I was simply going by remembered frequencies: every year since I started paying attention I've heard, at least once, something of the form "This year/season/month/day was (one of) the hottest on record in Ontario/Canada/America/the world." I therefore take the probability that at least one of these things happening to be quite high, and so the probability of specifically the U.S. having specifically a "historically hot" summer, although small, is by no means negligible. 10% is a reasonable rough estimate.
Did you know in certain parts of Europe, this winter was the first winter since 1945 where it has snowed for more than (some number) days before (some date) ?
Media like records, so they will report quantities that attain a record value.
As we did last year, use this thread to make predictions for the next year and next decade, with probabilities attached when practical.
Happy New Year, Less Wrong!