Another round of heavy thunderstorms and intense rainfall affected the Southern Plains over the weekend. This follows a month of excessive rainfall in the area, with frequent periods of organized and slow-moving intense thunderstorms. Widespread and severe flooding has affected many areas, including flash flooding in both the Houston and Dallas-Fort Worth areas. Many area rivers have yet to crest after the last round of heavy rainfall.
It will take some time to fully assess the scope and severity of impacts of this event. Media reports indicate at least 31 fatalities in Texas and Oklahoma, with an additional ten reported missing. Damage to autos, homes, businesses and infrastructure has been widespread and severe. Thousands of autos have been flooded, and at least 1,500 homes have been affected in Harris County, Texas (this number may increase as survey teams assess the area).
General outlook for significant flooding. Does not depict smaller flood areas. Source: U.S. National Weather Service.
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Radar-estimated May rainfall. Source: U.S. National Weather Service
Radar-estimated May percent of normal rainfall. Source: U.S. National Weather Service
Last week, organized and slow-moving thunderstorm clusters produced intense and long-lived rainfall events over the Southern Plains, including notably severe events over the Houston and Dallas-Fort Worth areas. These events were enabled in the presence of moist, unstable air and a potent upper-level low. Upper-level mechanics also were especially supportive of long-lived and slow-moving organized thunderstorm clusters (called mesoscale convective systems). Rainfall rates often exceeded two inches per hour, and 24-hour rainfall accumulations exceeded 11 inches for areas around Houston, and seven inches for areas adjacent to Dallas-Fort Worth.
Excessive rainfall in urban areas overwhelmed drainage systems and also caused rivers to overtop their banks and spill into adjacent flood plains. The Austin, Houston and Dallas metro areas suffered especially severe flash flooding, with a dramatic rise in flood waters reaching historic levels at some river gauges. The intense rainfall immediately contributed to flash flooding, but also has and will continue to contribute to river flooding further downstream.
Flood watches have been issued by the National Weather Service (NWS) in downstream communities along the Arkansas River and Red River in parts of Arkansas and Louisiana. Although attenuation is reducing the overall impacts of the flood waters, many river gauges are still at or above flood stage in the surrounding area. Interests further downstream are preparing for the coming flood peak expected early this week.
While the worst of the rainfall has passed, some Dallas lakes are beginning to overtop due to previous persistent rainfall, and flooding downstream locations.
The weekend passage of a frontal boundary brought another round of intense rainfall to the affected areas of Oklahoma and Texas, which has contributed to the ongoing flood threat. However, conditions have improved behind the front, bringing a period of much-needed relief from a month of excessive rainfall.
Weather patterns over the last month have frequently enabled organized clusters of heavy thunderstorms. These thunderstorm clusters have often moved slowly resulting in excessive rainfall totals on an ongoing basis. Most of Texas and Oklahoma have seen monthly totals for May exceeding normal by 200 percent, with some areas even exceeding 400 percent. Local monthly amounts exceeding 20 inches have been reported. A multi-year period of severe drought in the Southern Plains has been all but eliminated over a month-long period.
Oklahoma experienced the wettest month in history during the month of May. Reports indicate that the final statewide average rainfall for May was 14.41 inches, which according to the state climatologist is 9.59 inches above normal. According to the Oklahoma Climatological Survey, the previous record was 10.75 inches, set in October 1941. The previous record for the month of May was 10.54 inches in 1957.
Thousands of vehicles were reported flooded. Media reports indicate that loss estimates from flooded autos was around USD250 million, and could reach USD350 million, according to the Insurance Council of Texas. About two dozen roads and some interstate entrance ramps were closed in one of the busiest commercial areas in northwest Dallas. Reports indicate at least 155 roads underwater or closed due to damage, with about USD27 million in infrastructure damage according to Texas Transportation officials.
According to reports, the Office of Emergency Management warned some areas in Dallas to expect rising waters and street flooding. Approximately 1,500 homes in Harris County experienced flood damage, and this number is expected to increase as damage assessment teams survey the area. Southeast of San Antonio, reports indicate more than 150 homes near the city of Victoria have been flooded or threatened by rising waters. Media reports indicate at least USD25 million in damage to critical infrastructure and utilities in Harris County, with more the USD4 million to public buildings in the Houston area, according to state officials.
President Obama has declared a major disaster in the state of Texas. According to the Federal Emergency Management Administration (FEMA), Texas carries nearly 600,000 flood insurance policies in force statewide as of March 31, 2015. The figure represents nearly USD375 million in premium.
Media reports indicate significant damage to public infrastructure including roads and bridges across the state from excessive rainfall and flooding this month. Numerous properties have been affected. Dozens of state highways were closed on Tuesday in 20 counties from central and northern Oklahoma to the borders of Texas and Arkansas.
Sources: Insurance Journal, Reuters, Associated Press, Wall Street Journal, U.S. National Weather Service.
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