Nate Silver expects small convention bounces:
[M]y research suggests that the volatility of polls before the party conventions is correlated with the magnitude of the convention bounces. Volatile polling years seem to predict larger convention bounces, but they are smaller when the polling has been more stable heading into the conventions. Since the polls have been especially steady this year, we should probably expect below-average convention bounces: perhaps more like four percentage points rather than the long-term average of around seven points.
Because the conventions “introduce a lot of noise into the system” Silver advises “reading the polls with a more of a jaundiced eye during the next several weeks.”