Rescue crews continue to comb through the storm-swept streets of Joplin, Mo., following Sunday night's powerful tornado, which cut a swath of destruction through the center of the city more than six miles long and nearly a mile wide.
At least 116 people have died, according to emergency managers – the highest number of fatalities from a single tornado in over 60 years. At least 17 survivors have been pulled from the rubble the twister left in its wake.
The last twister with a similar toll appeared during a 1953 tornado outbreak in the Eastern US. Some 116 people were killed when a tornado struck Flint, Mich. The same outbreak sent a mile-wide tornado through Worcester County in central Massachusetts – a part of the country where tornadoes of that scale are rare – resulting in the deaths of 94 people.
On Monday, National Weather Service forecasters said the Joplin tornado packed winds of between 192 and 198 miles per hour, based on a preliminary survey of the damage. After more survey work the next day, they raised the estimated wind speeds to more than 200 m.p.h. on Tuesday evening, giving the tornado a rating of EF5 – the most-destructive designation, said Bill Davis, meteorologist in charge of the National Weather Service (NWS) forecast office in Springfield, Mo., who headed the survey team.
President Obama expressed his sympathy for the stricken and pleded federal support "until every home is repaired, until every neighborhood is rebuilt, until every business is back on its feet," according to a statement released by the White House on Tuesday.
Even as rescuers continued their search, the conditions remained ripe for additional strong thunderstorms – capable of spawning intense tornadoes – in a four-state area that included Joplin, according to the NWS Storm Prediction Center in Norman, Okla.
Severe weather over Monday night, during which two police officers searching for survivors were reportedly struck by lightning, hampered rescue efforts for several hours.
What's causing all the superstorms?
Tornadoes like the ones that hit Joplin on Sunday and Tuscaloosa, Ala., on April 27 are relatively rare, according to David Imy, a forecaster with the Storm Prediction Center. Some 75 to 80 percent of the tornadoes in the US are fairly weak, "with winds of 100 miles per hour, give or take some," he says. They tend to be short-lived.
More powerful tornadoes can see wind speeds above 200 m.p.h. and typically stay on the ground longer and cut longer paths, increasing the likelihood they can strike a populated area.
What has been triggering this Spring's spate of severe storms and chart-busting tornadoes?
The North Atlantic Oscillation, a see-saw circulation pattern over the North Atlantic, is as good a place to start as any, some researchers say.
The NAO has fallen into general pattern, during the past 23 months, that acts as a roadblock for the jet stream, a high-speed river of air energized by the temperature difference between polar air to the north and warmer air to the south. The jet stream in effect steers storm systems from west to east across North America.
Over the past 14 months, the jet stream's typical path across the Atlantic "has been blocked more often than it has in recorded history," says Jeff Weber, a researcher at the University Corporation for Atmospheric Research in Boulder, Colo., which oversees the National Center for Atmospheric Research there.
This roadblock pattern has remained strong and persistent through much of April and May, putting deep, south-dipping kinks into the jet stream. Storms move across the country more slowly, with more meanders than usual. The south-dipping kinks push the lingering storms
Meanwhile, powerful, cold storms have continued to roll in from the North Pacific. As these storms cross the Rockies, they are borne south along these kinks. The slowed jet stream allows storms to linger in the south, drawing more and more moisture and energy from the Gulf of Mexico.
When the cold and warm air masses meet, violent thunderstorms can develop along the storm front accompanying the system, especially in late spring, when the sun is higher in the sky and warms the surface longer than it does in winter, sharpening the temperature contrast.
For tornadoes to form, another ingredient must be present – wind shear, set up by changes in speed and direction of winds with altitude.
On May 24, for instance, the Storm Prediction Center forecast the strong likelihood for tornadoes in sections of Arkansas, Kansas, Missouri, and Oklahoma during the afternoon and evening, thanks in no small part to a strong jet stream aloft – moving in an east-northeasterly direction – atop lower-altitude winds, driven by the storm system itself that flew up from the south.
At this atmospheric crossroads, intense changes in speed and direction with height can set the entire, broad-scale "supercell" thunderstorm rotating, explains Mr. Imy.
Recent research has shown that a final necessary ingredient is additional shear within the first 2,000 to 3,000 feet above the surface, Imy adds.
This shear manifests itself as a vortex, oriented like a paper-towel tube sitting on a table. If the updraft in a thunderstorm is strong enough, it can in effect stand the vortex on its end. This upended vortex can ultimately spawn a funnel cloud.
The stronger the shear, the stronger any resulting tornado is likely to be – and the longer its track on the ground.
Supercell storms, like the one that struck Joplin, tend to generate the most powerful twisters.
With the storms that spawned the tornadoes across the eastern US in late April, as well as with the system that is still generating severe weather in the US heartland, "we had very strong shear," Imy says.
Superstorm-filled spring: Evidence of global warming?
The record-breaking burst of tornadoes at the end of April, as well as this week's events, have sparked speculation that global warming could be playing a role.
If that's the case, global warming's fingerprint is still too weak to pick out of the data, according to a preliminary analysis by Martin Hoerling, a researcher with the the National Oceanic and Atmospheric Administration's Earth Systems Research Laboratory in Boulder, Colo.
Tornadoes – even mile-wide twisters – happen on spatial scales far too small for climate models to deal with. But some scientists have started looking at the broader atmospheric conditions that can trigger the formation of thunderstorms capable of generating twisters.
In a draft analysis posted earlier this month, Dr. Hoerling analyzed data for the month of April for the lower Mississippi Valley between 1979 to 2010. At least for each April in his initial sample, the analysis shows no detectable trend in conditions that would generate tornadic thunderstorms.
Still, he writes, the lack of detection now "does not exclude that a future change" in the atmospheric conditions he analyzed "may occur" as humans continue to pump greenhouse gases into the atmosphere.
Another analysis by atmospheric scientist Roy Spencer at the University of Alabama in Huntsville suggests that over the past 60 years, the number of "intense" tornadoes in the US from March through August – twisters rated in the three highest categories of tornado intensity – have declined, even as global average temperatures have risen.