Moore Tornado Meteorology: Not Unusually Strong, No Definitive Climate Change Link Yet

on May 21 2013 11:38 AM
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A storm chaser posted this image of the massive tornado crossing the interstate. Ben Holcomb via Twitter

The tornado that ripped through Moore, Okla., on Monday may rank as one of the most destructive tornadoes in history, but it was not an unusually strong tornado, experts say.

The tragedy outside Oklahoma City comes amid a resurgence in tornado activity over the weekend, following what has been a relatively quiet season thus far.

The National Weather Service outpost in Norman, Okla., says that Monday’s tornado was at least an EF-4, meaning it had winds of between 166 and 200 miles per hour. But depending on the results of damage surveys, this rating could get bumped up to an EF-5, the highest on the Enhanced Fujita scale, which measures tornado strength, according to Weather Underground meteorologist Jeff Masters.

Violent tornadoes with a rating of EF-4 or higher are rare events, by definition, with about 10 per year occurring in the U.S., National Oceanic and Atmospheric Administration researcher Harold Brooks wrote in an email.

But “to the extent that a 10 [per] year event can be common, this looks like a normal violent tornado,” Brooks wrote. “The winds are [around] 200 mph or greater. It was on the ground for a not-unusual period of time. In general, a lot of bad luck.”

The tornado touched down at about 4 p.m. EDT on Monday and was on the ground for about 40 minutes. That seems pretty long, but it pales in comparison to the record for longest tornado duration, currently held by the 1925 Tri-State Tornado, which stormed through Missouri, Illinois and Indiana for at least 219 miles over 3 and a half hours.

The confirmed death toll from the Oklahoma tornado had at one point been given as at least 91 by the Oklahoma City medical examiner, but on Tuesday that number was revised to 24, according to the New York Times. It appears that some victims may have been counted more than once in the chaotic aftermath of the tornado. The new fatality figure includes at least seven children, according to the Associated Press.

Two elementary schools were hit by the twister, and entire neighborhoods in Moore were leveled.

Tornadoes in recent years have been deadly, but improved warning systems are probably the reason that twisters no longer kill hundreds at once. The 1925 Tri-State Tornado killed 695; a 1936 tornado that struck Tupelo, Miss., killed 216; another tornado in April 1936 killed 203 in Gainesville, Ga.

The National Weather Service noted that the tornado that devastated Moore took a path similar to another tornado that hit outside Oklahoma City on May 3, 1999:

moore_nws Preliminary, unofficial comparison of a May 3rd 1999 tornado path to today's tornado path, from the NWS outpost in Norman, Okla.  NWS via Facebook

That twister, called the 1999 Bridge Creek-Moore tornado, was also fairly destructive, killing 36 and injuring 583. At the time, it was the largest death toll from a tornado in more than 20 years. The 1999 storm had the highest wind speed ever recorded for a tornado, 318 miles per hour. The tornado also caused about $1.4 billion in damage, making it the fourth-costliest tornado in history. Monday’s tornado may also land on that list.

The similar tracks taken by the 1999 tornado and Monday’s twister might lead you to think that Moore is a magnet for tornadoes, but a National Center for Atmospheric Research tornado expert says the area’s geography (other than lying in Tornado Alley) likely isn’t to blame. "If there were geographic features that would tend to cause multiple tornadoes every few years… [then] why has this been happening only since 1999?" Bob Henson told NBC News.

In 2001, NOAA researcher Brooks published an analysis of the 1999 tornado and noted that 11 of the 36 deaths occurred in mobile homes – even though many more permanent homes were damaged.

“The problem of mobile home safety is the biggest obstacle to reduction of tornado deaths in the United States,” Brooks wrote. “The rate of death for mobile home residents is relatively close to the pre-1925 values in the United States.”

Some people may speculate on links between climate change and this latest tornado, but the scientific evidence isn’t quite there yet to make a connection.

In 2011, following a particularly active tornado season – the one that included the devastating tornado that struck Joplin, Mo. –  NOAA examined the climate-tornado link and found that there wasn’t enough data to support it.

“A change in the mean climate properties that are believed to be particularly relevant to major destructive tornado events has thus far not been detected for April, at least during the last 30 years,” NOAA researchers wrote. “So far, we have not been able to link any of the major causes of the tornado outbreak to global warming.”

However, global warming is thought to contribute to a greater uptick in severe weather events overall.

Weather Underground’s Masters noted that the tornado drought at the beginning of the season, two years after one of the most active seasons in memory, is an unusual pattern in and of itself.

“The extraordinary contrast underscores the crazy fluctuations we've seen in Northern Hemisphere jet-stream patterns during the past three years,” Masters wrote on May 8. “Call it ‘Weather Whiplash’ of the tornado variety.”

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