The hardest-hit community was Tuscaloosa. The top image, acquired on April 28 by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite, shows three tornado tracks through and around the city. The lower image, from April 12, shows the area before the storm. The tracks are pale brown trails where green trees and plants have been uprooted, leaving disturbed ground. Though faint, the center track runs from southwest of Tuscaloosa, through the gray city, and extends northeast towards Birmingham. Two other tracks run parallel to the center track. The northernmost track lies in an area where the National Weather Service reported a tornado, but no tornado was reported in the vicinity of the more visible southern track. In the southern region, strong winds were reported. The tornadoes were part of a larger weather pattern that produced more than 150 tornadoes across six states, according to the National Weather Service. The death toll reached 300 on April 29, making the outbreak the deadliest in the United States since 1974.
NASA images (April 12 and April 28) courtesy Jeff Schmaltz, MODIS Rapid Response Team at NASA GSFC. Caption by Holli Riebeek.
April 27 and April 28, 2011, brought the deadliest tornado outbreak in the United States since 1974. By dawn on April 28, at least 250 people had been killed in 6 states. Alabama was the hardest hit, with 162 confirmed dead as of Thursday morning.
The image below shows the storms at 1:45 p.m. U.S. Central Daylight Time on April 27, and the animation shows the development of the weather system that spurred the widespread tornadoes. The images are from the GOES satellite. The animation starts on April 26 and runs through the morning of April 28. The ingredients for severe weather are evident in the cloud patterns. A relatively stable mass of cold air—visible as a swirl of more-or-less continuous clouds—rotates in the north along the top of the image. Meanwhile, moist air pushes north and west from the Atlantic Ocean and Gulf of Mexico. The warm air contains small low clouds.
NASA image courtesy the GOES Project Science team. Animation by Jesse Allen. Caption by Holli Riebeek with information from George Huffman.
The collision between two such air masses is enough to generate severe weather, but the weather also was amplified by the jet stream on April 27. Though not directly visible in the image, the narrow band of fast-moving wind blew north and east between the two air masses. With surface winds blowing from the south and east, and the jet stream blowing from the west, powerful smaller-scale circulation patterns generated lines of intense thunderstorms. The thunderstorms shown in this image began to rise over Louisiana midway through the day on April 27—around 16:00 UTC or 11:00 a.m. local time—and moved across Mississippi and Alabama through the afternoon. Forming along and ahead of the turbulent boundary between the air masses, the storms move east and north with the jet stream. The bright white spots within the clouds are very active storms, quite probably the systems that generated tornadoes.
Throughout the day, many such storms arose over the South. The slow eastward progression of the upper-level system permitted successive storms to affect the same region for a much longer time span than usual. According to the National Weather Service, more than 150 tornadoes were reported throughout the day. The images are from the GOES satellite, a geostationary satellite whose orbit keeps it above the same spot on Earth at all times. Such an orbit allows the satellite to image the movement of clouds every few minutes, observations that are necessary for weather monitoring and forecasting. GOES is a joint NASA and NOAA satellite.
