A few minutes ago, CNN tweeted that 3,300 flights have been cancelled due to the impending blizzard. I was checking some information and ran across this at the American Airlines web site:
For example, assume you have a $300 "W class" fare. If all of the "W" seats are sold out (likely) they may charge, say, $600 Y fare on your new flight. I had this happen to friends of mine flying American Airlines back from Mexico after a hurricane caused a blizzard of cancellations.
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Addition at 9:50pm EST. Here is a discussion of this topic at Flyertalk. They are already noticing that most of the discounted inventory is already sold out.