The National Weather Service today expanded a tropical storm watch and a flash flood watch to include Oahu as a “weakening” tropical storm Felicia heads for the islands.

Despite the gradual weakening of the storm, forecasters warned residents to prepare to minimize damages from heavy rains, surf, and flooding.

The tropical storm watch was already in effect for the Big Island and Maui. A tropical storm watch means that tropical storm conditions are possible in the watch area within 48 hours.

At 5 p.m., the center of Felicia was 445 east-northeast of Hilo and 610 miles east of Honolulu. It had 50 mph winds and was moving west at 14 mph. Although the storm was weakening, it could reach the Hawaiian islands late tomorrow as a tropical depression or a tropical storm.  

“Regardless of its intensity, Felicia could produce very heavy rainfall with possible flash flooding beginning as early as Monday night,” a weather service hydrologic report said. The duration, amount and areas affected by rainfall are dependent on the track Felicia takes, the report said.

“There is considerable uncertainty in the locations that will be affected,” it said.

The weather service also issued a flash flood watch for the island of Oahu, beginning Tuesday morning.

“Felicia is expected to produce extremely heavy rainfall with the potential for widespread flash flooding, increasing on Tuesday and continuing into Wednesday,” the weather service said.

People in flood-prone areas need to prepare for the possibility of torrential rains and rock and mudslides.

Flooding rainfall could occur well away from the center of Felicia.

A building swell generated by the cyclone has already begun to reach the east-facing shores of the Big Island, the weather service said. The swell will reach other islands tomorrow and Tuesday.

A high surf warning has been issued for Maui and the Big Island from 6 p.m. tomorrow to 6 p.m. Tuesday. The surf will likely exceed the warning level of 15 feet tomorrow into Tuesday.

Public schools will remain open tomorrow, the Department of Education Web site said. The Education Department will continue to monitor Tropical Storm Felicia with state and county civil defense officials for possible school closures on Tuesday and Wednesday.

One charter school, West Hawaii Explorations Academy on the Big Island, will be closed tomorrow.

Felicia peaked as a category 4 hurricane with winds topping 140 mph on Thursday when it was still in the Eastern Pacific.