Hurricane Irma, which was upgraded to a Category 3 storm on Thursday afternoon — and is likely to become a high-end Category 4 or 5 beast of a storm — is moving west over the open ocean about 720 miles west of the Cabo Verde Islands. On its current track, Irma is forecast to begin affecting the Leeward Islands on Tuesday, with Puerto Rico, the Bahamas, and possibly the mainland U.S. in its sights after that. There are a few factors that worry hurricane forecasters more about this storm when compared to the myriad other tropical storms and hurricanes that tend to form in the Atlantic. 

The observed fluctuations in strength during the past day or so are
likely to continue for about another day while Irma remains over
marginally warm waters and in fairly close proximity to dry air.
Eyewall replacement cycles, like the one observed yesterday,
could occur, but forecasting the timing and duration of these are
not possible. After 24 hours, Irma is expected to move over
progressively warmer waters and into a more moist environment.
These more favorable conditions combined with low to moderate wind
shear should allow the hurricane to strengthen. The NHC intensity
forecast follows the consensus aids HCCA and IVCN, and it is fairly
similar to the previous advisory.

Irma is now moving due west at 12 kt. A subtropical high pressure
system to the north of the hurricane is expected to strengthen and
build westward during the next couple of days. This pattern should
cause Irma to move west-southwestward during that time. Thereafter,
a turn back to the west and then west-northwest is predicted in the
3-5 day time period when Irma moves on the south and southwest sides
of the high. Although the models agree on the overall scenario,
there remains about 200 n mi north-south spread among the
best-performing models on day 5. The NHC track forecast has been
adjusted to the south at the longer-range points, and it is about
halfway between the latest runs of the GFS and ECMWF models.