Scientist-astronomers who are going to be confronted with this should have some means of clearly communicating about it so as to clearly inform but not confuse or unnecessarily alarm the public. The likelihood of a collision is zero, or well below the chance that a random object of the same size will strike the Earth within the next few decades. This designation also applies to any small object to reach the Earth's surface intact. The chance of a collision is extremely unlikely, about the same as a random object of the same size striking the Earth within the next few decades. A somewhat close, but not unusual encounter.
Collision is very unlikely.
A close encounter, with one percent or greater chance of a collision capable of causing localized destruction. A close encounter, with a significant threat of a collision capable of causing regional devastation. A close encounter, with a significant threat of a collision capable of causing a global catastrophe. A close encounter, with an extremely significant threat of a collision capable of causing a global catastrophe. A collision capable of causing localized destruction.