A new asteroid, 2012 TC4 was discovered by astronomers on Oct. 4 and is projected to pass within 59,000 miles of the Earth today - a close shot in cosmic terms. The object is about 56 feet across or 17.9 m. The questions being asked again by concerned Earthlings are: 1) Is this thing a threat? and 2) If by some chance it did strike how bad would it be?
In answer to the first, though the distance is indeed a close call, the odds of it actually being deflected to Earth are essentially about 1 septillion to 1. Maybe less. In the absence of a third body to cause a gravitational deflection, there's simply no reason for alarm.
As to the second question, we look to the "Torino scale" of asteroid devastation, which is something analogous to the Richter scale for earthquakes. In this case, the size (diameter) of below 20m doesn't even make the first 'cut' for "regionally devastating impact". This would be along the lines of the Tunguska, Siberia event of 1908, which devastated about 10,000 sq. kilometers.
Having said that, the diameter is close enough to the lower limit (20m) to cause at least massive damage and probably fairly extensive localized impact. To be more specific, if it somehow managed to hurtle down and impact a city the size of Miramar, FLA, it would likely leave a burning impact crater of perhaps 1500 m diameter and lots of surrounding devastation.
Let's put it this way: You wouldn't want to be anywhere in Miramar if it struck!
What about an ocean strike, say 100 miles off Miami Beach? Very likely we'd see a 100m tidal wave hit the coast, and again, Miramar and most of Dade County south of it would likely be inundated. (Florida, the east coast, is already at sea level.)