town of Fairdale.

7:12:00 PM CDT

The tornado hits Fairdale. All homes on the northwest side of the community are destroyed, with damage growing less severe the farther southeast you go from the track. The top image shows a three-dimensional rendering of the precipitation/debris within the tornado (the outline of the funnel is readily visible), while the bottom image shows an aerial view of the damage in Fairdale. The streets in the community are oriented north-south, with north towards the top-left of the image.

98 minutes have elapsed since the storm developed.

7:27:43 PM CDT

The supercell begins to merge with an approaching line of thunderstorms coming in from the west, the strongest of which produced a tornado near Davenport, Iowa, just a few hours earlier. This merger begins to interrupt the supercell's structure, allowing rain-cooled air to slowly choke off the tornado and allow the larger storm structure to absorb into the squall line.

7:50:19 PM CDT

The original supercell essentially loses its identity as it moves into southern Wisconsin, fully integrating into the squall line chugging towards Lake Michigan. The storm would go on to produce damaging winds near Kenosha and Racine around 8:30 PM, nearly three hours after the original supercell first developed.

Tornado Scar

Satellite imagery of northern Illinois shows a visible scar left in the earth where fields, crops, and trees were torn away by the intense winds. As we see after most intense tornadoes, this scar will be visible from space for many years to come, especially when it snows.

The tornado that destroyed Fairdale is notable because it was towards the top of the Enhanced Fujita Scale, which is pretty rare when you take into account all of the tornadoes that develop around the country. We hear about these monsters more than the thousands of other tornadoes that have developed simply because of their intensity. High levels of media coverage create the illusion of a plague of violent tornadoes, when the odds that you'll ever see or take a direct hit from one of these storms are minuscule.

I did a very similar exercise after the "twin tornadoes" in Pilger, Nebraska, last June, where I went through radar imagery frame-by-frame to break down exactly what was going on within the supercells. As I noted then (as well as when the 2013 Moore tornado unfolded), a small shower or thunderstorm that's moving faster than the parent supercell can get sucked into the storm's inflow or brush by the hook echo, seemingly spinning a "regular" tornado into a much more dangerous event.

That appears to have been the case this time, but I can't be 100% sure.

Above is a loop showing the tornado before and after it grew into the intense feature it became. Note the strong shower that approaches from the southwest and the bold debris ball that appears after contact.

It's extremely interesting to study major events like this using radar data—it's mind-boggling that we're able to look inside of a thunderstorm and tell what kind of precipitation it's producing, the size and shape of the objects within the storm (whether it's rain, hail, or tornado debris), and use the velocity of the objects to determine the wind speed and direction within the storm.

Unfortunately, it's still pretty early in the year and we're just getting into the classic definition of "tornado season," which doesn't begin to peak until the first week in June. We have a long road ahead of us on which we'll likely see many more instances of dangerous weather between now and the onset of cooler months later in the year. Use the time before the next outbreak to make sure you're prepared for a natural disaster, no matter what it is or where you live.

[Top Image: AP | Surface Analysis: WPC | Model Graphics: GREarth | Rotation Diagram: NOAA | Satellite Images: NWS | Radar Imagery: Gibson Ridge | Video: The Weather Channel]

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