BOSTON – According to catastrophe modeling firm AIR Worldwide, following days of extended rainfall from a slow-moving storm system fed by tropical moisture, residents of several Southern U.S. states are facing historic flood levels that are still rising in some places. The flooding event began on March 8, when a southward shift in the jet stream led to the development of a low pressure system that pumped in water vapor and warm air from the Gulf of Mexico.
According to AIR, an anomalously strong upper-level low pressure system located over Mexico and the southern U.S. stalled out across the region, bringing multiple days of excessively heavy rain to Louisiana, Texas, and the surrounding areas of the Gulf of Mexico. The upper low originated from energy that moved across Southern California late in the weekend, a somewhat typical but slightly southward progression for a synoptic low pressure system. However, instead of crossing the Southeast, the subtropical jet stream dug progressively deeper, forming a strong trough that extended into Mexico. The low became strong enough to become cut off from the overall background atmospheric flow, which normally transports a system through a region. This slow-moving cut-off low tapped into a moisture source in the tropics over the Eastern Pacific, where sea-surface temperatures (SSTs) are at record warm levels, setting up an atmospheric river of moisture across the Southeast.
The upper low became one of the strongest ever recorded across Mexico with 500 mb heights at 5,580 meters, indicating deep cold aloft very far south. The gradient between this upper low and a strong upper ridge over the eastern U.S. helped intensify the southerly flow pumping moisture into the south-central states, according to AIR. As a result, a record-breaking atmospheric river set up across Louisiana and Texas bringing days of extensive, intense rainfall.
Record flood levels have been broken along several rivers and bayous, including the Sabine River at Deweyville, Texas (where a 130-year record was broken), Bayou Dorcheat at Lake Bistineau near Shreveport, Louisiana (where a 1991 record was topped by over 2 feet), Bogue Falaya River near Covington, Louisiana (where the river crested almost 3 feet above the 1993 record), Tchefuncte River near Folsom, Louisiana (where a 1983 record was broken), and Coldwater River at Marks, Mississippi (where a 1991 record was broken).
A “major disaster” was declared for the State of Louisiana on March 13. In southeast Texas, a State of Emergency was declared for 17 counties. A State of Emergency was also issued for Mississippi. Flood warnings remain in effect throughout northern Louisiana, and along rivers and streams in Mississippi, Arkansas, southeast Texas, and in southern Louisiana. Flood watches and warnings have affected more than 12 million people in the southern United States, according to reports.
At least 6,500 structures in Louisiana, Texas, and Mississippi have been damaged. In the small town of Deweyville, Texas, mandatory evacuations were enacted as the Sabine River rose to levels of 33.24 feet, breaking a 130-year old record by over a foot. On both sides of the Sabine River, which delineates the Texas-Louisiana border, authorities closed a section of Interstate 10.
More than 6,000 structures in Louisiana alone have been damaged by flooding, according to initial figures released on March 14 by the Governor’s Office of Homeland Security and Emergency Preparedness. Officials warned the count is likely to grow as damage is assessed in southeast Louisiana after water levels recede.
According to AIR, the vulnerability of buildings to flood damage depends on many factors, including construction and occupancy, and building height and age. In general, damage due to inundation tends to be nonstructural, affecting interior finishes, such as drywall, plaster, insulation, and flooring, in addition to building contents. In very severe inland flood events, high-velocity flood and debris can compromise the structural integrity of a building, which can lead to the collapse of foundations and the displacement of structural walls.
In Texas and Louisiana, more than 80% of the residential construction is wood, with an estimated 5% having basements. The presence of a basement increases the risk for contents and building damage. Over half of the commercial buildings are steel and concrete. Unlike residential structures, commercial buildings often are engineered and built to stricter standards, and are thus less vulnerable than single-family homes. Still, mechanical, electrical and plumbing (MEP) systems can experience severe damage, which results in high losses.
AIR will monitor the situation and will provide updates if warranted.
For more information, contact:
Kevin Long/Mary Bohenek
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