Struggling to Categorize an Epic Tornado Outbreak

May 9, 2011 · 1 comment

The numbers practically defy comprehension: 327 people killed in a single day; nearly that many reports of tornadoes that fateful day, April 27, 2011, when nature went on a spectacular rampage across the South; violent tornadoes in Mississippi, Alabama, Tennessee, and Georgia that generated winds of 180 mph, 190 mph, over 200 mph, ripping bark from trees, pavement from roads, even earth from the ground (video from The Weather Channel explains the scouring), and people from their homes and businesses, in cities and towns small and large; tornadoes that chewed up and spat out lives and belongings, mile after mile, for 132 miles straight in one twister. Having watched similarly violent yet lesser events unfold over the years and decades as warning times have steadily increased, in some cases long enough to jump in your car and drive out of harm’s way, it did not seem possible we’d ever again witness such devastation and widespread death from tornado winds, especially in a nation that has the best warning system for severe weather in the world. As has become painfully clear these last two weeks, however, one has to look way back to 1925—more than an average human lifetime ago—when either a single, marathon twister or a family of vicious vortices known as the Tri-State Tornado thundered across 219 consecutive miles of the Midwest and consumed nearly 700 lives—to find similarly overwhelming tragedy from tornadoes in the United States in a single day.

Annual tornado climatology and statistics for our nation make the historic events of April 27, 2011, even more difficult to believe: 60 people on average lose their lives to tornadoes each year–and for the past decade we’ve considered that number to be excessive; it takes 365 days annually to produce the 1,300 tornadoes we usually have, yet almost a quarter of those may now have happened in just a single 24-hour period (there were 292 reports of tornadoes on April 27, and the NWS has confirmed 143 so far); and fewer than 1% of all tornadoes recorded on our soil over decades ever churn with such violence that they are rated in the top tier of the Fujita tornado intensity scale, regardless of its evolving criteria … yet we had at least three of these most destructive tornadoes strike Mississippi and Alabama on this recent deadly April day.

Stunned weather enthusiasts and meteorologists alike have been searching for more than a week for ways to classify this maelstrom of tornadoes. Some have tried to fit this event into the mold created by the most prolific and extraordinary tornado event in U.S. history—the Super Outbreak of April 3-4, 1974, which in just 18 hours delivered 148 tornadoes that tore up more than 2,500 miles of 13 Midwestern, Southern, and Eastern states and  included 30 tornadoes rated violent 4s and 5s at the top end of the original Fujita tornado intensity scale.

Bloggers are beginning to offer comparisons between April 27 and that historic day, nearly four decades ago, especially because the Super Outbreak’s toll of 330 killed is comparable. The event two weeks ago is considered a “textbook” tornado outbreak: “I mean, literally what I learned from a textbook more than 30 years ago,” writes Senior Forecaster Stu Ostro of The Weather Channel (TWC) on his blog. “Not only were the (atmospheric) elements perfect for a tornado outbreak, they were present to an extreme degree,” he notes. It was of the caliber defined by the Palm Sunday outbreak of 1965, the blitzkrieg of twisters across the Ohio Valley on May 29-30, 2004, as well as the Super Outbreak, which Greg Forbes, TWC Severe Weather Expert, says has since become the benchmark for all tornado outbreaks. Indeed, just last year Storm Prediction Center (SPC) tornado and severe weather forecaster Steve Corfidi, et al., published the article “Revisiting the 3–4 April 1974 Super Outbreak of Tornadoes,” in which they state: “The Super Outbreak of tornadoes over the central and eastern United States on 3–4 April 1974 remains the most outstanding severe convective weather episode on record in the continental United States.” What happened on April 27, 2011, however, may have rewritten the tornado textbook, and reset that benchmark, states Jon Davies, a meteorologist and noted Great Plains storm chaser who has been studying severe weather forecasting for 25 years. He wrote in a blog post last week that the setup for tornadoes on April 27, 2011 was “rare and quite extraordinary.” The combination of low-level wind shear and atmospheric instability was “optimum,” creating a “tragic and historic tornado outbreak [that] was unprecedented in the past 75 years of U.S. history, topping even the 1974 ‘Super Outbreak.’”

That comparison is the ongoing debate as the severity of the tornadoes defining the outbreak of April 27 comes into better focus. So far, 14 of its tornadoes have been rated 4 or greater on the Enhanced Fujita (EF) Scale. With more than twice that many in the Super Outbreak, which were rated using the earlier Fujita tornado intensity scale (compare the two scales), it would seem this new event falls short. But Forbes cautions that likening the April 27 tornado outbreak to the Super Outbreak simply using number of violent tornadoes has its pitfalls.

“I was part of the team that developed the EF Scale, and it’s a system that more accurately estimates tornado wind speeds. But it troubled me then (and still does) that it might be hard to compare past tornado outbreaks with future ones and determine which was worst. It’s apples and oranges, to some extent, in the rating systems then and now.”

That’s a statement that could be applied to today’s severe weather forecasting, technological observation, and warnings for severe weather, which have improved significantly in the last 37 years. In his blog post, Recipe for calamity, NCAR meteorologist Bob Henson looks beyond the ingredients that caused the April 27 tornado outbreak and seeks to consider why the human toll of this tragedy was so massive. He mentions that some parts of the South had been hit by severe thunderstorms and damaging winds earlier that day, which knocked out power and may have disrupted communications in areas that tornadoes barreled through later in the afternoon and evening. The speed of the twisters as well as the lack of safe places to hide from such extreme winds also likely fed the outcome. Of note, too, is mention that perhaps the density of the population centers hit by such dangerous winds resulted in a far greater toll than otherwise would have been realized. This insight comes from Roger Edwards, a forecaster and tornado researcher with SPC. Edwards has a long track record of trying to better educate a populace about the realities of severe storms and tornadoes. He blogged about the catastrophe on April 27, 2011, offering this matter-of-fact consideration: “When you have violent, huge tornadoes moving through urban areas, they will cause casualties.” Edwards then follows this statement, in a subsequent post on the April 27 tornado outbreak, with a poignant look at where we seem to be in the inevitable collision of an expanding population with bouts of severe weather:

“I can say fairly safely that a major contributor here clearly was population density. Even though 3 April 1974 affected a few decent sized towns (Xenia, Brandenburg) and the suburbs of one big metro (Sayler Park, for Cincinnati), a greater coverage area of heavy developed land seemed to be in the way of this day’s tornadoes. How much? I’d like to see a land-use comparison. Harold Brooks and Chuck Doswell [current and retired tornado scientists with the National Severe Storms Laboratory, respectively] have discussed the phenomenon of sprawl as a factor in future tornado death risk and the possible nadir of fatalities having been reached…and the future seems to have arrived.”

Viewing this spring’s deadly onslaught of severe weather through that lens makes it all too clear we’ve been here before: recall a late-August Monday on the central Gulf Coast nearly six years ago shattered by an almost unimaginable, hurricane-whipped surge of seawater that drowned coastal communities in southeast Louisiana, Mississippi, and Alabama. Ostro prefers to use the perspective that event gave us when sizing up the April 27 tornado event:

“So that makes this the Katrina of tornado outbreaks, in the sense that it’s a vivid and tragic reminder that although high death tolls from tornadoes and hurricanes are much less common than they were in the 19th and first part of the 20th centuries, we are not immune to them, even in this era of modern meteorological and communication technology.”

A focus on the human toll of the April tornado tragedy might be the facet that really sets this outbreak apart. One of the most remarkable reports came from Jeff Masters’ blog entry of May 5, 2011—eight days after the outbreak. Noting the extreme number of deaths from the tornadoes and how the toll had fluctuated because some victims were counted twice, he wrote: “There are still hundreds of people missing from the tornado, and search teams have not yet made it to all of the towns ravaged by the tornadoes.” Hundreds missing. Even if that number is off by a factor or 10, this outbreak will be historic for its toll in human lives. Almost two weeks since the outbreak, Alabama’s Emergency Management Agency continues to post daily situation reports in which it states that the number of casualties in the state is “to be published at a later time.” The reasons are spelled out vividly in this blog post from The Huntsville Times, which details accounts from survivors of perhaps the day’s most violent tornado, an EF5 with winds estimated at 210 mph that destroyed much of Hackleburg, a town of 1,600 people in northwest Alabama. At least 18 people lost their lives there. Twenty-one-year-old resident Tommy Quinn said only one person died on his street, the Times reports. But six days after the tornado obliterated the area, a neighbor still hadn’t been found. “We can’t even find her house,” Quinn said.

As is evident in this blend of writings and perceptions, the devastatingly deadly tornado outbreak of April 27, 2011, was an epic event, rivaling even the most revered such disaster in the modern era. What these experts and others are finding, though, is that while the April 27 outbreak was indeed exceptional in many ways, and history will likely reveal it to be a meteorological event unprecedented in some aspects, it seems the Super Outbreak of 1974 won’t be stripped of its meteorological prominence, at least not fully. Instead, the tornado outbreak that occurred on April 27, which was truly remarkable, will be referred to in its own way, likely earning a signature nom de guerre through its shockingly violent legacy.