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Thu Apr 24, 2014 7:55am EDT

1 of 24. A large TVS (tornadic vortex signature) thunderstorm supercell passes over storm chaser Brad Mack in Graham, Texas late April 23, 2014. The thunderstorms were a precursor of what's forecast for this coming weekend, which could be the most significant multi-day tornado outbreak in the U.S. since 2011.

Credit: Reuters/Gene Blevins

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