Julio gathered enough power in the Pacific to be upgraded to hurricane status Wednesday as it trailed Hurricane Iselle, which could hit Hawaii as early as Thursday.
Hawaii has been directly hit by hurricanes only three times since 1950, though the region has had 147 tropical cyclones over that time.
The National Hurricane Center classified Hurricane Julio as a Category 1 storm, with winds of about 75 mph (120 kph). The storm was expected to strengthen slowly over the next day and a half.
Lixion Avlia said the system was swirling about 1,650 miles (2,655 kilometres) east of Hilo and was on course to pass north of the Hawaii islands in three to four days. But Avlia said it's too early to predict its actual path.
Hurricane Julio is just behind the slightly weakened Hurricane Iselle, which could bring heavy rains, high surf and strong winds to Hawaii's Big Island Thursday. Iselle loomed about 600 miles east of the Big Island early Wednesday, spinning at 85 mph.
Hawaiian Airlines will waive reservation change fees and fare differences for passengers who need to alter travel plans because of the storms. The airline said fees will be waived for those who are ticketed to travel on Thursday and Friday. They will be allowed to change reservations for flights through Aug. 12.
The clustered storms are rare but not unexpected in years with a developing El Nino, a change in ocean temperature that affects weather around the world.
Much of Hawaii's archipelago is under a tropical storm watch or warning. Residents were stocking up on essentials Wednesday, and weather officials asked the whole state to prepare for flash flooding.
The last time Hawaii was hit with a tropical storm or hurricane was in 1992, when Hurricane Iniki killed six people and destroyed more than 1,400 homes, said Eric Lau, meteorologist for the National Weather Service in Honolulu.