A large TVS (tornadic vortex signature) thunderstorm supercell passes over storm chaser Brad Mack in Graham, Texas late April 23, 2014. The thunder storms on Wednesday were a precursor of what's forecast for this coming this weekend that could be the most significant multi-day tornado outbreak in the U.S. since 2011. Picture taken April 23, 2014. REUTERS/Gene Blevins
Storm chaser photographer Brad Mack checks out the radar on his laptop as thunderstorms supercells pass through areas in Erick, Oklahoma late April 23, 2014. The thunder storms on Wednesday were a precursor of what's forecast for this coming weekend Saturday, Sunday, and Monday that could be the most significant multi-day tornado outbreak in the U.S. since 2011. REUTERS/Gene Blevins
Huge lightning strikes cross the skies as thunderstorms supercells pass through areas in Archer City, Texas late April 23, 2014. The thunder storms on Wednesday were a precursor of what's forecast for this coming weekend Saturday, Sunday, and Monday that could be the most significant multi-day tornado outbreak in the U.S. since 2011. Picture taken April 23, 2014. REUTERS/Gene Blevins
Storm chaser photographer Brad Mack watches thunderstorms supercells pass through areas in Kamay, Texas late April 23, 2014. The thunder storms on Wednesday were a precursor of what's forecast for this coming weekend Saturday, Sunday, and Monday that could be the most significant multi-day tornado outbreak in the U.S. since 2011. REUTERS/Gene Blevins
Huge lightning strikes hit the ground along a highway as thunderstorms supercells pass through areas in Kamay, Texas late April 23, 2014. The thunder storms on Wednesday were a precursor of what's forecast for this coming weekend Saturday, Sunday, and Monday that could be the most significant multi-day tornado outbreak in the U.S. since 2011. Picture taken April 23, 2014.REUTERS/Gene Blevins
A storm chaser photographer looks at thunderstorms supercells pass through areas in Vinson, Oklahoma late April 23, 2014. The thunder storms on Wednesday were a precursor of what's forecast for this coming weekend Saturday, Sunday, and Monday that could be the most significant multi-day tornado outbreak in the U.S. since 2011. Picture taken April 23, 2014. REUTERS/Gene Blevins