Julio gathered enough power in the Pacific to be upgraded from tropical storm to hurricane status Wednesday as it trailed Hurricane Iselle, which could hit Hawaii as early as Thursday.

The National Hurricane Center classified Hurricane Julio as a Category 1 storm, with winds of about 120 km/h. The storm was expected to strengthen slowly over the next day and a half.

Lixion Avlia said the system was swirling about 2,655 km 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's eye was about 1489 km from Honolulu on Wednesday morning.

Meanwhile, Canadians were told to avoid non-essential travel to Hawaii because of the storms. Canada’s Department of Foreign Affairs also warned any Canadians already in Hawaii to monitor local weather reports.

"Canadians seeking emergency consular assistance in the area should contact the Consulate General of Canada in San Francisco or the Emergency Watch and Response Centre in Ottawa," the department said on its website.

Hawaiian Airlines said it 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.

Ahead of this year's hurricane season, weather officials warned that the wide swath of the Pacific Ocean that includes Hawaii could see four to seven tropical cyclones this year.

"The central Pacific doesn't see nearly the activity that the Atlantic sees," said James Franklin, chief of hurricane specialists for the National Hurricane Center.

With files from CBC News