Pepeng added to the wreak caused by the earlier Typhoon Ondoy (Ketsana), affecting thousands of families on the north, especially on Pangasinan province where the San Roque Dam inadvertently released water to prevent its breach. However on the first week of October, Parma interacted with the incoming Typhoon Melor on the Pacific (via a Fujiwara interaction), rendering it stationary as it made landfall on Southern Taiwan. Days later, the greatly weakened Parma retreated back to Luzon making further landfalls on Ilocos Norte and Cagayan. The now severe tropical storm Parma then began to wane its strength as it crossed Luzon island for the second time. It then emerged on the South China Sea as a tropical depression. So far, typhoon Parma became one of the deadliest typhoons to hit the Philippines in a decade.

When the cyclones approach each other, their centers will begin orbiting cyclonically about a point between the two systems. The two vortices will be attracted to each other, and eventually spiral into the center point and merge. When the two vortices are of unequal size, the larger vortex will tend to dominate the interaction, and the smaller vortex will orbit around it.

The effect is often mentioned in relation to the motion of tropical cyclones, although the final merging of the two storms is uncommon. The effect becomes pronounced in these storms when they approach within about 1,450 km (900 mi) of each other and are at tropical storm strength or stronger.