April 14-15 U.S. Severe Thunderstorm Outbreak

First Posting | Summary
Posting Date: April 16, 2012, 1:00:00 PM

A series of severe thunderstorms produced damaging wind, heavy rain, up to baseball-sized hail, and many tornados across six states over the weekend of April 14 and April 15, 2012. Preliminary data from the National Weather Service (NWS) indicate that 122 tornadoes touched down during this most recent outbreak, 99 of which occurred in Kansas. Although a majority of these twisters tracked over sparsely populated areas, there were a few notable exceptions. In Thurman, Iowa, officials report that up to 90 percent of homes experienced some degree of damage. Across southwestern Wichita, Kansas, thousands were left without power. Five fatalities were reported in Oklahoma, where several tornadoes caused widespread property damage. Meteorological Summary This weekend's outbreak of storms is typical for this time of year in the central and Midwestern United States. The storm developed when a low pressure system and associated cold front that moved out of the Rocky Mountains and into the Central Plains interacted with warm, humid air from the Gulf of Mexico. Unstable conditions around the dry line—the boundary separating hot, dry air to the west from warm, humid air to the east—prompted thunderstorm formation by midday on April 14. Potent jet stream winds energized the system, producing widespread supercell thunderstorms that spawned tornadoes from Oklahoma up through Kansas, Nebraska, and Iowa during the afternoon. In addition to tornadoes, strong straight-line winds, heavy rain, and large hail were observed across the affected region. The largest hail associated with the system, 4.5 inches in diameter, was reported in Randolph, Kansas. A second burst of jet stream activity further enhanced the system, leading to the development of a squall line and additional tornado activity late Saturday night in Oklahoma. According to data provided by NOAA's Storm Prediction Center (SPC), as of April 16, the NWS received 110 tornado reports, 137 hail reports, and 72 straight-line wind reports on Saturday, April 14. An additional 11 tornado, 19 hail, and 18 straight-line wind reports were associated with the storm system on Sunday. To date, seasonal activity in 2012 is slightly higher than normal. However, it should be noted that a single strong event often offsets the annual total level of activity from the normal level. Similarly, quiescent periods could trend overall activity downward. The current NOAA forecast is calling for a period of quieter activity, which could revert the current anomaly back to near normal levels. Reported Damage Nearly 100 tornadoes were reported in central and southern Kansas, with the most severe damage occurring in the greater Wichita city area, where a state of emergency has been declared. Preliminary reports found damage consistent with an EF-3 tornado in Wichita (estimated population of 382,368 in 2010); this tornado ravaged heavily populated residential neighborhoods, and a few mobile home parks southeast of the city, in the Oaklawn district. Subsequently, the tornado hit the aviation firms Boeing and Spirit AeroSystems, which are located south of the city, damaging dozens of buildings and prompting the facilities to shut down. The McConnell Air Force Base also suffered tornado damage to facility buildings and a hangar. Unconfirmed reports suggest Wichita Mid-Continent Airport and the Kansas Aviation Museums were also damaged by tornadoes. Additional tornado damage was reported across Kansas. In the rural town of Kanopolis, preliminary reports indicate that an EF-4 tornado ripped apart a farm and damaged a few residential buildings. Tornadoes also caused significant damage across southern Iowa. The small town of Thurman (estimated population 207 in 2009) was especially hard hit by a tornado with a preliminary rating of EF-2, with winds up to 135 mph. It is estimated that up to 90 percent of homes experienced some level of damage. In the town of Creston, a tornado reportedly struck a hospital, damaging windows and the roof. The NWS reported around 10 tornadoes in Oklahoma. A deadly twister swept through Woodward city (estimated population 12,051 in 2009), killing five people. The fatalities are attributed to the town's failed warning system. Expected Pattern of Damage Residential structures in the affected area typically comprise wood-frame construction, which are more vulnerable to high winds and windborne debris than masonry structures. Commercial buildings are, on average, less vulnerable than residential structures or automobiles, but exhibit a broader damage distribution due to wide variations in construction practices and design. Light-metal structures are the most vulnerable to high winds and can suffer severe to complete damage from tornadoes categorized as EF-2 or higher. For all types of structures, roofs are often the first part to be damaged by tornado winds, as once a single shingle is removed, neighboring shingles can easily be penetrated and lifted. Tornado winds can peel off unsecured slates, roll metal roofs, and damage windward overhangs and eaves. In the direct path of the tornado, failure of the roof system weakens lateral support of walls, contributing to their collapse. On the periphery of the track, lower damage ratios are expected. Ongoing Damage Assessment Damage assessments from the National Weather Service are ongoing and it will take weeks to develop a more complete picture of the destruction. The AIR severe thunderstorm team is continuing to monitor reports as they come in to the NOAA Storm Prediction Center and the NWS. Additional information may be made available to ALERT subscribers, as warranted.

First Posting | Downloads
Posting Date: April 16, 2012, 1:00:00 PM