I'm not a pilot -- but I am a meteorologist

Turbulence on a TPAC -- on a specific day and route -- is nearly impossible to predict a few days in advance, assuming there will be no significant storms in the departure or arrival terminal area. With the exception of a vigorous storm enroute (not likely this time of year), the two things that can cause significant turbulence are:

• Passing through the edges into the core of the jet stream. But on an eastbound, remember that the jet stream is also your friend -- it gives you the the best tail wind to shorten your trip.
• Flying right in the tops of the clouds. Often going up a few thousand feet can alleviate the bumps, but especially early in a flight when the plane is heavy with fuel, this is not possible. And flying at a lower altitude means fuel burn penalty -- costly and not in the best interest of making it to the destination without a fuel stop. It also takes significant coordination with Oceanic ATC to change altitudes outside of the flight plan.