Deadly twister outbreak smashes records

The tornado outbreak that devastated a vast swathe of the US south last month broke the previous record for the highest number of tornadoes in one event.

Three hundred and five tornadoes were recorded during the outbreak between 0800hrs on April 27 and 0800hrs on April 28. The figure easily surpasses the previous record of 148 twisters recorded in an outbreak that happened between April 3 and 4 1974.

Last month’s outbreak of storms also proved to be packing one of the most deadly tornadoes in memory. The Tuscaloosa-Birmingham tornado that happened during the event left at least 65 fatalities in its 80 mile long, 1.5 mile wide wake.

That’s the highest number of deaths from a single tornado in the US since 1955 when 80 people were killed in southern Kansas, 75 of which were in the small town of Udall.
The deadliest single tornado on record is the so-called Tri State tornado of March 18 1925 which left 695 dead in Missouri, Illinois, Indiana.

Altogether the April 2011 outbreak of tornadoes caused 340 fatalities during the 24-hour period from 0800hrs April 27.

The National Weather Service gave the event a rating of EF 5 on the Enhanced Fujita Scale, the highest possible and reflecting estimated wind speeds of at least 200mph.

 

Deadly confluence of conditions

But what are the ingredients for such a freakishly powerful tornado outbreak?

“This outbreak of tornadoes resulted from the confluence of atmospheric conditions that together created a prolonged period of tornadic thunderstorms in the southeast,” said Dr. Tim Doggett, principal scientist at catastrophe model consultants AIR Worldwide. “Colder air from the northern plains moved southward in the form of a cold front that became established throughout the southern Gulf States. This cold front interacted with warm, humid air from the Gulf of Mexico and resulted in the formation of thunderstorm activity.”

But the primary factor that controlled the severity of the event was the large and very strong jet stream disturbance that moved over the region on Wednesday (27th) and Thursday (28th), according to Dr. Doggett.

“The strength of this disturbance provided the needed energy to sustain a high level of activity and thus supported the development of high intensity tornadoes. At the same time, the large size of the disturbance allowed strong storms to develop over a larger area than normally is the case,” he explained.

The series of developments was similar to those of the April 1974 outbreak. That event also saw a very big, intense jet stream disturbance create both a high number of tornadoes, many of which were intense.

Experts point out that none of the individual weather elements – a strong jet stream and persistent frontal features - that created the outbreak are particularly unusual. What is rare is for all the risk factors to come together optimally in the same place at the right time.

The peak of US tornado season varies by geographic region but usually begins in the southern states during April moving through the southern plains in May and June. The peak time for twisters in the midwest and northern plains typically comes during June and July.
Insurers to foot the bill

Modelling insured losses from tornadoes, compared to hurricanes, is notoriously difficult. The hundreds of micro-events that make up an outbreak often happen beyond the range of weather stations, so wind speed measurements are not available. Also damage itself can be extremely localised, with a row of properties destroyed while houses across the street are left intact.

Insurers and reinsurers will help finance the recovery of areas devastated by the twister outbreak by paying claims on property insurance coverage.

Most weather-related losses are covered by homeowners and commercial insurance policies.

Eqecat, the cat modelling consultant, estimates that the insured losses resulting from the outbreak will be in the region of $2 billion to $5 billion.

Useful links:
http://www.noaanews.noaa.gov/april_2011_tornado_information.html
http://www.eqecat.com/catWatchREV/secureSite/report.cfm?id=318

www.air-worldwide.com

Comments

No comments