Well, the simple answer to your question is: yes. But that doesn't really say a whole lot.
Early on, the high has more influence. Once the storm reaches the midlatitudes -- or a trough reaches into the tropics, whether the trough actually reaches the tropics or just erodes part of the ridge in the tropics -- the trough patterns tend to take over, leading to a more northward component of motion. Of course, there isn't always a trough to impact a storm's motion...but more often than not, there is.
To date, Ivan has been steered by the ridge. Right now, it is rounding the base of that ridge and should soon begin the feel the effects of the trough. In essence, in time, the ridge and trough will "combine" to form a channel for Ivan to follow to the north -- and either north-northwest or north-northeast, depending on the sharpness of the trough.
And that will be critical for the final landfall of the storm. North-northwest (~340ish degrees) will take it towards MS/AL. North-northeast (~020 degrees) will take it towards the central FL panhandle.
For effect, let's take a 20 degree difference averaged out from the T+48 hour position, 22.3 N and 85 W. The Gulf coast is roughly at 30 N. Assuming a due north motion, it is 855 or so km to the coast. A 20 degree shift to the left or right results in a 311km -- just under 200mi -- difference in landfall position. If you center this over a landfall point of Ft. Walton Beach from the 11pm NHC advisory, you could see a landfall anywhere from near Mobile, AL to Panacea, FL along the coast. This serves to highlight how crucial the timing and sharpness of the trough that will ultimately pick up Ivan is to the ultimate landfall position...as well as shows how difficult track forecasting can be, despite very precise measurements of the track.
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