In the most unpredictable presidential election cycle in living memory, even oddsmakers are scrambling to keep up with the myriad of twists and turns as Donald Trump and Hillary Clinton battle for the White House. Clinton had appeared to be heading toward the Nov. 8 election as an almost prohibitive favorite.

However, the FBI’s disclosure on Friday that it had discovered and was reviewing additional emails related to its investigation of her handling of classified information has caused another dramatic shift.

That latest “October surprise” immediately saw movement in the polls, with many suggesting that Trump had closed the gap nationally to just 1 or 2 points. Data analysis sites have followed suit. While Clinton is still the favorite, Nate Silver’s FiveThirtyEight now gives Trump a 24 percent chance of becoming president of the United States, double what it was two weeks ago.

Hillary Clinton U.S. Democratic presidential nominee Hillary Clinton holds an unscheduled news conference to talk about FBI inquiries into her emails after a campaign rally in Des Moines, Iowa, U.S. Oct. 28, 2016. Photo: Reuters

Betting sites, which tend to be less emphatic in their predictions as a result of having real money on the line, have also seen a significant change. Such was the drama of the news on Friday the FBI was looking at emails found during an investigation into former U.S. Rep. Anthony Weiner, estranged husband of Clinton aide Huma Abedin, that online gambling site Bovada initially suspended betting.

When it reopened the book, Trump’s odds had come in from +375 (15/4) to +200 (2/1), equating to a 33.3 percent chance of winning. The odds of a win for Clinton, meanwhile, lengthened from -550 (2/11) to -300 (1/3), which translates to a 75 percent chance. More than 10 days after the final debate, such a shift is far from the norm.

“In previous elections, we wouldn’t expect to see much line movement this late outside of a final debate bump for either candidate,” said Bovada’s head oddsmaker, Pat Morrow. “Of course, this hasn’t been a typical election cycle.”

While the odds still suggest that a Clinton victory is highly likely, it may be worth bearing in mind that just ahead of the Brexit vote in June British bookmakers gave a "Remain" vote a 76 percent chance of prevailing.

And oddsmakers have far from ruled out another significant alteration ahead of Americans going to the polls.

“Whether it’s the FBI investigation, another WikiLeaks dump or the next Trump gaffe, we assume there’ll be much more line movement before Election Day,” Morrow added.