I'm so-so at predicting swings in the price of BTC but this one's so gimme, I have to.  Shortly before the split, people will buy up BTC to hold onto for the long term because eventually the price will go up significantly.  That means a gigantic price spike!  Then, when the blocks actually do split, the price will start to float upwards until too many decide that's "good enough" and dump off their coins.  A little sustained $0.50 dip after weeks of slow raising will spook everyone into thinking they missed the opportunity to sell off their BTC at top price so there will be a massive dump.  The miners waiting for the price to stop going up to sell and pay off their ASIC mining hardware debts will dump em off too.  That seems extremely likely to happen and similar things have happened in the past.