That is a $10 million dollar question. If airlines could answer it they would all be flying there. Turbulence occurs because of principally three atmospheric traits: Frontal systems, jet stream and oragraphic (terrain) factors. The last one is the easiest to answer. When strong winds blow over large mountain ranges it will cause turbulence. So part of the answer lies in where are the mountains, but the turbulence will only occur if a strong wind mostly the jet stream is striking those objects. So now we have boiled it down to two issues fronts and the jet stream. Both of these phenomena move and change hourly like the burbles in a river.

A research being conducted by NOAA has been trying to determine the answer to your question for years and at the cost of millions of dollars. See http://adds.aviationweather.gov/turbulence/
If you look at this site over a month or so you will see that turbulence occurs everywhere but at different times. If my eye is correct though the further south you go the less likely turbulence is to occur. The Jet streams are more a northerly phenomena as are strong weather systems driven by the Jet.

Now having said all that the Pilot observation over years is that there often is turblance along the west coast near Port Hardy, BC; over southern OR and Northern CA where the jet encounters the west coast and mountains, and over the Fair Weather range in AK.

Hope that helps.

PS if I knew how I would move this question to the Pilot question thread as it is just what that one is about.