I disagree with the sentiment that this story takes place "after the action." The action in this story isn't an object smashing into North America. The action is a global civilization coming to terms with an unprecedented catastrophe. That's very interesting to me, and very relatable. The (fortunate) reality is that more people have to come to terms with surviving catastrophes than actually get killed by them.
Unrelated, but I can't resist the opportunity to tease out some of the logistics of a continent-wide evacuation with six months to prepare.
How many people are we talking about? Well, per Wikipedia the population of North America is 538 million today. Reasonable projections put the population in 2025 somewhere in the 600 million range. We have six months to get 600 million people off a continent.
First, let's try ships. According to this article:
http://www.hellenicshippingnews.com/News.aspx?ElementId=984ef639-7f94-4d62-88a9-f80b3ecc6fb9 the gross tonnage of the world's commercial fleet is 1.01 billion (note that despite the confusing name gross tonnage is a measure of volume and carrying capacity). The last time that a government undertook to move people from one continent to another en masse without a lot of concern for comfort was a World War. So I looked into gross tonnage to passenger ratios for troopships. Now, it turns out that the US sends troops overseas in planes most of the time these days. But during the Cold War, the US built the SS United States, a passenger liner with the theoretical capacity to become a troop carrier. It's gross tonnage was 53,333 and it could carry 15,000 troops. That ratio comes out to 3.55 gross tons per person. So if you converted the entire shipping capacity of the planet to carrying people, it looks like could stuff about 284,000,000 folks on ships. But we only have six months to do it and we still need to feed everyone and move fuel around (the project of moving a continents worth of people is going to eat a lot of fuel). I would be surprised if the world, however united, could muster a third of that capacity for the task.
How about planes? Well, in 2012, 815 million US passengers flew an average of 1,389 miles each, for a total of 2.2ish trillion miles. So let's say for roundness that in the half year we have to get people out, we can get 1 trillion passenger miles of evacuation travel. (This is basically ignoring any travel within North America, say to get people from the middle swath of the continent o ports where all those ships are waiting). I suspect the mean center of population of North America is somewhere south of the current mean center of population of the US, Wright County, Missouri. So let's arbitrarily decide that the mean evacuee has to fly from Little Rock, AK. That's about 4,500 miles from either Europe or West Africa, 5,000 miles from Rio de Janeiro, and 6,500 miles from Tokyo. Let's figure the average evacuee has about a 5,000 mile trek ahead. Based on our 1 trillion passenger mile estimate earlier, that's another 200 million people, give or take.
Just from a raw carrying capacity standpoint, it is not only not unrealistic for the death toll in this scenario to be hundreds of millions, it is more likely than not. In the real world, with limited resources about to get a lot more limited when North America's food and oil production come to an abrupt halt, I think we could count it a major global accomplishment to get 50 million folks out.