Tornado Activity More Likely Over Multiple Days, As Evidenced By This Week's Storms
May 1, 2014

Tornadoes Most Likely To Occur During Multi-Day Spans

Brett Smith for - Your Universe Online

Earlier this week, tornadoes ravaged the Midwest and southern states from Nebraska to Mississippi and a new report from a Purdue University tornado expert has found that substantial tornado episodes and particularly strong tornadoes are more inclined to take place within sustained bouts of storm activity lasting three or more days.

In a report published by the journal Monthly Weather ReviewJeff Trapp, a planetary sciences professor at Purdue, said a bout of 20 or greater reported tornadoes had a 74 percent chance of occurring throughout a period of tornado activity sustained for three or more days. Throughout those exact same periods, a tornado with an intensity that scored 3 or higher out of 5 on the Enhanced Fujita scale had a 60 percent chance of hitting, the report added.

"Two extreme tornado events last year led to 32 deaths, injured more than 377 and cost $2 billion in damage and inspired this study," Trapp said in recent statement. "Unfortunately, the devastating tornadoes these past few days, tragically, seem to be bearing out the results."

On Sunday, tornadoes touched down in areas of Arkansas, Oklahoma, Kansas, Missouri, Nebraska, Iowa and Mississippi. On Monday, Mississippi, Alabama and Tennessee were hit and North Carolina saw tornado activity on Tuesday.

The recent spate of tornados took place during the time multiple-day periods are most likely to occur – April through July, according to the new report which was inspired by Trapp’s participation in the National Science Foundation's Mesoscale Predictability Experiment (MPEX) to improve predictions of severe weather.

"The encouraging news is that the larger, more slowly evolving and moving systems that appear to contribute to multiple-day tornado periods may be more predictable," Trapp said. "The weather system responsible for the tornadoes this week falls in this category and was revealed in the forecast models at least five days in advance with good fidelity."

The Purdue professor based his report on National Oceanic and Atmospheric Administration's historical data spanning from 1983-2012, which included more than 3,100 tornado days and over 1,400 distinct periods of tornado activity. Multiple-day periods comprised 24 percent of the distinct periods of activity.

Data on this week’s tornado activity from NOAA’s GOES-East satellite was just released by NASA and an animation based on the data showed the progression and motion of the weather system that spawned the twisters. NASA’s Aqua satellite also gathered information on the storm system using its Atmospheric Infrared Sounder (AIRS) instrument. AIRS data revealed very chilly cloud top temperatures suggesting that the thunderstorms had powerful uplift which pressed cloud tops to the upper reaches of the troposphere.

“AIRS data shows spatial extent of strong convection (rapidly rising air that condenses and forms clouds) in the slow-moving severe storm system that spawned tornadoes in Arkansas, Oklahoma and Iowa,” said Ed Olsen, creator of the AIRS image at NASA JPL.

AIRS revealed that the most frigid cloud top temperatures spanned from eastern Nebraska, through western Iowa, western Missouri, northern Arkansas and southeast into northern Mississippi and Alabama.