Hurricane Lane weakened into a tropical storm as it moved closer towards the centre of the Hawaiian islands.
The system brought torrential rain to the Big Island, resulting in severe, widespread flooding, officials said.
Lane, with maximum sustained winds diminishing to near 110 kph, is forecast to make its nearest approach to land just west of the island chain over the weekend, bringing tropical storm-force conditions to Maui and the state's most populous island, Oahu.
Further north, a tropical storm watch was posted for the island of Kauai.
The biggest immediate danger was posed by flooding and mudslides that could grow worse the longer the storm lingers close to the islands.
"We do anticipate lots of rain from Lane in the coming hours," Governor David Ige said.
Emergency crews were responding to reports of people trapped in vehicles and homes along with landslides and fallen trees that have blocked major roadways on the island of Hawaii, popularly known as the Big Island, according to accounts on social media.
But forecasts made clear that Hawaii had been spared from the threat of its first direct hit by a major hurricane in a quarter of a century.
Lane was downgraded to a Category 2 hurricane, then to a Category 1, the lowest ranking on the five-step Saffir-Simpson scale, as it churned through the Pacific south of Oahu, the National Weather Service said.
It was downgraded again to a tropical storm shortly as its maximum sustained wind speeds fell below 119 kph.
The storm was expected to continue drifting slowly northward and weaken further still before trade winds overtake its forward momentum and begin pushing it sharply off to the west and away from land, forecasters told a news conference in Honolulu.
They said Lane, which peaked as a Category 5 hurricane earlier in the week, to diminish into a tropical depression by tomorrow.
More than 60cm of rain fell in a 36-hour period on the Big Island's windward side, where the Weather Service reported "catastrophic flooding" and wind gusts peaking at 108kph.
Forecasters said some parts of the island had received around 91cm of rain.
A number of structures on the Big Island were destroyed and some residents were reported to be fleeing their homes, said Melissa Dye, a Weather Service spokeswoman in Honolulu.
Fire department personnel have conducted several rescues of people stranded by high water on the Big Island since Thursday, mostly around its biggest city, Hilo, said Kelly Wooten, a Hawaii County civil defense spokeswoman.
Overnight, the National Guard and fire department rescued six people and their dog, and two hikers were rescued by helicopter near the camping destination of Waimanu Valley, Ms Wooten said. There were no injuries.
She said two Hilo-area neighborhoods were evacuated.
Flash flooding and mudslides on the Big Island have also forced a number of road closures and Governor David Ige urged residents to avoid any unnecessary travel.
