I remember Gilbert (a September 1988 storm). I disagee, there were several troughs that entrenched into the northern gulf, that perplexed the experts. you are correct, Gilbert did not vary at all from its 290 path. What Gilbert showed was that some storms can get so massive that nothing can influence them. As was Andrew in 1992
It has some to do with the size of the storm...and some with the strength. Its false to say that nothing can influence them. Andrew moved west because it had a ridge to its north. It recurved later. Gilbert did not stay on a 290 just because... The meteorology behind it is that the storm was so strong it puts large quantities of air out the top that subsided and warmed. This built the ridge to its north. However, had it been in front of a long wave trof...it would have recurved...like David, Gloria, you name it...they did it. Isadore is not that massive and not that strong yet. Since the steering currents are weak, it may build a slight ridge north of it that could lead to a westward movement. However, if it was clipping along at 14 kts to the WNW, you could bet your house that it would recurve if it was sitting at 33N/72W right now. There is a huge longwave to the west of that position. I don't care if Izzy was a 180KT Cat 5...with TS force winds out 300 NM from the center...it would not miss that trof. Storms do not miss major trofs in their path because they are too massive. They miss trofs because they put out enough air to warm the environment to the north of them and create a ridge. When you are dealing with major trofs...the air transport out in front of the trof is enough to carry the energy away from the storm...and the falling heights ahead of the trof are more than enough to take care of any environmental ridging produced by the storm.
Correction about Gilbert. Those were not major trofs. They were minor. Also...they never materialized. The models of the day kept showing them...but they were what we call minor short waves. They heat tranport from Gilbert warmed the environment enough to the north to create a small thin ridge by which it traveled. Again...David is a perfect example of a very big, strong storm that recurved. If the trof is strong enough and deep enough...you could put SuperTyphoon Tip up next to it and it is recurving.