Posted on 07 Jun 2011
Catastrophe modeling firm AIR Worldwide estimates that the outbreak of severe thunderstorms that struck the United States between May 20-27 this year will result in insured losses to residential, commercial, and industrial properties and their contents, and to automobiles of between $4 billion and $7 billion.
“The month of May, normally the most active month for tornadoes, began quietly,” said Dr. Tim Doggett, principal scientist at AIR Worldwide. “For three weeks, only a handful of isolated tornadoes were reported. But on May 20, severe thunderstorms in eastern Texas and parts of Arkansas and Oklahoma brought high winds, hail, and five reported tornadoes. Over the next seven days, more than 150 confirmed tornadoes raged across the heart of the country.
The severe weather funneled across a corridor that stretched from Lake Superior to central Texas and east through Missouri, Tennessee, Kentucky, Ohio, and to the East Coast, impacting more than 20 states in all. Thousands of buildings were damaged, hundreds more were completely destroyed, and more than a thousand people were injured.
”??It should be noted that none of the individual meteorological elements that gave rise to the outbreak of severe thunderstorms is unusual. Dr. Doggett commented, “Large, strong, jet stream disturbances happen occasionally; persistent low pressure frontal systems are common, especially in spring; and the storms that developed occurred where they are expected to occur at this time of year.
What is unusual is for all of the factors that contribute to the development of severe thunderstorms to have aligned themselves so optimally in the same place at an opportune time. To get optimal intense instability, shear, and lift all in the same place for a long period of time is a relatively rare circumstance.”
“However, this outbreak of tornadoes coupled with the unusually high number of tornadoes in April this year has turned what began as an unremarkable year into a year—so far—that has produced almost twice as many preliminary tornado reports as the average since 2005, and that is on track to rival the very active 2008 season. It is also becoming quickly apparent that 2011 will surpass 2008 in terms of insured losses from severe thunderstorm activity. Indeed, the two major outbreaks of this year—the first in late April, the second in late May—are the costliest on record.
”??According to AIR, the focus of this severe thunderstorm activity was a pair of strong low pressure systems that were preceded by one weaker low pressure center. The weak center of low pressure developed in eastern Colorado on May 20, giving rise to scattered severe thunderstorms in eastern Texas and Arkansas. As a cold frontal boundary became more pronounced in the area, a strong low pressure center formed in northern Nebraska, initiating much of the thunderstorm activities in that region on May 21.
Over the next two days, this extratropical low (a low pressure system that exhibits many of the characteristics of a migratory frontal cyclone—but at middle or higher latitudes) drifted northwestward into Canada. In its wake, its trailing cold front pushed eastward. On May 22nd, as enhanced winds in the jet stream moved over the cold frontal boundary, conditions became very conducive for the formation of rotating supercell thunderstorms over the entire region, from Minnesota to Texas. That afternoon there were widespread reports of hail and tornadoes, and the devastating EF5 tornado struck Joplin early that evening, just before 6:00 pm.
Dr. Doggett continued, “By May 24th, the first low pressure system had moved into the Canadian Maritimes, and a new low pressure system began to form in West Texas as another pulse of higher jet stream winds moved overhead. The coupling of the low pressure, the jet stream, and a dryline (a boundary separating humid Gulf air from arid West Texas air) triggered more supercell thunderstorms in Kansas, Oklahoma, and Texas. These storms produced strong, long-track tornadoes in central Oklahoma and very large hail in the Dallas/Fort Worth area. The low pressure system strengthened as it moved to the northeast over the next two days, producing numerous reports of tornadoes, large hail, and strong straight-line winds, including hail in excess of 4” in St Louis. As this second system finally moved off the east coast on May 27, large hail and strong winds were reported all along the east coast, from Vermont in the north to Georgia in the south.”
Damage by State
Characteristic of the similar but varying impact of the severe weather in the different states, Minnesota suffered a moderate amount of significant damage across portions of the Twin Cities metropolitan area, where more than 100 houses and several commercial properties were damaged, and many trees and power lines were knocked down. ??In Kansas, there were damaging winds and hail over much of the state, as well as 14 reported tornado touchdowns on May 21 alone. The city of Reading (population 231) was hardest hit, where 26 homes and ten commercial buildings were destroyed. The state capital, Topeka (population 127, 473 in 2010), suffered brief tornado touchdowns in its south and east sections, and hail as large as baseballs fell in some locations. ??Texas was impacted mainly in the north.
Severe thunderstorms, funnel clouds, and hail the size of tennis balls bombarded the Dallas and Fort Worth areas through the early evening of May 24th, smashing car windows, and damaging roofs. Eight confirmed tornadoes touched ground in the state, one of them in the town of Denton, about 40 miles north of Dallas and Ft. Worth. ??The severe thunderstorms impacted Indiana mostly in the southern part of the state. In Bedford, a small town (14,000 population) near Bloomington, several homes were destroyed and much of the town sustained significant damage. ??Missouri was most severely hit. The city of Joplin (population 49,000) lies in the southwestern corner of the state, just a few miles from the Kansas border.
In the early evening of May 22, an extraordinarily violent tornado—later rated an EF5, the highest possible on the Enhanced Fujita Scale with winds of at least 200 mph—touched down just inside the Missouri border. It cut straight across Joplin, then continued to the east. Several other tornadoes also touched down in Missouri that day. ??The tornado left a flattened path through Joplin three-quarters of a mile wide and 14 miles long. In nine minutes, more than 8,000 homes and apartment units, and more than 500 commercial properties were heavily damaged or destroyed. It was the deadliest tornado to hit the United States in more than half a century.