NBC Nightly News with Brian Williams
Tornado watch issued for central and eastern Oklahoma, including Moore, Oklahoma City and Tulsa until 10pm CDT - via The Weather Channel

Tornado watch issued for central and eastern Oklahoma, including Moore, Oklahoma City and Tulsa until 10pm CDT - via The Weather Channel

BREAKING NEWS: Ricin letter mailed to President Obama containing threats similar to those sent to NYC Mayor Bloomberg, sources tell NBC News. Letter was intercepted by U.S. Secret Service.

BREAKING NEWS: Letters sent to NYC Mayor Michael Bloomberg preliminarily test positive for ricin; letters referenced debate on guns, NYPD says

Severe weather forecast for tomorrow through Friday - via The Weather Channel

Severe weather forecast for tomorrow through Friday - via The Weather Channel

Brian Williams interviews 6 amputee survivors of the Boston Marathon bombing. Interview airs Friday.

Brian Williams interviews 6 amputee survivors of the Boston Marathon bombing. Interview airs Friday.

Today’s severe weather forecast via The Weather Channel

Today’s severe weather forecast via The Weather Channel

Boston Marathon bombing victim attends senior prom, voted prom queenStory/video: http://nbcnews.to/146tJ9s

Boston Marathon bombing victim attends senior prom, voted prom queen

Story/video: http://nbcnews.to/146tJ9s

Just in: 4.6-magnitude earthquake 12 miles west of Santa Barbara, CA 

Just in: 4.6-magnitude earthquake 12 miles west of Santa Barbara, CA 

Boston Marathon bombing survivor Jeff Bauman throwing out the first pitch tonight at Fenway. (via @Boston_Police)

Boston Marathon bombing survivor Jeff Bauman throwing out the first pitch tonight at Fenway. (via @Boston_Police)

Fire burning after freight train derailment in Baltimore County, MD