What Lixion's getting at with the "trough interaction aiding intensification" -- hopefully in layman's terms -- is that if the trough is of the right size and distance from the storm, it can interact constructively with the storm, whether via enhanced outflow (more often an upper-low), superposition (almost always an upper trough and not a cutoff upper-low), or some other unknown technique. Usually, the narrower (in width/scale) of the trough, the more likely that it is going to match the width/scale of the tropical system and be able to enhance development. This most often is via baroclinic enhancement leading to an enhancement of the processes that lead to tropical development -- maybe I'll go into the nuts and bolts of it later in the winter, but I'm too drained this evening (anyone who watched the football game tonight will know why!), so my apologies. Generally accompanying this interaction is a reduction in the negative impacts of the trough, primarily shear. Elena from 1985 in the NE Gulf is the best-known case and there is a lot of research out in the field about that one...I'm pretty familiar with it, so if anyone has any further questions about this process, please feel free to ask!

These trough interactions generally tend to aid moderate tropical storms to weak hurricanes, allowing them to reach up to borderline category 2/3 intensity (although this high end isn't reached that often). The other type, briefly alluded to above, occurs when you get upper-lows distant from the storm that aid in enhancing outflow. We saw that several times this year -- Rita is the best case, but Katrina and Wilma had it too -- and is most often seen with major hurricanes, allowing them to reach higher intensities than they might otherwise. Generally accompanying these situations is a strong upper-ridge building over a very deep circulation, further enhancing the favorable environment. Such an environment is generally pretty resilient to change, leading to eyewall cycles being big.

Of course, Zeta here is a case of the former rather than the latter. Hope this helps clear it up a tad.