On Friday 2 March 2012, there was a tornado outbreak in my area in the mid afternoon. Because I live in a house with no basement, I went to school. The university closed at 1:30 but of course, as a faculty member, I have access to my building. The storms came in sooner and faster than I expected; I hoped to get something done in my office before the weather forced me to the basement, but all I did was watch the weather coverage from the local NBC affiliate, streamed on my computer. As it happened, there were two very ugly supercell storms that followed the same path just 10 minutes apart, 15 miles north of me. I watched these as they came past my location; this was at about 3 p.m. The sky was very dark (streetlights came on), and the clouds overhead to the north were churning. I was concerned about a third storm that was going to track over my location. That storm did not prove to be very intense (severe, but routine), but the other two storms were far worse. In fact, they were both tornadic. By the time they were to my north, it was clear that a powerful tornado was on the ground---the weather service issued a tornado emergency warning. This was a long-track tornado, produced by the first cell---it was on the ground for 52 miles, and it was widely visible (not hidden behind rain or hail). I was not able to see it, either because I wasn't quite close enough or because I did not have an unobstructed view of the northern horizon (probably both). But a variety of images of it soon surfaced online. It was 150 yards in diameter.
By 4 p.m. the line of storms had passed to the east, allowing news helicopters to fly over the affected areas. The damage proved to be extensive and extreme. Most of Henryville, Indiana, was destroyed, including the high school. Fortunately, the students had already been sent home, and there was little loss of life considering how extensive the damage was (Henryville has about 2000 residents). But Henryville is close enough to Louisville that many people here, including friends and colleagues of mine, were affected. One of the couples in my Episcopal men's group lost siding and windows on their house (they live near Pekin, west of Henryville). A colleague of mine who lives near Henryville was not hit by the tornado, but they had bad hail damage to their barn (fortunately humans and animals were all right). This fellow was able to take a picture of the tornado from what appeared to be a very uncomfortably close distance, from below their deck. And a friend of mine from church has a son whose friends lost their house in the tornado.
Today, the weather service was able to confirm that the Henryville tornado was an EF4, the next-to-highest category on the Extended Fujitsu Scale (EF0--EF5). Tornadoes in this category produce winds between 175 and 200 mph (EF5 is above 200 mph). Tornadoes of this strength are designated "violent." To determine tornado intensity, a carefully-designed system of damage surveying is used. (The following is from the Wikipedia article.) There are 28 types of structures (e.g., number 2, FR12 = one-family or two-family residence; or number 20, IB = Institutional Building [Hospital, Government or University Building]). These are each given precise "Degrees of Damage," with associated wind speeds. For example, FR12 has 10 degrees of damage; e.g., "2, loss of roof covering material (<20%), gutters and/or awning; loss of vinyl or metal siding." This corresponds to wind of 79 mph. Total destruction of the entire building is degree 10; this corresponds to 170 mph wind. By contrast, IB has 11 degrees of damage, the highest of which is "significant damage to building envelope" and corresponds to 210 mph wind. (This is why I go to campus when severe weather approaches.) The images from Henryville clearly meet the respective criteria for EF4.