Residents of a vast stretch of Western Australia's coastline are being warned to prepare for two possible tropical cyclones developing offshore. A cyclone advice alert has been issued by the Department of Fire and Emergency Services for an area spanning from Onslow to Jurien Bay in the state's Pilbara, Gascoyne and Mid West regions. Tropical Cyclone Seroja is tracking south from its current position 720km northwest of Broome and is expected to make landfall as early as Sunday. A separate tropical low lying well to the south of Christmas Island could develop cyclone intensity in coming days as it tracks east, the Bureau of Meteorology said on Wednesday.