This is a part of the "discussion" that the NHC posts with their updates. This discusses some of the steering forces for Irma: ( source:
Hurricane Irma Forecast Discussion)
Irma has been moving a little south of due west today, and the
longer-term motion estimate is 265/11 kt. The hurricane will
reach the southwestern portion of a strong mid-level ridge that is
centered over the central Atlantic later today or tonight. This
should result in a westward, then west-northwestward turn over the
next 24 to 36 hours. This motion is expected to bring the hurricane
near or over the northern Leeward Islands on Tuesday night or early
Wednesday. A large mid-latitude trough that is predicted to deepen
over the eastern U.S. during the next few days is forecast to lift
northeastward late in the week, which is expected to cause the
subtropical ridge over the western Atlantic to build westward.
As a result, Irma is predicted to remain on a general
west-northwestward heading on days 3 through 5. The dynamical
model guidance is in excellent agreement through 72 hours, with
some increase in spread late in the period, however the typically
more reliable ECMWF and GFS are in very good agreement through day
5, and the new NHC track forecast lies very close to those models.