Hurricane Olivia became a tropical storm Monday but the Big Island and Maui are still bracing for what is forecast to be the worst initial impacts to the state from the second hurricane threat in four weeks.

The Central Pacific Hurricane Center said Olivia could bring some areas “significantly worse impacts” than Hurricane Lane, which spared the state devastating winds but caused destructive flooding last month.

Intense flooding was concentrated largely in Hilo. Olivia, however, is on a track to come closer to both Hawaii island and Maui while also delivering potentially damaging winds and rain to other islands.

Hawaii island, Maui County and Oahu are under tropical storm warnings, and the National Weather Service issued a flash flood watch for the whole state through late Thursday night. Such flooding can be life-threatening.

“Rain events of this size can cause widespread flooding and can affect areas that do not usually flood,” the weather service said. “Numerous landslides are expected, especially along the Hana Highway on Maui and the Mamalahoa Highway on the Big Island.”

Olivia is forecast to bring 10 to 15 inches of rain in isolated areas along windward portions of Maui and the Big Island, with some spots getting up to 20 inches.

The Hurricane Center said people on islands east of Kauai should finish preparing tonight for storm impacts that could include large surf in addition to flooding rainfall and damaging winds.

As of 11 p.m. Monday, the hurricane center reported that Olivia was packing winds near 70 miles per hour with higher gusts. Tropical-storm-force winds, defined as between 39 and 73 miles per hour, extended outward up to 105 miles from the center of the storm. Gradual weakening was forecast over the next 48 hours.

The storm was moving west toward the islands at about 9 miles per hour and was about 360 miles east-northeast of Hilo and 505 miles east of Honolulu.

The center of the storm is expected to move over the main islands late tonight into Wednesday, the hurricane center said.

On Oahu, Mayor Kirk Caldwell warned residents to remain vigilant.

“A tropical storm is just as dangerous as a Category 1 (hurricane),” Caldwell said. “So please folks don’t let your guard down. …. Be prepared.”

Caldwell said the city was preparing to open seven shelters on city and county land and one provided by a private organization, as opposed to using public schools for shelters as was the case for Hurricane Lane.

Hawaii County declared a state of emergency Monday afternoon to better react to storm impacts.

“I want you to know how unpredictable this is,” Mayor Harry Kim told emergency workers at a Hawaii County Civil Defense briefing Monday.

Hawaii County Civil Defense Administrator Talmadge Magno said impacts from the storm are expected to last about 18 hours with windward areas receiving the heaviest rains. County officials and the Hawaii Red Cross were preparing to open one emergency shelter per district for the island, Magno added.

Maui County officials did not announce any shelter openings Monday.

The state Department of Education said Hana High and Elementary schools will be closed today, and that all after-school activities — including interscholastic, athletic events and “A Plus” programs — are canceled today in Maui County.

State schools superintendent Christina Kishimoto said in a statement that the department understands that school closures may cause scheduling difficulties and asked parents to remain flexible and to check the department’s website for updates throughout the week.

“The safety of our students and staff is the highest priority and will guide any decision to alter school schedules,” she said.

Hawaii’s Public Charter School Commission website said Connections Public Charter School in Hilo will be closed today, and parents of children attending charter schools are advised to contact their schools for closure information.

The Hawaii Tourism Authority advised tourists to contact companies providing their air travel, lodging and activities to see if any adjustments are needed because of dangers.

“To Hawaii’s visitors, please listen to our airline, hotel and tourism industry professionals,” George Szigeti, HTA president and CEO, said in a statement. “They are well-trained for handling situations involving severe weather and do an excellent job of looking out for and taking care of our guests.”