May 20-27 U.S. Severe Thunderstorm Outbreak
6/6/2011 10:30:00 AM
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First Posting 6/6/2011 10:30:00 AM
Posting Date: 6/6/2011 10:30:00 AM
AIR Worldwide estimates that the outbreak of severe thunderstorms that struck the United States from May 20th to the 27th this year will result in insured losses to residential, commercial, and industrial properties and their contents, and to automobiles of between USD 4 billion and USD 7 billion. ISO’s Property Claim Services® (PCS®) has issued the event Catastrophe Serial Number 48.

The Event

The month of May, normally the most active month for tornadoes, began quietly. 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. More than 160 people were killed, most in Joplin, Missouri.

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 20th, 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 21st.

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. (For a NASA satellite animation of the line of thunderstorms that generated the Joplin tornado, click here.)

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 27th, large hail and strong winds were reported all along the east coast, from Vermont in the north to Georgia in the south.

Representative Summary

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. The governor declared a State of Emergency.

In Kansas, where the governor also declared a State of Emergency, there were damaging winds and hail over much of the state, as well as 14 reported tornado touchdowns on May 21st alone. The city of Reading (population 231) was hardest hit, where 26 homes and ten commercial buildings were destroyed, 30 homes and four commercial buildings sustained major damage, and several people were injured. 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, reportedly causing one injury. Windows were broken, air conditioners were blown off buildings, at least one large semi-trailer was overturned, and many trees were downed. Several thousand people lost power.

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, damaging roofs, and leaving nearly 70,000 homes and businesses without electricity. About 10,000 passengers were stranded at Dallas-Fort Worth International Airport when hundreds of flights were canceled. 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 twister caused substantial damage to numerous homes in a Denton subdivision, while in the nearby towns of Argyle, Azle, and Watauga, 100 mph winds smashed car windows and stripped roof tiles from homes.

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. At least 12 people were injured. Near Greensburg, farther to the north nearer Indianapolis, several houses and farm buildings were heavily damaged and buildings at the local fairgrounds were destroyed, while on the opposite side of town several trees were downed and about 15 to 20 homes sustained roof and/or siding damage. Farm buildings and equipment, trucks and other vehicles, and other homes and buildings also were damaged or overturned throughout the area, and many trees were uprooted or broken.

Joplin, Missouri

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 22nd, 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 governor declared a State of Emergency.

Figure 1. Devastation in Joplin, Missouri. Source: Missouri State Emergency Management Agency

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. Two fire stations were destroyed and eight primary schools were damaged or destroyed. St John’s Regional Medical Center was heavily damaged; although still standing, the entire hospital was shifted four inches off its foundation. Cell and landline telephone service was severely impaired, as was the city’s communication system. Almost 1,500 people were injured, and 138 killed. It was the deadliest tornado to hit the United States in more than half a century.

Putting It into Perspective

It should be noted that none of the individual meteorological elements that gave rise to the outbreak of severe thunderstorms is unusual. 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, as seen in Figure 2, 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 (Figure 3). It is also becoming quickly apparent that 2011 will surpass 2008 in terms of insured losses from severe thunderstorm activity; according to PCS, the industry sustained more than USD 10.5 billion in losses in 2008. Indeed, the two major outbreaks of this year—the first in late April, the second in late May—are the costliest on record.

Two things are important to note, however. First, the current count will be adjusted downwards as data that actually refer to multiple sightings of the same event are eliminated. Second, there is nothing to suggest that the higher-than-normal activity seen thus far this year will continue for the remainder of the season.

Figure 2. 2011 Daily Tornado Reports Compared to Running Average Trend. Source: Storm Prediction Center

Figure 3. Annual Trends of Preliminary Tornado Reports. Source: Storm Prediction Center

AIR Modeled Loss Estimate

Estimating losses caused by severe thunderstorm events in real time is an extremely challenging task. Because of the long duration of the entire phenomenon and the sheer number of preliminary reports coming in to NOAA’s National Weather Service (NWS) Storm Prediction Center (SPC), substantial data cleaning and validating is required to create a set of events to input into the AIR Severe Thunderstorm Model for the United States.

AIR's research team has processed SPC data from hundreds of "microevents" (individual tornadoes, hailstorms, and straight-line winds) that made up the outbreak—the "macroevent." This raw data, reported by local NWS branches, is likely to contain many duplicate reports since different witnesses will report the same event; for this reason, it is difficult to determine in real time whether an individual report is unique or not. To remove the duplicate reports, AIR researchers look for overlapping events.

In addition to the challenge that identifying and locating unique microevents presents, many of the events typically are of short duration (sometimes lasting only minutes) and usually occur out of the range of weather stations or anemometers, thus complicating the effort. Generally, exact intensities associated with these localized events—such as wind speeds and hailstone size—are not known. For tornadoes, storm parameters like damaging wind speeds and tornado path length and width, are estimated based on damage observed during on-going post-event NWS surveys. Usually this data-gathering and analysis process takes several months, only after which does the cleaned and validated data enter the official SPC database.

At the current early stage of this process, detailed information from the NWS is assigned to the raw data when available. However, many storm parameters remain missing. AIR simulates these parameters by using the distributions of event characteristics that are built into the AIR Severe Thunderstorm Model for the U.S.—while being guided by the available meteorological information about the event. Of the more than 100 simulated scenarios that were developed, five have been posted to the ALERT site to reflect the range of uncertainty.

A map of the resulting cleaned data (starting locations) is shown in Figure 4. Of these, non-loss-causing events—such as those that occurred in open fields and forests—were eliminated from the final simulation.

Figure 4. AIR Cleaned Dataset of Microevents Spanning the Period from May 20–27. Source: AIR

Despite the uncertainty surrounding the data in its current form, the AIR team has run the severe thunderstorm model using a variety of assumptions regarding storm parameters. As a result of their analysis, AIR estimates that losses to insured property from the activity spanning May 20 to May 27 are expected to be between USD 4 billion and USD 7 billion.

AIR's insured loss estimates reflect:

• Insured physical damage to property (residential, commercial, industrial, auto), both structures and their contents;

• Additional living expenses (ALE) for residential claims;

• Business Interruption losses;

• Effects of demand surge.

They do not reflect:

• Non-modeled losses, including loss adjustment expenses.

AIR will continue to monitor developments from this disastrous severe thunderstorm outbreak and will provide updates as warranted.

May 20-27 U.S. Severe Thunderstorm Outbreak
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