There is currently a story circulating, allegedly from a Professor Herbert Raymond Brown, that a 300 meter diameter asteroid called DA2013cl2 will impact the Atlantic Ocean (or Canada or Mexico) on 25 March. This impact will allegedly kill millions first off, then exterminate the rest of humanity later on.
This is a hoax. There are a number of ways you can tell it is a hoax.
1) The story is illustrated with a picture of comet C/2011 L4 PanSTARRS, not an asteroid.
2) It does not have a real asteroid name, asteroids that do not have proper names (like 4 Vesta), are named year, alphabetical identifier, numerical identifier (eg 2012 DA14). There's a 2013 CL2, but that comes nowhere near us, there's a 2013 CL22, which has a 1/10,000,000 chance of hitting us in 2064, but nothing that fits the hoax description.
3) A 300 meter asteroid impact would be serious, with an impact energy of 1110 Mt or TNT, creating a crater 4.6 Km in diameter, but it's no civilisation killer. The Creataceous/Tertiary impactor that did in the dinosaurs (at least in part) was around 10 Km in diameter.
4) Outside of the various posts promulgating this story, there is no astronomy Professor Herbert Raymond Brown
5) There is no high risk asteroid of any sort in the asteroid hazard lists.