But why the sudden turn?
Because of the passage of an upper-level trough to the north, with Irma starting to feel the steering effects of the westerly flow of the midlatitudes. The forecast 500 hPa chart (around 18,000 ft) with heights and winds illustrate the predicted environment as the hurricane approaches Florida.
The trouble with such tropical-midlatitude interactions as hurricanes move north is that such situations can produce a lot of uncertainty, something suggested by the ensembles. It is sort of like jumping on to a rapidly moving merry-go-round. You know generally where you are going once you get on, but you are not sure which horse you will end up on. A very challenging forecast. But with a storm this large and powerful, anywhere in southern Florida will have serious impacts.

As I noted in my last blog, this is a VERY hard forecast, with the storm predicted to make a sharp right turn near a relatively narrow peninsula. Small errors in position and time of the turn have HUGE implications for the forecasts. The storm has also weakened substantially (now a category 3), with the latest model runs suggesting some intensification (perhaps to a cat 4, but NOT a cat 5).