Tropical Storm Ian, a strengthening storm moving through the Caribbean Sea, should reach peak intensity later this week as a category-4 hurricane.

According to a bulletin from Guy Carpenter, at this early stage its track and strength remain uncertain, though it suggested that the storm should begin a period of rapid intensification before passing near or over the Cayman Islands early today (26 September) and then moving into the south-eastern Gulf of Mexico on Tuesday. 

It added that a range of landfall scenarios from the southwest Florida Peninsula to the western Florida Panhandle are very plausible this week:

“The NHC best forecast splits the difference with a track through the Florida Big Bend region, but explicitly noting a lower-confidence forecast than usual after 72 hours and a need to stay informed as the event unfolds.”

The broker added that Ian is now a tropical storm with a somewhat disorganised structure but showing some signs of increased activity and organisation with gradual drop in pressure:

“Ian is moving through a moist environment with reduced wind shear over very warm waters, with additional storm ventilation from upper-level wind patterns. Accounting for these factors, the NHC best forecast takes Ian to hurricane status later today, and major hurricane status late Monday.”

“Rapid intensification is probable per the NHC. Ian should affect the Cayman Islands Monday as a major hurricane and retain this status as it emerges into the southeastern Gulf of Mexico on Tuesday. Further strengthening over very warm waters is expected after clearing Cuba, and Ian should reach peak intensity of at least a category-4 hurricane.”

It added that Later next week, wind shear should cause Ian to weaken and expand in size, although specifics will depend on the exact track and land interaction which remains a low confidence forecast.

Guy Carpenter said that hurricane to major hurricane conditions are probable for the Cayman Islands, and very possible from the southwest Florida Peninsula to the Florida Panhandle. Downed trees and power lines with resulting power outages can be expected along with variable property damage. Areas subjected to major hurricane wind conditions will experience more substantial damage to property and infrastructure. 

It said that storm surge and excessive rainfall are also probable, regardless of the track and intensity of the storm, which will cause coastal and inland flood impacts with resulting damage to property and infrastructure, though it t is too early to determine specifics for Florida as this will depend on the still uncertain track and intensity of the storm.