Forecast to strengthen to a Category 1 hurricane, Tropical Storm Ana was 545 miles southeast of Hilo as of 11 p.m. Wednesday, prompting the National Weather Service to issue a tropical storm watch for Hawaii County.

In anticipation of Ana’s arrival, the weather service issued a flash flood watch for Hawaii island from noon Friday through Sunday evening. “Tropical storm Ana is expected to bring heavy rain and thunderstorms to the Big Island then likely sweeping from east to west across the remainder of the island chain this weekend,” the advisory said.

At about 11 p.m., Ana’s sustained winds slowed to 60 mph, below Category 1 hurricane strength of 74 mph. The storm was about 545 miles southeast of Hilo and 755 miles southeast of Honolulu.

The storm is expected to take a slight northwest turn — toward Hawaii — tonight and become a hurricane Thursday, continuing to strengthen through Friday, forecasters said.

Ana’s latest track has it passing south or skirting the southern edge of the Big Island on Sunday and hitting Oahu and Kauai Sunday and Monday with hurricane-force winds of 75 to 90 mph.

The storm is moving west at 9 mph with tropical storm force winds extending out 65 miles from the center.

Forecasters caution that hurricanes can “wobble” and the track and intensity could change significantly over the next five days.

“There’s still uncertainty with the movement and strength of Tropical Storm Ana,” said Tom Evans, a meteorologist with the Central Pacific Hurricane Center.

The best-case scenario is that the storm passes far south of the islands and only brings high surf. The worst-case scenario is that it makes landfall on one or several Hawaiian islands bringing damaging winds, heavy rain leading to flooding, and storm surge, Evans said.

Large swells produced by Ana are expected to arrive in Hawaii starting late Thursday.

A NASA Gulfstream jet was expected to fly near Ana Wednesday to give forecasters more information about conditions around the storm. An Air Force hurricane hunter plane will fly through Ana on Thursday.

Forecasters are watching a high pressure system near the islands that could keep Ana on a more southerly path, Evans said. Another developing atmospheric weather system between Ana and the islands, called a trough, could create wind shear that might weaken the storm.

Hawaii island’s high mountains may also change the direction and intensity of the storm.

“The Big Island could have some impact, as it did with (Tropical Storm) Iselle,” Evans said. “But if the storm passes further south, it will have less of an impact.”

The weather service and civil defense officials say now is the time to prepare for the storm and residents of all islands should be ready for a direct hit from the storm.

Forecasters are likely to issue either a tropical storm watch or hurricane watch Wednesday. Watches are usually issued at least 48 hours before the storm arrives. A warning is issued about 36 hours before landfall, Evans said.

“Be prepared and stay informed,” Evans said. “The forecast is going to change over the next 48 hours.”