Study: Spring tornadoes different from 1974 outbreak

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Published: July 21,2011

Tags: La Nina, severe weather, tornadoes, Weather

STARKVILLE — This spring’s rash of deadly tornadoes bears a striking resemblance to the Super Outbreak of 1974, but the summer weather that followed both of these La Nina springs is very different.

Grady Dixon, an associate professor of geosciences at Mississippi State University, said this spring’s tornadoes were caused by a predictable weather pattern.

“A strong low-pressure system moving across the Midwest, coupled with warm, humid conditions over the East and Southeast and a strong jet stream, caused the early spring tornado events we had,” Dixon said.

He said there is some evidence suggesting that such weather systems are more likely and possibly even stronger during La Nina years. Both 2011 and 1974 had La Nina springs.

“Through the end of May, 1974 saw 79 tornado days produce 515 tornadoes,” Dixon said. “For the same period, 2011 saw 65 tornado days yield an estimated 1,200 tornadoes.”

Dixon said the increased tornado count is a reflection of improved technology, public awareness and an expanded population. In other words, better meteorological tools and more people with cell phone cameras mean more tornadoes are spotted today than in previous years.

“Tornado numbers prior to 2000 may be quite low compared to reality,” Dixon said. “The tornado day values are considered more reliable.”

Nancy Lopez, U.S. Department of Agriculture physical scientist at MSU’s Delta Research and Extension Center in Stoneville, said La Nina, El Nino and neutral are designations that refer to weather patterns influenced by water temperatures in the equatorial Pacific Ocean.

“El Nino has warmer-than-normal sea surface temperatures in the equatorial Pacific. The opposite, or La Nina, is typically cooler than normal, or it can be considered neutral,” Lopez said. “All refer to episodic periods or phases in the Pacific that can affect weather patterns.”

In 1974, the summer following the severe tornado outbreak was relatively cool from June 1 through Sept. 30 across Mississippi, especially the Delta, Dixon said.

“1974 stood out as one of the lowest-yield years for U.S. agriculture, in part because of mild temperatures and an early September frost in the upper Midwest,” Dixon said.

Based on that pattern, the odds were better than 50/50 that this summer would also see relatively mild temperatures and ample precipitation, but that hasn’t been the case. Despite both 1974 and 2011 being La Nina years, the weather patterns for the two have been almost opposite.

“1974 was cooler and wet in the summer in Stoneville, but so far this year, 2011 has been very warm and dry despite coming off a La Nina phase,” Lopez said. “This hot streak has been a little prolonged.”

Joe Street, MSU Extension Service state leader for agriculture, said the success of any crop is dependent on a variety of factors, not just the weather at any one point in the growing season.

“Farmers are optimists, and they plant each year expecting to harvest good crops,” Street said. “Our scientists at Mississippi State are involved in ongoing research and development to ensure that producers have the best technology available that will allow their crops to withstand a variety of challenges from weather, disease and insects.”

This summer’s temperatures through July were showing just how difficult it is to predict weather, even with good historical data and sophisticated analysis.

“Most of this year has been above-normal for temperatures, but below-average in rain, except for April, which had more than average rainfall,” Lopez said.

Lopez said temperatures in the Mississippi Delta were 4.3 degrees above normal in June and have stayed hot in July.

“We were in La Nina this spring, and we’re neutralizing and supposed to stay neutral until fall,” Lopez said.

Source: MSU

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