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Making Sense of the Moore Tornado in a Climate Context

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The devastating tornado that ripped apart Moore, Okla., on Monday now joins the ranks of America’s strongest twisters on record, coming almost exactly two years after a similarly extreme and deadly tornado struck Joplin, Mo. In trying to make sense of the tragedy that unfolded in Moore, here are some of the things we know and don't know about tornadoes, and whether or how climate change may be influencing them now and into the future.

Tornadoes are no stranger to the U.S., which sees the majority of the world's tornadoes. The heart of "Tornado Alley" — where warm, moist air collides with cooler, drier air coming from the West — runs right through Moore, and the town was severely damaged by a tornado in 1999 as well as by less damaging twisters since.

Based on data from 1982-2011, Oklahoma City was the likeliest spot in the country for seeing severe thunderstorms on May 20. Tornado statistics show that the Oklahoma City metro area has had the most direct tornado hits of any American city, with at least 100 since 1890. That's according to the Storm Prediction Center in Norman, Okla., which is situated just down the road from Moore, and whose forecasters were forced to take shelter as the storm moved through.

Probability of severe thunderstorms within 25 miles of a location as averaged from 1982-2011. This shows the highest odds of severe weather on Monday were in Oklahoma.
Credit: Storm Prediction Center.

In recent years, tornado researchers and climate scientists have been trying to make advances in unlocking the secrets of what causes such monster storms, and how manmade global warming may already be affecting them now and in the future. 

While the understanding of climate and tornadoes is progressing slowly, far more rapid gains have been made in the ability to forecast tornado outbreaks days in advance, and to detect and warn people when (or preferably before) tornadoes touch down. During the past two years, the National Weather Service upgraded its nationwide network of Doppler radars to take advantage of dual-polarization technology, which allowed forecasters in Oklahoma to provide an exceptional lead time of 16 minutes before the tornado struck Moore — far more than the average tornado warning lead time of 13 minutes. 

Tornado data does not reveal any clear trends in tornado occurrence or deaths that would suggest a clear tie to global warming, at least not yet. A recent paper published in the Bulletin of the American Meteorological Society found that the occurrence of EF-1 and stronger tornadoes on the Enhanced Fujita Scale has shown no trend since 1954, which was the first year of near real-time data collection. Instead, an increase in tornado counts of EF-0 or stronger tornadoes has been attributed to an uptick in observations of very weak tornadoes. The Enhanced Fujita Scale measures tornado strength based on the extent and type of damage that they cause (no surface weather station has ever survived a direct tornado strike to take wind measurements from inside a twister).  

Number of annual EF-3 or greater tornadoes from 1954 to 2012.
Credit: Storm Prediction Center.

It's difficult to tease out trends from historical tornado data, due to changing reporting practices, population growth, and the advent of advanced radar technology that has allowed meteorologists to spot more tornadoes now than ever before. It's thought that many tornadoes — particularly weaker ones — were missed in the early decades of recordkeeping, and construction methods have also changed with time. That's important since the strength of a tornado is determined by post-storm damage surveys.

“We have little confidence in the accuracy of trends in the meteorological occurrence of severe thunderstorms (including hail storms) and tornadoes,” according to a recent study published in the Bulletin of the American Meteorological Society. 

Similarly, there is no evidence to indicate that EF-4 and EF-5 tornadoes — like the one that decimated a large swath of Moore — are becoming more frequent or severe. Such tornadoes are rare — they comprise less than 1 percent of the total number of tornadoes — yet they are the most reliable killers, accounting for 70 percent of tornado fatalities. The record annual number of EF-4 and EF-5 tornadoes occurred in 1974, when 36 such tornadoes scarred the landscape of the Midwest and Great Plains. Between 2000 and February of this year, there were 129 EF-4 and EF-5 tornadoes, according to a Storm Prediction Center database.

The number of tornadoes and tornado-related fatalities varies greatly from one year to the next, owing largely to natural climate and weather variability and where tornadoes happen to strike. (Given population growth, the chance that a tornado will strike a heavily populated area, such as Moore, is increasing.)

For example, the record 2011 tornado season, which featured a whopping 1,691 tornadoes — the second highest number on record — and 553 fatalities, was followed by one of the least active seasons on record the next year. According to the Storm Prediction Center, in 2012 there were just 939 tornadoes and 70 fatalities.

Large-scale climate patterns can influence tornado seasons. In 2011, there was a La Niña event in the tropical Pacific Ocean, which has been linked to active tornado seasons in the U.S. due to the way it influences the jet stream. In 2012, much of the country was hit with widespread drought, which dried up the severe thunderstorm season just as it dried up wheat fields across Tornado Alley. The drought continued into the first half of this year, before shrinking back to the west, and this tornado season got off to a very slow start. In fact, between 2012 and 2013, the U.S. had the longest-ever streak of days without a tornado fatality

Because historical tornado data is not considered very reliable or consistent, scientists have focused especially closely on how a warming climate is altering the balance of ingredients that go into producing a tornado.

The results of a computer-modeling study comparing the projected summer climate in 2072–2099 from 1962–1989. CAPE is predicted to rise enough to overwhelm a slight decrease in vertical wind shear, leading to an increase in severe thunderstorm days, particularly in the eastern states. 
Credit: NASA Earth Observatory.

The predicted decrease in vertical wind shear from the same modeling study.
Credit: NASA Earth Observatory.
 

A key ingredient for producing tornadoes is a warm, moist, and unstable atmosphere. That means there needs to be high levels of humidity, and conditions in the middle to upper layers of the atmosphere need to encourage a rapidly rising motion of air. Such conditions were present on Monday, as demonstrated by the sky-high reading of one metric that meteorologists use to quantify atmospheric instability — known as “Convective Available Potential Energy,” or CAPE.

The CAPE reading in central Oklahoma was approaching 4,000 joules per kilogram on Monday. CAPE is a measure of the potential energy available for storms, and how much warm, moist and buoyant air is present in a given area. A CAPE value above 2,500 is considered to be an indication of extreme instability in the atmosphere, which means that if a trigger comes along for a storm, such as a cold front, storms could develop rapidly and quickly turn severe.

The high amount of instability on Monday helped thunderstorms explode along a slowly moving frontal boundary separating hot, humid air to the east from comparatively cooler and drier air to the west. This satellite loop shows the massive mushroom-like clouds, which reached heights of up to 60,000 feet above the surface, forming suddenly, as if lit by a fuse on Monday afternoon.

Satellite loop of the severe thunderstorms that spawned the Moore tornado. Credit: NOAA.

Climate studies show that atmospheric instability has already been increasing in some parts of the U.S., although not by a sufficient amount to make it distinguishable from natural variability, at least not yet. Computer-model projections of how instability may change in the future, though, show that this tornado ingredient is likely to increase because of warming surface temperatures and the addition of moisture in the air through evaporation. That increase in atmospheric instability could boost the number of days with severe thunderstorms in parts of the U.S.

One study, published in the Proceedings of the National Academy of Sciences in 2007, found that a doubling of greenhouse gases in the atmosphere would boost the number of severe thunderstorm days in parts of the U.S., particularly in the Southeast and along the East Coast. Some cities, including New York, could see twice as many potential severe thunderstorm days by the end of this century as they experience today.

Another necessary ingredient for tornadoes to form is atmospheric wind shear, which occurs when winds change in speed or direction with height. Wind shear was also present in abundance on Monday, which is why weather forecasters had highlighted that region as the most at risk for significant tornadoes.

In fact, some studies, including a paper published in the journal Atmospheric Research in April, have shown that wind shear is a more critical ingredient for strong tornadoes to occur than atmospheric instability, suggesting that a decrease in shear could limit the potential for the strongest tornadoes to occur in the future.

The author of that paper, Harold Brooks of the National Severe Storms Laboratory in Norman, Okla., gave a video interview at a tornado and climate research conference held at Columbia University earlier this year:


Credit: IRI.

While a warmer climate is likely to feature more opportunities for thunderstorms to form, studies also show a lessening of atmospheric wind shear, which would suggest a decrease in the potential for tornadoes to form. How these two trends play out — one increasing the odds of tornadoes, the other reducing them — is a subject of active scientific research. 

A fact sheet from the National Oceanic and Atmospheric Administration on tornadoes and climate change describes the counteracting trends of decreasing shear and increasing instability in a warming world as a "tug of war."

One inference from the research to date is that given the likely abundance of instability in coming decades, when adequate wind shear to produce tornadoes is also present, tornado outbreaks could be even more extensive than they are today, although by how much is a question yet to be addressed. Advances in computer modeling will may soon enable scientists to better simulate small-scale events such as tornadoes, and the conditions that trigger them. 

The bottom line? So far, there’s simply not enough information to say anything definitive about the future of tornadoes under climate change. But every thunderstorm, and every tornado, now takes place in a warmer, wetter atmosphere due to the increasing concentration of greenhouse gases in the atmosphere. 

Related Content
Tornadoes Rake Oklahoma, Kansas as Storm Threat Continues 
U.S. Sees Record Low Tornadoes and Tornado Deaths
Deadly Georgia Tornado First In a Record 220 Days
Tornado Outbreak Raises Climate Change Questions

Comments

By Eric Peterson (Front Royal, VA 22630)
on May 22nd, 2013

Outstanding article Andrew.  You are solid as always on this topic.  The only thing I would add is that while the air in western OK was only a bit cooler, it was a lot drier.  The push of that dry air into the midlevel of the supercell (from the sheer you talked about) is a key ingredient in this type of event.  Geographically speaking the dry air east of the Rockies along with the warm moist air from the Gulf is what gives us tornado alley, unique in the world.

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By Jan Galkowski (Westwood, MA 02090)
on May 22nd, 2013

Yes, people unfortunately need to be a bit sophisticated about this.  If there is a probability distribution of extreme weather events, and there were a set of fixed parameters describing such a distribution when CO2 was 288 ppm, the pre-industrial level, then when human-cause climate change kicks in, due to CO2 to high 390 ppm, it will change the parameters of that distribution.  Consequently, an inference which attributes specific weather phenomena to climate change needs to be able to differentiate the present state of affairs and parameters from those at the 288 ppm level. 

There are a couple of implications of this.

First, depending upon variability of the phenomenon in question, and assuming variability is the same at 288 ppm and now, it may or may not be possible to “see” the change in a statistical manner.

Second, climate may REDUCE the frequency of extreme events as well as increase them.  This is something I first saw pointed out by Professor Myles Allen. The following is a hypothetical example, but consider what happens to Bangladesh if Himalayan snow pack evaporates.  Their incidence of flash floods may decrease, due to drought, but the incidence of flooding due to larger storms and sea-level rise may increase.

Third, all this said, it exposes a limitation of traditional fairly knowledge-free statistical approaches to inferring attribution.  If, now, mechanisms of global mesoscale weather are brought into the inference as a model, the inferential power can increase.  As far as I know that hasn’t as yet been done, but there are mesoscale models of this kind being constructed by Professor Jennifer Francis and students at Rutgers, sketched in her presentation earlier this year at http://www.youtube.com/watch?v=xugAC7XGosM

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By Michael Fjetland (Sugar Land)
on May 22nd, 2013

Being prepared for these killers means improving our building codes. Architects still ignore the weakest link of every building - its windows. When debris hits them they blow out, causing internal pressure that blows the roof off ,etc. Check out our blog on the science of how this works at the link:
www.ArmorGlass.com

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By Eli Rabett (washington DC 20003)
on May 22nd, 2013

There are at least three climatic effects, humidity, wind shear and jet stream.  All three are being affected by climate change, but in different directions in different places.  Less humidity and windshear in OK, but the jet stream shifting north (see J. Francis) will increase the risk.  In the northern MW and northeast all three will positively increase the probability of tornadoes.

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By Eric Peterson (Front Royal, VA 22630)
on May 22nd, 2013

The jet stream shifting north will decrease the risk, not increase it.  Jennifer Francis is (loosely) talking about increased meridional flow, i.e. more southerly jet stream.  Thus traditional (pre-Francis et al) models predict fewer strong tornadoes because the jet stream would be very far north.

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By Jan
on May 22nd, 2013

Thanks once again for giving climate change deniers the chance to say there is nothing to worry about. No wonder people think this is a hoax. A paragraph I read in a report by Jeff Masters of Weather Underground stated that the Moore tornado was on the ground for 40 minutes moving about 20 to 25 MPH allowing for more damage (which was unusual), whereas violent tornadoes move at about 60 MPH. I find that to be a potential correlation to the fact that Hurricane Sandy which was tied to extenuating circumstances due to climate change was also moving at about only 10 MPH because of a meandering jet stream influenced by Arctic sea ice loss that kept the front in place longer. For the life of me I can’t understand why the eggshells are being walked on regarding the ability of tornadoes to be more intense or held in those types of fronts as well as they are influenced by the same atmosphere that all other weather events are. And also considering where we now stand globally regarding the effects we are seeing do we really have time to sit debating whether it or isn’t? So 97 % of climate scientists are in consensus that humans have changed the atmosphere and that more moisture has been added to it… yet, tornadoes somehow manage to escape being part of that same atmosphere. It strains logic.

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By Andrew
on May 22nd, 2013

Jan - Thanks for your comment. There is absolutely no scientific evidence to conclude that global warming caused the Moore tornado to move more slowly than many other violent tornadoes, and Jeff Masters didn’t tie that factor to global warming either. Violent tornadoes and weak tornadoes move at varying speeds. No two tornadoes are exactly the same, they’re kind of like snowflakes that way.

Also, Hurricane Sandy was moving closer to 25 mph when it made landfall, not 10 mph.

My job is not to think of how climate change skeptics will interpret a story - it’s to tell the scientific facts about climate change, and in this case, climate researchers and tornado experts are working hard to learn more about tornadoes and climate change, but not much is clear at this time.

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By Steven McQuinn (Salt Lake City UT )
on May 23rd, 2013

I wonder if any researchers are approaching the issue from the other end, asking what kind of climate change signal, other than the obvious ones like patterns of intensity and frequency, should we be looking for in the record of weather events like thunderstorms and tornadoes. I know this question seems obscure, because it is asking what obscure evidence are we missing because we don’t think of it as evidence. Perhaps the focus on extreme events is too limiting. Perhaps there has been a shift in the fractal scaling of events for a given weather type. Perhaps the geographical distribution or timing of events has shifted subtly. I’m not a climate scientist but I think that data often has a lot more to tell than we usually ask of it.

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By Chris Machens (ClimateState.com)
on May 23rd, 2013

As with hurricanes, I think frequency needs to be separated from intensity.

Climate change increases the available energy for tornadoes through a warmer and moister atmosphere. Wind shear decreases in the global mean, but this might be irrelevant locally when the jet stream dives southward like it did last weekend across the Plains.

“I believe there is evidence that the strongest tornadoes are getting stronger. They are certainly getting longer and wider.
James B. Elsner, an atmospheric scientist at Florida State University http://climatestate.com/2013/05/21/humid-air-and-the-jet-stream-help-to-fuel-more-intense-thunderstormstornadoes/
Humid air and the Jet Stream help to fuel more intense thunderstorms/tornadoes

Re above image “Number of annual EF-3 or greater tornadoes from 1954 to 2012.” - yet the impression you get when accoutning for “all” tornadoes since the 50’s is the opposite, see today’s Master’s post, graph on this http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=2415

“In 2011, there was a La Niña event in the tropical Pacific Ocean, which has been linked to active tornado seasons in the U.S. due to the way it influences the jet stream”
This article here forgets to outline shortly the broader impacts on the jet stream, from climate warming. Because as Eli Rabbit points out above, the jet stream is part of the puzzle.

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By Mike Armstrong (OKC/OK/73111)
on May 23rd, 2013

The advancement of dual-pol had nothing to do with the lead time of the tornado warning. This is not what dual-pol radar is designed to do. It is designed to classify the various types of hydrometeors, identify debris signatures, and improve rainfall estimates.

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By Andrew Freedman (Brooklyn)
on May 24th, 2013

Mike - you are totally right that dual-pol was not designed to improve lead times on tornado warnings. However, it is still used in the warning process, since it can confirm a tornado is on the ground and doing damage. In the case of the Moore tornado, it helped the NWS decide to issue very strongly worded statements and warnings (along with spotter reports, of course).  -A

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By Eric Peterson (Front Royal, VA 22630)
on May 26th, 2013

Climate change increases the available energy for tornadoes through a warmer and moister atmosphere. Wind shear decreases in the global mean, but this might be irrelevant locally when the jet stream dives southward like it did last weekend across the Plains.

Chris, your second sentence that I quote above is the appropriate context for the first.  It is true that global wind shear decreases don’t matter, but the models also show local wind shear decreases (map of the US with lots of blue above).  The key is your phrase “when the jet stream dives southward”.  That is primarily a natural phenomenon.  See the recent article at skepticalscience: http://www.skepticalscience.com/news.php?n=1967 The argument in the article that surface gradient dictates the strength of the jet is weak.  More likely the dynamics of the jet are related to the vertical gradient and the upper troposphere gradient.

Both of those gradients are increasing .  The increase in those gradients means a stronger jet and a stronger jet has fewer southward dives.  In fact climate models predict a stronger jet with fewer southward excursions.  It is one of the reasons that climate models predict so much warming (more than has been observed).  In fact a diving jet and blocking patterns are associated with global cooling.

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By Daniel Miles (Weyburn, Saskatchewan, Canada S4H 1W7)
on May 26th, 2013

What seems to be ignored by all the data crunchers is climate changes from nice to nasty when saying ‘rule of law’, or saying “Peace and safety” in ‘rule of law’, especially in the age of grace; And it changes suddenly, as noted in 1Thessalonians 5:3, with not escape for the they sayers. We’ve seen it with Katrina paying a visit to New Orleans, and with Sandy paying a visit to New York; both places having ‘New’ in their name; but not the new and living way, rather the new and dead way, aka still law contained in flaw. Law worketh wrath: Romans 4:15, and wrath is the second of four consequences in Romans 2 for those remaining contentious. But wrath is not what God has appointed us to in 1Thessalonians 5:9. So the solution to these tragedies is flush law as dung and tornadoes, hurricanes, etc will vanish. If not, then the next consequence in Romans 2 is tribulation, followed by anguish. Selah America.

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By David Werth (Salem, OR 97302)
on May 27th, 2013

Eric - I’m not sure you can ignore the apparent effects of diminishing Arctic sea ice on the jet stream.

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By Ashjaei (Tabriz / East azarbijan / Iran)
on June 8th, 2013

With best regards
I’ll be glad to receive your articles about weather and climate and any news about weather forecasting ( methods and ...) and any other useful document you can send .
Sincerely yours
ashjaei

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