Look at the direction of the airflow on the left side of this image - northward. Hurricanes, even major ones, do not steer themselves, they are like spinning tops, and react to the air masses around them. The NHC has forecast that Irma has reached the westernmost edge of the Atlantic high that has guided Irma so far, and will now start reacting to that trof. The only question is just where that starts, and in what direction does it take her? But that trof would not allow any major westward deviations, unless Irma stalls completely until it passes, which is unlikely given Irma is already in contact with it.
