ΣΕΡΑΦΕΙΜ ΚΟΤΡΩΤΣΟΣ 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.
REUTERS/Gene Blevins