 However, hot, dry air flowing into the system from the south, combined with vertical wind sheer, kept the storm from strengthening as much at its maximum potential rate.By 19 March Fay's track had turned to the west-southwest, and over the next day it continued to strengthen in due to favourable upper level outflow and weak vertical sheer.On 21 March, Fay became a Category 5 cyclone on the Australian Region Tropical Cyclone Intensity Scale.