 Around the same time , the JTWC upgraded the system to a tropical storm , estimating the cyclone to have attained peak winds of 65 km / h ( 40 mph 1 @-@ minute sustained ) . On 19 May , satellite imagery of the system depicted that a new low pressure centre had developed roughly 300 km ( 190 mi ) south of the original low . Several hours after the relocation , the JTWC downgraded Herbie to a tropical depression as the system 's movement began to accelerate towards the southeast . As the storm moved at a rapid speed towards the coastline of Western Australia , it began to undergo an extratropical transition . During a 24 @-@ hour period ( 20 – 21 May ) Herbie tracked roughly 1 @,@ 500 km ( 930 mi ) , with the movement of the storm reaching 70 km / h ( 43 mph ) at times . 
