Wednesday, April 23, 2014
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A rescue worker checks the rubble in a residential area in Moore, Oklahoma May 21, 2013 after a massive tornado struck the area May 20. Emergency workers pulled more than 100 survivors from the rubble of homes, schools and a hospital in an Oklahoma town hit by a powerful tornado May 20, and officials lowered the death toll from the storm to 24, including nine children. (REUTERS/Richard Rowe)
An aerial view of destroyed homes in Moore, Oklahoma May 21, 2013, in the aftermath of a tornado which ravaged the suburb of Oklahoma City. Rescuers went building to building in search of victims and thousands of survivors were homeless on Tuesday, a day after a massive tornado tore through Moore, wiping out whole blocks of homes and killing at least 24 people. Seven children died at the school which took a direct hit in the deadliest tornado to hit the United States in two years. (REUTERS/Rick Wilking)
Still, the conditions needed to form such a violent and devastating tornado were there and forecasters knew it, warning five days in advance that something big could happen, Brooks said.
By Monday morning, forecasters at the National Weather Center, home of the storm lab and storm prediction center, knew "that any storm that formed in that environment had the potential to be a strong to violent tornado," he said.
"This is a pretty classic setup," Brooks said.
Tornadoes have two main ingredients: moist energy in the atmosphere and wind shear. Wind shear is the difference between wind at high altitudes and wind near the surface. The more moist energy and the greater the wind shear, the better the chances for tornadoes.
But just because the conditions are right doesn't mean a violent tornado will form, and scientists still don't know why they occur in certain spots in a storm and not others, and why at certain times and not others.
On Monday, the moist energy came up from the Gulf of Mexico, the wind shear from the jet stream plunging from Canada. "Where they met is where the Moore storm got started," Brooks said.
With the third strong storm hitting Moore in 14 years — and following roughly the same path as an EF5 that killed 40 people in 1999 and an EF4 that injured 45 others in 2003 — some people are wondering why Moore?
It's a combination of geography, meteorology and lots of bad luck, experts said.
If you look at the climate history of tornadoes in May, you will see they cluster in a spot, maybe 100 miles wide, in central Oklahoma, Houston said. That's where the weather conditions of warm, moist air and strong wind shear needed for tornadoes combine, in just the right balance.
"Central Oklahoma is a hot spot and there's a good reason for it," Houston said. "There's this perfect combination where the jet stream is strong, the instability is large and the typical position for this juxtaposition climatologically is central Oklahoma."
And the timing has to be perfect. Earlier in the year, there's not enough warm moist air, but the jet stream is stronger. Later, the jet stream is weaker but the air is moister and warmer.
The hot spot is more than just the city of Moore. Several meteorologists offer the same explanation for why that Oklahoma City suburb seemed to be hit repeatedly by violent tornadoes: Bad luck.
Of the 60 EF5 tornadoes since 1950, Oklahoma and Alabama have been struck the most, seven times each. More than half of these top-of-the-scale twisters are in just five states: Oklahoma, Alabama, Texas, Kansas, and Iowa. Less than 1 percent of all U .S. tornadoes are this violent — only about 10 a year, Brooks said.
The United States' Great Plains is the "best place on Earth" for the formation of violent tornadoes because of geography, Markowski said. You need the low pressure systems coming down off the Rocky Mountains colliding with the warm moist unstable air coming north from the Gulf of Mexico.
Scientists know the key ingredients that go into a devastating tornado. But they are struggling to figure out why they develop in some big storms and not others. They also are still trying to determine what effects, if any, global warming has on tornadoes. The jet stream can shift to cause a record number of tornadoes — or an unusually low number of them.
Early research, much of it by Brooks, predicts that as the world warms, the moist energy — or instability — will increase, and the U.S. will have more thunderstorms. But at the same time, the needed wind shear — the difference between wind speed and direction at different altitudes — will likely decrease.
The two factors go in different directions and it's hard to tell which will win out. Brooks and others think that eventually there may be more thunderstorms and fewer days with tornadoes, but more tornadoes on those days when twisters do strike.
"Tornadoes are perhaps the most difficult things to connect to climate change of any extreme," said NASA climate scientist Tony Del Genio. "Because we still don't understand all the factors required to get a tornado."