Cyclone Pali is currently above the equator in the ocean between the Marshall Islands and Kiribati, about 1300km southwest of Hawaii.
A forecaster at the US National Weather Service in Hawaii, Alistair Foster, says Pali is one of the earliest recorded in the central Pacifc, where the cyclone season is not due to begin for another five months.
Mr Foster says it's not unprecedented to have out-of-season cyclones in an El Niño year, with warm ocean temperatures allowing them to develop.