Well the "ridge" is from the high pressure. The basics are troughs and ridges. On the WV you can see the trough of low pressure squeezed in between the two high pressure systems. With the high pressure(ridge) to the east of Florida building in closer to the state. Currently it's over part of the Bahamas. What will happen is that Ivan will follow along the bottom of the high pressure until it finds the weakness between the two caused by the trough and it will make its turn there and head north. Once it makes the turn that is where it will be going. It will be pretty close to a due north movement once it's turned. Right now it's just a wait and see where the storm makes it turn. Ridge builds further west and the turn comes later, ridge recedes and the turn comes sooner.