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April 27 and 28, 2011 brought the deadliest tornado outbreak in the United States since 1974.

For the latest information on how to help tornado survivors and apply for assistance, please visit the Federal Emergency Management Agency’s (FEMA) blog.

The above animation is from from NASA Earth Observatory:

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.

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.