La Niña, Tornadoes Linked
Plains Affected by Weather 5,000 Miles Away

By Joseph B. Verrengia

                 The Associated Press
                 May 6 — Oklahoma’s catastrophic tornadoes
                 were influenced by La Niña, the weather
                 phenomenon 5,000 miles west of the prairie in
                 the Pacific Ocean, scientists say.
                      It’s part of the same widespread weather trend that
                 dumped a record 91 feet of snow on Mount Baker in
                 Washington this winter and triggered severe drought and
                 wildfires in Florida.
                      “The signal is there,” said Steve Byrd, science officer
                 for the National Weather Service in Omaha, Neb. “The
                 incidence of tornadoes on the central Plains is slightly
                 higher during La Niña.”
                      La Niña refers to a large, unusually cold pool of water
                 in the equatorial Pacific Ocean that develops every several
                 years, and influences weather patterns around the world.
                      It works in reverse of the more common and
                 better-understood El Niño, which is a warming of Pacific
                 waters.
                      La Niña has generated a stronger, colder jet stream in
                 the upper atmosphere, while warm, wet air at lower levels
                 has been pulled from the Gulf of Mexico into the Plains
                 states.
                      This combination generated the ferocious
                 thunderstorms that spawned dozens of twisters, including
                 a historic F-5 tornado in Oklahoma City. An F-5 is the
                 most powerful tornado there is. At least 41 people have
                 died in Oklahoma alone as the result of the severe
                 weather.
                      The current La Niña has persisted for nearly a year.
                 Most La Niñas last only several months, and there are
                 signs that this one is beginning to wane.
                      The weather pattern comes on the heels of an El Niño
                 that dissipated in 1998. Studies suggest tornadoes were
                 reduced by as much as 24 percent in some regions during
                 the most recent El Niño.
                      El Niños increase storm activity in California and South
                 America while calming things down in the Atlantic and
                 Caribbean. La Niñas, by contrast, reduce storms in
                 California but stir up trouble in other parts of the country
                 as well as in India and southeast Asia.
                      This spring, meteorologists in Nebraska and Iowa
                 already have tallied 17 tornadoes — five more than
                 normal by early May. “We’re already living up to
                 predictions,” Byrd said.
 
 

                 Copyright 1999 The Associated Press. All rights
                 reserved. This material may not be published,
                 broadcast, rewritten or redistributed.


 
UNL
University of Nebraska-Lincoln
High Plains Climate Center