Ridges (high pressure) act as walls. For instance, when there’s a Bermuda high over the mid-Atlantic, that ridging causes tropical systems to stay to the south and not break north/east quickly.
Troughs (low pressure) act as steering currents, for the most part. Think of it as a big ditch and the tropical system as a big spinning ball. As the ball approaches the ditch, it falls into it and while it might spin enough to push through or “fill” the trough, it usually slows and follows the path of that ditch out to sea.
If you look at that forecast map, you’ll see that stationary front (trough) parked over the northern Gulf. As it degrades, it leaves behind weaker steering currents that help to guide the system to the east.
Since Isaac is going over both Hispaniola and Cuba at some point, the shear created from the mountainous terrain will tear apart the outer convection currents, and depending on how close it gets, the eye as well. If Isaac didn’t hit all that land first, I’d be concerned for a more forceful entry into the very warm Gulf and a battering-ram forward movements into the northern Gulf states.
Since it will be so weakened once it emerges in the Florida Straights, it’s likely to just meander up the west coast and break to the east somewhere over the Florida peninsula.
Very interesting! Thanks for the lesson....
Well.....this thing is moving west as I predicted. :( It just might end up in Louisiana after all.