The Central Pacific Hurricane Center said the system's maximum sustained winds slowed from 70 mph to 60 mph. Iselle is expected to weaken further over the coming two days.
The Big Island remains under a hurricane warning, while a tropical storm warning extended across Hawaii's other islands.
The storm is expected to inundate Hawaii with 5 to 8 inches of rain, with up to a foot of rain in some spots.
Wind and rain has already knocked down trees, caused power outages and road blocks on the big island. No deaths or major injuries have been reported.