Ooh goody, something new to worry about on the weather front!
Have you ever heard of the Fujiwhara effect? No, me neither. That is until I was perusing the YBW site earlier this morning.
It seems that with the current pile up of Hurricanes Igor and Julia in the Atlantic conditions could be conducive to the formation of this rare and potentially lethal phenomena.
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.
Seems that makes predicting the path that much more difficult. It'll be interesting to see what happens with these two...