Timeline of a tornado
Jeffrey Frame, a University of Illinois clinical professor in atmospheric sciences, has traveled throughout the Midwest, Plains states and other regions of the country tracking and observing tornadoes, lake effect snowstorms and other severe weather. With a statewide tornado drill set for 10 a.m. today (Tuesday, March 4), we shared storm chaser Jessie Starkey's photos of the Gifford tornado with Frame, who explains how the tornado formed and forged its path through East Central Illinois.
(See accompanying photos)
12:47 p.m. Nov. 17, 2013: This is the wall cloud that produced the Gifford tornado. A wall cloud is a lowered cloud attached to the base of a thunderstorm cloud. It often signifies the location of the most intense updraft, or current of rising warm, moist air that feeds the thunderstorm. If a wall cloud is rotating, it means that a tornado may form in a few minutes, or even sooner. In this image, the wall cloud is just left of center. One minute later, this wall cloud produced the tornado that struck Gifford and the surrounding farmland.
12:48 p.m. Nov. 17, 2013: The Gifford tornado touched down just east of Thomasboro and moved rapidly northeastward at nearly 60 miles per hour. Tornadoes form when sinking air (the rear-flank downdraft) brings rotation from aloft down to the surface, then this rotation is stretched in the storm's updraft (think of a figure skater bringing her arms in and spinning faster). Tornadoes are often visible as funnel clouds or as swirls of dust and debris on the ground. The rotating winds, not the funnel cloud, cause damage. Sometimes, damaging winds can extend beyond the visible funnel, or there can be no funnel visible at all. The tornado was about 6 miles away at the time of this image.
12:49 p.m. Nov. 17, 2013: The tornado grows in size. Within minutes of touching down, the Gifford tornado intensified to an EF-2 tornado on the Enhanced Fujita Scale (111-135 mph winds), capable of removing large sections of the roof and collapsing the exterior walls of well-built homes. A combination of moderate atmospheric instability (warm, moist air at the surface and cold air aloft) and strong vertical wind shear (changes in wind speed or direction with height) put central Illinois at a high risk for long-tracked violent tornadoes on Nov. 17, 2013.
12:51 p.m. Nov. 17, 2013: The tornado becomes a wedge tornado, appearing wider than it is tall, although the circulation actually extends at least a few miles up into the cloud. The tornado reached its maximum intensity of EF-3 with 140 mph winds southwest of Gifford and produced EF-2 damage in town. The tornado was on the ground for 29.7 miles, had a maximum path width of 0.5 miles, and lasted for 30 minutes. It is important to remember that you can't tell the intensity of a tornado just by looking at it; if you are under a tornado warning, seek a substantial shelter.
Frame will deliver a talk at 7 p.m. Wednesday about severe weather and preparedness at the Beckman Institute, 405 N. Mathews Ave., U.