"The JPL Center for NEO Studies (CNEOS) computes high-precision orbits for Near-Earth Objects (NEOs) in support of NASA’s Planetary Defense Coordination Office. These orbit solutions are used to predict NEO close approaches to Earth, and produce comprehensive assessments of NEO impact probabilities over the next century."

When assessing which end of the world scenario to make an event for, the approach of an asteroid came to mind - there's already a whole area of studies devoted to finding and neutralising potential impact threats... then, the concept of how an impact would be ~announced~ or communicated came to mind. Such a catastrophic event is so beyond human comprehension, it's almost impossible to imagine the Earth-wide mourning that would ensue.
Studying test-scenarios as provided on the 'Sentry' web site gives us a little insight on how things would proceed.

the red bar with the disclaimer is delightfully dramatic... something about this warning in itself gives the page an air of misplaced doom... don't you think?

The plain language and the clearly contrasting colours, the language of emergency - clear communication. How would one react to reading "This page DOES describe a real asteroid impact"? The sterile scientific language of end of the world scenarios is endlessly fascinating...

The press release is crucial - plain wording, again, the language of hazard and catastrophe needs to be carefully calculated - you must understand that everything will be destroyed, you must understand without fail or confusion...