South-Central Severe Thunderstorm Outbreak

First Posting | Summary
Posting Date: April 28, 2014, 12:30:00 PM

Just two days after a series of tornadoes caused destruction in parts of North Carolina, a massive storm system on April 27, 2014, spawned dozens of tornadoes across at least seven states in the south-central U.S. On Sunday, an outbreak of intense thunderstorms accompanied by high winds, quarter-to-softball sized hail, and tornadoes flattened hundreds of homes and caused more than 100 injuries and at least 16 fatalities. While tornadoes were sighted in several states from Texas to Nebraska and Iowa, the most damaging twisters occurred in Arkansas. According to preliminary filtered reports from the NOAA Storm Prediction Center (SPC), 30 tornadoes have been produced by this severe thunderstorm outbreak so far. However, the SPC cautions that this is a preliminary number and the total may therefore be revised over the next several days.

Meteorological Summary

The outbreak was caused by a strong low pressure system that moved out of the southern plains and over the Great Lakes on Friday, April 25. This storm system was accompanied by a strong cold front that swept eastward over central Oklahoma and northern Texas. Ahead of the front, warm, humid air was drawn northward from the Gulf of Mexico. Deep and low-level shear provided the fuel needed to support long-lived, long-tracked supercells capable of producing strong tornadoes, large hail, and damaging wind gusts.

The SPC issued a “High Risk” warning for potential severe weather in Arkansas on Sunday. This was the first “High Risk” outlook issued by the SPC in 2014.

The visualizations below are provided by AIR’s sister company, Verisk Climate. The image shows radar-derived hail (yellow) and straight-line wind (blue) activity on April 27; the black triangles represent tornado observation reports. Verisk Climate clients receive these storm updates multiple times a day.

The radar-based analysis by Verisk Climate of the Mayflower / Vilonia Tornado in Arkansas (location indicated by the grey arrow) indicates that tornado damage was possible over a 48 mile long path, which was up to 1.0 mile in width (radar-indicated) near the city of Mayflower. Spatial analysis conducted using Verisk Climate's Respond™ service (swath not shown) indicates that commercial property west of Interstate 40 in Mayflower may have been the most heavily affected area in the town. The most intense portion of the tornado covered an area of approximately 19 square miles. The Respond service, which provided insight into this tornado as early as 10 p.m. ET Sunday, provides comprehensive, rapid information on the extent and severity of weather-related and other causes of loss for use by insurance claims and enterprise risk divisions. This radar content is provided exclusively to Verisk Climate by their partner Weather Decision Technologies.

Radar derived hail (yellow) and straight-line wind (blue) activity on April 27, with reported tornado observations (black triangles) overlaid. Source: Respond™ from Verisk Climate

Exposure at Risk

Residential houses in the affected areas typically comprise wood-frame construction, which are more vulnerable to high tornado winds and windborne debris than engineered structures. Commercial buildings are, on average, less vulnerable than residential structures or automobiles, but exhibit broader roof and window damages due to high wind and windborne missiles. Light-metal structures are the most vulnerable to tornadoes and can suffer severe to complete damage from tornadoes categorized as EF-2 or higher.

For all types of structures, roofs and curtain walls are often the first elements to be damaged by tornado winds. The high negative pressure of tornado creates suction load on the building, which can cause roof uplifting. Failure of the roof system weakens lateral support of walls and frames, leading to their collapse. Tornado winds can peel off unsecured slates, drip edges, roll metal roofs, roof-to-wall and wall-to-foundation connections, and damage overhangs and eaves. A direct hit by a tornado causes a catastrophic damage to a residential building and major damage to engineered structures. On the periphery of the tornado track, lower damage ratios are expected as the intensity of the tornado weakens.

Reported Damage

Until the National Weather Service completes damage surveys of the affected regions, the full impact of these tornadoes will not be known. Arkansas was most severely hit. Powerful winds unseated roofs from buildings, destroyed homes, and tossed empty big rigs nearly 100 feet into the air. In Vilonia, a town of 4,000, about 15 miles north of Little Rock, entire neighborhoods were leveled. While power has been restored to some customers, Entergy Arkansas has reported that over 16,000 homes and businesses are still without power as of April 28. Interstate40, littered with overturned vehicles more than 20 miles northwest of Little Rock, was shut down in both directions. The American Red Cross has been dispatched to the hardest hit areas, to assess the need for emergency shelters and other services. The state has requested a federal disaster declaration.

The severe storm also tore roofs off of homes across southeast Kansas. As of Monday morning, about 70 homes and 20 businesses in communities near Baxter Springs (population 4,200) have been damaged. Governor Sam Brownback has declared a state of disaster emergency for Baxter Springs. The storms also toppled a grain elevator onto railroad tracks, derailed six train cars, and damaged six homes in the southeastern Kansas town of Hammond, about five miles north of Fort Scott.

In Oklahoma, a tornado touched down near the town of Quapaw, a small town of about 900 residents. According to local officials, the tornado damaged or destroyed about 60 buildings, including Quapaw's volunteer fire department station and a bank.

Putting It into Perspective

While it has been a quiet start to the spring severe thunderstorm season, the long term average for tornadoes is relatively constant. One thing that may be happening recently is more variability around that long-term mean, and also variability in the starting date of the severe thunderstorm “season.” However, even a relatively quiet thunderstorm season does not necessarily translate into low losses. A single severe outbreak affecting metropolitan areas could cause atypically high losses.

Forecast

The severe weather threat continues today through much of the Southeast and Midwestern states, with the highest risk for tornadoes through Mississippi, Kentucky, Alabama, and Tennessee. High levels of instability, coupled with deep layer shear, will once again present the possibility of supercells and long-tracked tornadoes. An upper-level low has been slow to leave the area, providing ample destabilization of the atmosphere. Further north into Illinois, Indiana, and Ohio, up-scale growth of convection into more linear squall lines presents the risk of strong winds and large hail, with isolated tornadoes possible.

Ongoing Damage Assessment

As stated above, the National Weather Service is currently assessing the damage from these tornadoes, and a complete picture of the destruction may not be available for several days. The AIR severe thunderstorm team is continuing to monitor reports as they come in to the SPC and the National Weather Service, and will continue to follow the evolution of the current weather system. Additional information may be made available to ALERT subscribers, as warranted.

The visualizations provided by Verisk Climate are from the Respond™ service which provides rapid analysis after each storm. Users access the Respond analysis through seamless integration into their enterprise GIS systems or easy-to-use Benchmark™ web and mobile platforms. For more information, please contact Patrick Pollard, VP of Insurance Solutions, 781-761-2211 or ppollard@veriskclimate.com.

First Posting | Downloads
Posting Date: April 28, 2014, 12:30:00 PM