May 24, 2011

US Experiencing Deadliest Tornado Year In Six Decades

A top weather expert said Monday that the US is experiencing the deadliest year for tornadoes in nearly six decades, the AFP news agency reports.

The reasons for the spiking death tolls are more likely due to the rise in the number of mobile homes and the chance paths taken by a series of tornadoes that have targeted populated areas.

"This year is an extraordinary outlier," Harold Brooks, research meteorologist at the National Oceanic and Atmospheric Administration's (NOAA) National Severe Storms Laboratory in Norman, Oklahoma, said in a statement.

"This is the deadliest year for tornadoes in the US since 1953," he said, referring to June of that year when a tornado killed 90 people in Worcester, Massachusetts.

A tornado tore through Joplin, Missouri over the weekend, killing 116 people less than a month after the storms struck across seven states and killed 354 people.

Brooks said the contemporary U.S. tornado record, which dates back to 1950, "can be a difficult thing to work with."

However, he said when scientists examine the most complete records available and adjust for changes in how tornadoes were reported over time, "we see no correlation between global or US national temperature and tornado occurrence."

"Tornado deaths require two things. You have to have the tornado and you have to have people in the right or the wrong place," Brooks said in a statement.

"The biggest single demographic change that probably affects things is that the fraction of mobile homes in the United States has increased over the years," he said.

Tornadoes are formed when atmospheric conditions come together in a certain way.  The atmosphere is warm and moist at low levels, coupled with cold dry air above.

Winds must be increasing in speed from the Earth's surface up to elevations of about 20,000-feet.

"In April, essentially we were stuck in a pattern where that was the way things were for a couple of weeks, and that pattern didn't move so we had repeated episodes that were favorable for producing significant tornadoes," Brooks explained.

He said the weather phenomenon known as La Nina may have had a "relatively small impact" on producing the right conditions, but that is not entirely to blame.

"It's an area of research to try to identify why the pattern was so favorable and why it was favorable for so long."

He said the tornado record does not show a steadily increasing trend toward bigger deadlier storms.  "2009 was a really low year for tornadoes. Some recent years have been big, some recent years have been small," he said.

The deadliest outbreak on record was on April 3, 1974.  The "Super Outbreak" claimed 310 lives when 148 tornadoes over a 24-hour period swept across 13 states.

The single deadliest tornado in U.S. history killed 695 people when a massive twister tore up parts of Missouri, southern Illinois and southwestern Indiana in 1925.


Image Caption: NOAA satellite image of a thunder storm minutes before a large tornado formed over Joplin, Missouri. Credit: NOAA


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