The depression strengthened into Tropical Storm Isaac on September 28, just twelve hours after the cyclone's formation. Deep convection was slow to form that day since the system remained over cold waters that had upwelled in the wake of Hurricanes Gordon and Helene. The next day, the cyclone moved west-northwestward and started gaining subtropical characteristics while its convection deepened. Vertical shear weakened as the system moved away from the cold, upwelling waters and the core of Isaac started re-developing. Atmospheric conditions remained favorable and Isaac reached hurricane status on September 30. Isaac continued strengthening a little bit during the next day near Bermuda, reaching its peak intensity with sustained winds of 85 mph (135 km/h) and minimum central pressure of 985 mbar. Isaac's winds quickly weakened to 80 mph (130 km/h), but its central pressure did not change.