Billy Burns stands on his front porch talking to a neighbor after a tornado hit Pleasant Grove just west of downtown Birmingham yesterday afternoon on Thursday, April 28, 2011, in Birmingham, Ala. Firefighters searched one splintered pile after another for survivors Thursday, combing the remains of houses and neighborhoods pulverized by the nation’s deadliest tornado outbreak in almost four decades. At least 280 people were killed across six states — more than two-thirds of them in Alabama, where large cities bore the half-mile-wide scars the twisters left behind. (AP Photo/Butch Dill)

WASHINGTON
(AP) — Some of the killer tornadoes that ripped across the South may
have been among the largest and most powerful ever recorded, experts
suggested, leaving a death toll that is approaching that of a tragic
“super outbreak” of storms almost 40 years ago.

“There’s
a pretty good chance some of these were a mile wide, on the ground for
tens of miles and had wind speeds over 200 mph,” said Harold Brooks of
the National Severe Storms Laboratory in Norman, Okla.

It
may have been a single long-ranging twister that battered Tuscaloosa,
Ala., and then covered the 60 miles to Birmingham, Brooks said.

Only
1 percent of twisters reach the most powerful readings, but Brooks
thinks several of those that left death and destruction in Alabama and
five other states Wednesday fall into that category.

That speculation hasn’t been confirmed yet, but if it is, it’s no wonder so many homes were flattened and scores were killed.

Most
tornadoes are weak, so most reasonably built structures survive them.
The typical tornado is on the ground for a couple of miles and is a
couple hundred yards wide with half the wind speed of the storms that
barreled through the region on Wednesday.

It
was the deadliest day for tornadoes since a series of twisters killed
more than 300 people in 11 states in 1974, Brooks added. The death toll
from Wednesday has surpassed 250 and is rising. The worst day in
recorded history for storm fatalities is March 18, 1925, with 747
deaths.

“A big question is — the tornado in Tuscaloosa and Birmingham, is it the same one? I think they are the same,” he said.

This aerial photo shows the devastation of The Rosedale Court housing community in Tuscaloosa, Ala. on Thursday, April 28, 2011. A powerful and deadly tornado cut through Alabama Wednesday evening. President Barack Obama said he would visit Alabama Friday to view damage and meet with the governor and families devastated by the storms. Obama has already expressed condolences by phone to Gov. Robert Bentley and approved his request for emergency federal assistance. (AP Photo/The Tuscaloosa News, Dusty Compton)

Chris
Weiss, a tornado expert at Texas Tech University, said the storm that
spawned that tornado formed in Mississippi and “lasted over 300 miles,
and even for a super cell that’s pretty long.”

Tornado
outbreaks happen just about every year somewhere in the country. But
this time conditions were just about perfect for the series of powerful
storms, explained Jerry Brotzge, a senior research scientist at the
Center for Analysis and Prediction of Storms at the University of
Oklahoma. He noted that a deadly tornado in Oklahoma in 1999 also was
almost a mile wide.

Brooks
noted there was a trough in the mid-levels of the atmosphere over the
western U.S., with a strong jet stream coming across the southern U.S. A
trough to the west means winds blowing to the south, turning and then
moving back north at the same time a powerful jet of wind blows from the
west above.

And
that, explained Brotzge, results in an area “to the east of the trough
where you have warm, moist southeast winds at the surface and strong dry
winds from the west above … that creates the perfect scenario for
strong thunderstorms” and tornadoes.

Why was there such an active weather pattern?

“Causes
are always difficult to assign,” Brooks said. “A little bit has
probably been the weakening La Nina in the Pacific, but not all
weakening La Ninas are associated with lots of tornadoes, and we get
lots of tornadoes in other situations as well.”

La
Nina is an unusual cooling of the water in the tropical Pacific Ocean
that can change weather patterns around the world. The federal Climate
Prediction Center said last month that La Nina conditions were weakening
but could continue to affect weather for months.

Weiss
said there is no scientific consensus on whether climate change played a
role in this series of powerful storms. “The problem is trying to
relate a climate signal to a specific weather event is always
dangerous,” he said.

Deaths
from twisters have been declining in recent years because of improved
forecasts and increased awareness of them by people living in
tornado-prone areas, especially in smaller and rural communities.

Chad Willis searches through the remains of his mobile home which was rolled and crushed by Wednesdays, tornado near Tanner, Ala. on Thursday, April 28, 2011. (AP Photo/The Decatur Daily, Gary Cosby Jr.)

While
most Americans live in cities, urban areas actually cover only a
relatively small percentage of the country. The result is that tornadoes
occur more often in rural, sparsely populated areas.

That’s
led some people to believe twisters don’t strike cities. But the
National Climatic Data Center in Asheville, N.C., calls that a myth:
“Tornadoes have hit several large cities including Dallas, Oklahoma
City, Wichita Falls, St. Louis, Miami and Salt Lake City. In fact an
urban tornado will have a lot more debris to toss around than a rural
twister.”

While
May is historically the busiest month for tornadoes, they surge sharply
upward in April as warm weather begins setting in and dry western air
collides with warm moist conditions moving north from the Gulf of
Mexico.

Indeed,
the biggest tornado outbreak on record occurred April 3-4, 1974 when
147 confirmed twisters touched down in 13 states, claiming more than 300
lives in the United States and Canada.

However,
April 1957 was more like this year, recording several days with large
numbers of deadly twisters, said Brooks. By contrast April 1974 was a
relatively average month, he said, with one “ridiculous” day.

The
extraordinary swarm of tornadoes battering the country this month seems
bent on proving Mississippi State University professor Grady Dixon’s
point — Tornado Alley is a lot bigger than people thought.

While
that’s traditionally seen as a north-south swath of the nation from the
Dakotas to Texas with a second twister center — Dixie Alley — extending
across the South from Arkansas to Georgia, Dixon argues they are really
one big tornado risk area.

“Our
goal is to show that there really are no separate regions, it’s all one
large risk area that’s connected,” Dixon said, describing a study
scheduled to be published in the Bulletin of the American Meteorological
Society.

And the near record number of tornadoes reported this month has obligingly swept across both “alleys.”

Eaton reported from Norman, Okla.

SOURCE: The Associated Press