December 30, 2011

Was the Joplin Tornado the Deadliest We Can Expect?

Filed under: Severe — Erin @ 10:58 pm

Meteorologists and weather-watchers are bidding the year 2011 a less-than-fond farewell.  While it was certainly a banner year from the point of view of storm chasing—6 EF-5 tornadoes, 17 EF-4s, and many of them highly photogenic, as the dozens of home videos on Youtube illustrate—it was a catastrophe in terms of the human impact.  With 552 fatalities, this year is tied for the second-deadliest tornado year in the U.S.  The death toll is an order of magnitude greater than even most of the “bad years” of the 1975-2010 period.  Two events are primarily responsible for this:  the April 27 Dixie Super Outbreak, which killed over 300 people (breaking the 1974 Ohio Valley Super Outbreak’s grim record by a hair), and the Joplin, MO EF-5 tornado, with approximately 160 fatalities.

With the 2011 Super Outbreak, meteorologists are starting to work out an approximate historical return period for these large-magnitude events.  Before the 1974 event, the last comparable event occurred in 1936, with an outbreak popularly known as the Tupelo-Gainesville outbreak for the violent tornadoes that occurred in Mississippi and Georgia.  It seems that these huge events occur approximately every 35-40 years.  Obviously, a comparable event could occur next spring, but statistically, it seems that they are a 35- to 40-year event.  And, given that the 1974 Super Outbreak and 2011 Super Outbreak saw comparable death tolls, I think we can also estimate what the human toll for such an event will unfortunately be as long as the affected communities have unsuitable safety options for EF-4 and EF-5 tornadoes.

The Joplin tornado is a different beast.  We do not have a comparable modern event.  Individual tornadoes in 1953 killed over 100 people in Waco, TX and Flint, MI, but that year was something of a catalyst of public outrage, for a third tornado in Worcester, MA killed 94 people.  Public sentiment that year was essentially, “DO something so that this never happens again!”  And for 57 years, no single tornado in the U.S. did kill over 100 people.  Then… it happened again.

Was the Joplin event a worst-case scenario?  Is this the deadliest (give or take) that a single tornado can actually be now?

I think the answer to the first question is a guarded “yes,” at least for the specific case of a tornado striking a city.  The tornado was about as strong as they come; its winds were estimated to be up to 250 mph.  They can get more intense than that, but it doesn’t make a lot of difference in terms of structural damage.  The tornado rapidly intensified precisely as it entered the heavily populated regions of Joplin, and it passed right through residential and commercial shopping areas—the worst areas it could strike.  Examination of the track shows that there was also a pretty large corridor of EF-4 and EF-5 tornado damage, which would be expected for a wedge tornado.  Sometimes the area of violent damage is comparatively small, but this was not the case with this tornado.  Storm cellars were rare in this area, making survival above ground mostly a matter of good luck.  The tornado was also rain-wrapped for much of its existence.  In terms of the storm’s power and the location of impact, you can’t get much worse than this.  However, I should note that it occurred on a Sunday.  Some have argued that if it had happened at the same time of day on a work day, it could have been worse.  We don’t know for sure, and let’s hope we don’t find out.  I tend to think it probably would not have been much worse, given that residential areas (not a likely area for commuters to be stranded) and the shopping district (which probably would get more foot traffic on weekends than work-week afternoons) were such a large part of the damage zone.  In my opinion, the Joplin tornado was essentially a worst-case scenario for a tornado striking an urban area.  A comparable tornado striking an urban area probably would have a comparable human toll.

Unfortunately, the second question—is the death toll of ~160 the highest we could see for a single tornado in the modern era—has a different answer.  There are two ways that a single tornado could kill a lot more people than that.

One is the possibility of a weak, poorly-built or dilapidated high rise building taking a direct hit from a violent tornado and collapsing with a lot of people inside it.  Generally, these buildings are not supposed to collapse even in EF-5 events.  Images of collapsed high rises on hurricane landfall sites are misleading; these buildings mostly had shallow foundations and were undermined by the storm surge.  They were not blown over by wind alone, and storm surge is obviously not a factor for tornadoes.  The St. John’s Hospital building in Joplin took a direct hit from the tornado when it was at EF-4 intensity and it did not collapse.  However, a poorly-constructed or dilapidated one could.  (As an aside, one does have to wonder about the possibility of a tornado tearing up ground several feet deep, as happened in the EF-5 tornado on April 27 in central Mississippi. This could definitely undermine a slab foundation on a house, resulting in the foundation being ripped from the ground—the supposed hypothetical “F6 intensity” signature that one heard bandied about prior to the adoption of the Enhanced Fujita Scale.  However, high-rise buildings have much deeper foundations than residential homes.)

The other possibility is that of a violent tornado striking a crowded spectator event, such as a sports game, a fairground, a speedway, etc.  This possibility has been discussed at length by meteorologists such as Dr. Roger Edwards of the Storm Prediction Center.  It’s almost happened before, in fact; in 2008 an EF-2 tornado in Atlanta, GA struck the Georgia Dome while a basketball game (involving my college team) was going on.  It had gone into overtime, so people were not milling around outside.  Still, there are videos from that event of pieces of the roof collapsing and falling to the floor while the spectators were left to fend for themselves in the stands.  A stronger tornado could very easily have taken that roof off.

So yes, although the Joplin tornado was very likely a worst-case event for a tornado strike on a city, thereby representing an approximate limit on fatalities for that type of disaster, the potential exists for individual tornadoes to kill far more people than that in a different sort of disaster.  Let us hope that we can deal with the infrastructure and the safety considerations of large venues so that these greater disasters do not occur, either in 2012 or years to come.

February 5, 2011

Lessons of the Super Tuesday Tornado Outbreak of 2008

Filed under: Severe — Erin @ 5:59 pm

2008 OutbreakAt the moment, Arkansas is apparently under a blanket of snow in many areas, and more cold weather and possible winter precipitation is apparently expected. This is interesting to me because at this time not too long ago, Arkansas was essentially “ground zero” for an entirely different kind of destructive meteorological phenomenon, which had also occurred in a La Nina winter. Three years ago, a violent, extensive tornado outbreak began in the South and lower Midwest. Because the outbreak began on “Super Tuesday,” the day when the largest number of state presidential primaries were held, the outbreak was dubbed the Super Tuesday Outbreak. It left in its wake 87 confirmed tornadoes, which killed 57 people—a death toll unheard of for almost 25 years.

The system that produced the outbreak, a low pressure center with a deep warm sector and powerful trailing cold front, moved from Texas to Missouri during the outbreak. Upper-level jets provided decent support, with a jet approaching 120 knots located over eastern Kansas, Arkansas, Missouri, and Oklahoma. Instability was sufficient to promote severe weather in the form of supercells, with 1000 J/kg of CAPE observed. This is not particularly spectacular for the usual tornado season, but it is not often found in cold-season outbreaks. High shear as a result of the jet further fueled the outbreak. The tornadoes occurred on the east side of the cyclone, as is usually the case.

Though it was not as intense as the Super Outbreak of 1974 that produced either 6 or 7 F5 tornadoes (depending on what source you believe), this outbreak was typical of large-scale tornadic outbreaks in that it produced a lot of violent and strong twisters, which normally occur in comparative isolation. In this outbreak, five EF4 tornadoes were observed, including a long-tracked tornado in Arkansas that had a path length of over 120 miles. This tornado alone was responsible for 13 of the fatalities. Another long-tracked tornado, this one an EF3 in Tennessee with a 51-mile path, was responsible for 22 deaths.

This relates intimately to a point that I have been making for several years, and what I think is the principal lesson of this outbreak and its calamitous death toll (for modern times). The South, particularly AR, LA, MS, TN, and AL, is subject to far more violent long-tracked tornadoes than any other region of the country. I mean it isn’t even close. Here is a PDF of a paper by two professional meteorologists about the statistical patterns indicating the real “tornado alley,” at least as far as F3-F5 long-tracked tornadoes are concerned. If you are frightened of statistics, this is a graphic from Jackson, MS that puts up the raw data, the paths of all long-tracked tornadoes rated 4 or 5. It’s not exactly the same data being looked at in the graphic and the paper, but nonetheless, the point that is coming through loud and clear is that the Deep South is the primary breeding ground for the most dangerous, destructive, and deadly kinds of tornadoes that the atmosphere can produce: strong-to-violent long-tracked twisters.

Furthermore, the South is more prone than any other region of the country to have substandard housing that is particularly vulnerable to tornadoes. There is no reason for anyone in a house to die in a weak tornado (an EF0 or EF1), but it still happens, and poor construction or deterioration is to blame for a lot of it. Mobile homes do not help the problem, and it is sickening to me (I guess I still have fragments of a social activist in me) that one’s survival in a tornado, in the United States in the 21st century, could depend on one’s financial status, but this is how it is. Also, most buildings in the South do not have tornado shelters. Even large venues in the South often have no severe weather plan. I’m going to pick on one, the Georgia Dome, which was brushed by an EF2 tornado while a basketball game was going on later in 2008. There was little official word given to the spectators about what was happening until the tornado was actually upon them, making the ceiling shake! There is an infamous YouTube video from inside the stadium as the tornado was passing by, so you can see for yourself what I mean. The only reason that didn’t result in a major disaster was that the game went into overtime, keeping the fans inside the stadium. And finally, unlike the other parts of Tornado Alley (“Dixie Alley” and “Hoosier Alley,” the South and Midwest respectively, are not really separate from Plains Alley), the South does not, in many locations, have an effective local warning system. I don’t mean the issuance of warnings. The weather office in Jackson is very effective at producing warnings. If anything, they overwarn! There is not a problem with the warnings being generated. The problem, rather, is with dissemination to the public during times when the public is most likely not able to know what is going on.

Some areas are better than others at issuing tornado alarms. I can think of one time when Oktibbeha County, Mississippi sounded its alarms for apparently the whole county when the tornado in question was on a far corner and headed out. This is how it happens more often than not when a county has tornado sirens. This kind of thing, in my opinion, is overwarning, and it can lead to complacency, but it is preferable to the alternative. I witnessed that on the New Year’s Eve 2010/2011 tornado outbreak when a very strongly rotating supercell entered Noxubee County, MS around 2:00 AM. It was quickly warned by the National Weather Service. However, the county did not sound its alarms until the tornado was actually on the Mississippi/Alabama border! It was an EF3 tornado, and by that time, it had already done its damage in the rural part of the county. I’ll repeat this: 2:25 in the morning on New Year’s Day, and the tornado alarm was not sounded until the tornado had already passed. This is inexcusable. It is fortunate that no one was killed. Some other areas do not have tornado alarms at all. The major problem in the South, which contributes to the higher death tolls from tornadoes in the South than in any other region of the country (including the other two parts of Tornado Alley), is that of public awareness.

However, in general I do not mean awareness of the event as it is ongoing. Unless it is a nighttime event or is occurring when people are likely to be occupied with something else (such as watching a basketball game or being drunk, as the case may be), I think most people are aware if there is a tornado warning for their county. The local media are still the primary source of people’s information about weather warnings (most people are not plugged into the Internet 24/7, and as horrifying as it may seem to some of you, most of the South is still on dial-up). Most TV stations here interrupt regular programming to provide information. WCBI in the Golden Triangle region of Mississippi is good at providing detailed information about what is happening (though, in my opinion, there is a slight urban bias in what events warrant cuts in programming). When I say that there is a public awareness problem, it is referring more to long-term awareness. Awareness of the risk, which (as the paper and the graphic I linked above indicate) is much higher than they seem to realize. This is the sort of awareness that might lead people to build a tornado shelter underground, designate a specific room in the house to be shored up (which, if done properly, generally provides shelter against up to EF4 winds), and have a plan.

The affordability of shelter is a separate problem, and it is one that, as I said, positively sickens me. Back in the day, after the F5 tornado of May 3, 1999 that hit near Oklahoma City, there was a government rebate for people who purchased tornado shelters. It’s difficult in this day and age to make the case that the government should spend money on ordinary people, it seems, but it really wouldn’t hurt that much for it to at least be a tax write-off.

The Super Tuesday Outbreak demonstrated that, even in the modern era of NEXRAD, real-time weather forecasts, high-speed Internet, cell phones, and PDAs, a tornado outbreak lasting essentially a day and a half can still kill over 50 people. A single tornado, one that was not even rated “violent,” can kill over 20 people. Meteorologists for several years have been saying that there is a tornado awareness problem in the South, which is manifest in public complacency, poor shelter options, and ineffective tornado siren policies. Given that the Deep South is, quite clearly, the bull’s eye for the worst kind of tornado that the atmosphere can produce—long-tracked violent ones—this state of affairs is unacceptable. The Super Tuesday Outbreak will be repeated at some point—if not in the exact same geographical location in the South, then somewhere else in the region, and it will have an aftermath that is the same or worse unless something is done about the problem.
