Posted on 20 Apr 2011
According to catastrophe modeling firm AIR Worldwide, the severe thunderstorm outbreak that swept across the Midwest and southeastern United States over the weekend will likely produce significant insured losses in the states of North Carolina and Virginia. Although severe thunderstorm activity occurred in Oklahoma, Arizona, Alabama and Mississippi, this activity—with some isolated exceptions—primarily impacted rural areas; indeed, many of the tornadoes that occurred here touched down in open fields, and much of the damage was to trees, not properties. By contrast, in North Carolina, high winds caused significant damage to suburban neighborhoods as well as business districts.
According to Dr. Tim Doggett, principal scientist at AIR Worldwide, “Although April and May are the worst time for tornadoes in the South, this weekend’s storm system was unusual because of its size and duration. The storm formed when a strong area of low pressure that moved out of the Rocky Mountains and into the Central Plain last week dragged colder air southward from the northern plains to clash with warm, humid air from the Gulf of Mexico. These unstable conditions set up a potent frontal system that, as it moved eastward, triggered widespread supercell thunderstorms. As the jet stream overhead energized the system on Thursday, April 14, numerous tornado, hail, and wind reports were associated with this system.”
“The weekend’s severe thunderstorm outbreak began on Thursday, April 14, lasting through Saturday, April 16. It brought high winds, large hail and tornadoes,” continued Dr. Doggett. “Thousands of trees were toppled or snapped, and downed power lines cut electricity to tens of thousands of households. Roofs were ripped from homes and, in the most severe cases, homes and businesses were flattened.”
According to preliminary data issued by the National Weather Service’s (NWS) Storm Prediction Center (SPC), there were more than 240 reports of tornadoes over the course of the three days. On Thursday, an EF-3 tornado (wind speeds of 136 to 165 miles per hour) destroyed two schools in the small town of Tushka, Oklahoma, 125 miles south of Tulsa. Streets on which virtually every home was destroyed mark the twister’s path. Straight-line winds damaged barns and outbuildings in Wagoner, Oklahoma. At times during the course of the day, more than 40 counties in Arkansas and Oklahoma were under a tornado watch. In Texas, golf ball-size hail was reported in Cooke, Grayson, Denton, Fannin and Dallas counties.
According to AIR, on Friday, several homes were destroyed and several damaged by a tornado in the town of Sumter, Alabama. At least 50 homes were reported damaged and 10 destroyed by a tornado in Greensboro, and several mobile homes at a dealership in Hale, Alabama, were turned over.
By evening, 30,000 households were reported without power in the state, 12,000 of them in Tuscaloosa.
By Saturday, the severe weather was wreaking havoc in the Carolinas and southern Virginia. More than 250,000 households in North and South Carolina were left without power as strong winds downed power lines. In Moore County, North Carolina, where a tornado that would become an EF-3 first touched down, homes were blown off their foundation in one neighborhood. Nearby, the tornado blew the second story off several two story homes and the top floors off two apartment buildings. As the storm in Moore County continued northeast, gathering strength, it crossed into an industrial complex, destroying at least three large warehouses and several steel retail buildings. A report from the NWS’s preliminary damage survey of the EF-3 storm in this area indicates that wind speeds likely exceeded 160 miles per hour.
Dr. Doggett commented, “Though it had weakened en route to Raleigh, the tornado regained strength near the large city (population 400,000), achieving EF-2 strength and winds of 110 mph. It damaged nearly all homes in a Raleigh mobile home park; two mobile homes were tossed up to 50 feet from their foundation. Several two-story homes in a nearby neighborhood experienced heavy roof damage, and in downtown Raleigh, several major buildings were damaged.”
Golf ball sized hail was reported near Burlington, North Carolina, in the central part of the state. Also in central North Carolina, 82 homes were destroyed in Bladen County, which was particularly hard hit. On average, North Carolina normally gets about 19 tornadoes a year, according to the National Climatic Data Center. There are 97 preliminary reports of tornadoes in the state from this latest outbreak.
In Virginia, where damage was also significant, five tornadoes—including EF-2 tornadoes in Halifax and Gloucester counties and EF-1 tornadoes in Dinwiddie and Augusta counties, and an EF-0 in Rockbridge County—downed trees, disrupted power lines and caused minor to moderate damage to homes, prompting the governor to declare a state of emergency.
The effects of the tornados in the southeast were felt as far away as New Jersey, Pennsylvania, New York and Connecticut, where high winds and isolated flash flooding occurred in certain locations on Saturday night.
“On Friday, the storm moved eastward with widespread reports of tornados from Illinois to Mississippi and Alabama, as well as some isolated reports of hail in excess of 4″ in diameter,” continued Dr. Doggett. “As the low pressure system was finally steered to the east coast on Saturday, the warm front strengthened on the leading edge of the low pressure, which enhanced the thunderstorm activity. This enhancement occurred as the storm moved into the Appalachian Mountains, and it resulted in severe weather from South Carolina to Virginia. The long track lengths of the North Carolina tornadoes were due to their occurrence along the warm frontal boundary, which allowed these storm cells to maintain their inflow of warm, humid air for a longer duration than storms along the cold front.”
“The weekend’s storm system, while potent and producing widespread storm activity, is typical for this time of year, when unstable surface air masses and strong jet stream activity are present aloft. However, the activity in the mid-Atlantic region over the weekend was enhanced due to a more favorable orientation of the jet stream in that location, which increased the wind shear and vertical lift occurring in the system at that time. Increased wind shear and vertical lift are important ingredients for strong rotating supercell thunderstorms, and they helped to maintain last week’s severe thunderstorm outbreak all the way to the east coast.”
Modeling the losses from actual severe thunderstorm events in real time is an extremely challenging task. The hundreds of “microevents” (individual tornadoes, hailstorms and straight-line windstorms) that comprise the outbreak are typically of short duration and are, more often than not, out of the range of weather stations or anemometers. When no measured wind speeds are available, scientists must estimate wind speeds from observed damage.
AIR is continuing to monitor reports as they come in. Additional information may be made as warranted.