WASHINGTON Winter tornadoes that ripped across
parts of the American South this week were unusually lethal but
not particularly rare, a .government meteorologist said on
Wednesday as the death toll mounted.
Tornado season in the United States generally starts in
March and continues through the summer months but winter
tornadoes have become an almost annual occurrence, according to
Harold Brooks of the National Oceanic and Atmospheric
"While this is not a normal event, it's not an incredibly
rare event," Brooks, based at the agency's National Severe
Storms Laboratory in Norman, Oklahoma, said by telephone.
Tornadoes that rolled through Arkansas, Alabama,
Mississippi, Tennessee and Kentucky this week killed more than
50 people. Brooks said tornadoes in the southeastern United
States occur in winter "roughly once a year," he said.
The current tornado outbreak, which Brooks estimates
includes some 30 to 40 tornadoes, is similar to a March 1,
2007, outbreak that killed 20 people in and around Enterprise,
There were previous deadly tornado outbreaks on March 12,
2006, in Missouri and Illinois and on January 1, 1999, in
Arkansas and Tennessee, Brooks said.
The difference between these other three outbreaks and the
recent one is the death toll, he said.
Tornadoes develop in warm, moist air ahead of east-moving
cold fronts. There are 800 tornadoes reported in the United
States in an average year, resulting in 80 deaths and over
1,500 injuries, according to the weather agency's Web site
Big differences in temperature help fuel tornado
development by whipping up strong winds aloft where masses of
cold air and warm air meet. This year's cold northern
temperatures and warm air in the U.S. south created good
conditions for tornado formation, Brooks said.
Does climate change play any role in the frequency or
intensity of tornadoes? Brooks said no, adding that the
historical record of tornadoes is insufficient to let
scientists figure out what impact, if any, climate change has.
"Our current physical understanding of how tornadoes work
(is that) some of the ingredients that are important to make a
tornado will increase in a greenhouse-enhanced world, some of
them will decrease and the balance is unknown," Brooks said.
(Editing by Bill Trott.)