Emergency sirens blared along Oahu's North Shore, where rising waters damaged homes in a community world-renowned for its surfing.
Honolulu officials told residents on Friday to leave the area downstream of Wahiawa dam — long known to be vulnerable — saying it was "at risk of imminent failure".
There were no immediate reports of deaths or injuries, but some homes had been swept away, said Ian Scheuring, a spokesperson for Honolulu.
Crew searched by air and by water for people who had been stranded — efforts that were hampered by people flying personal drones to get images of the flooding, he said.
Dozens — if not hundreds — of homes had been damaged but officials have not been able to fully assess the destruction, Honolulu Mayor Rick Blangiardi said. Some 5500 people were under evacuation orders.
"There's no question that the damage done thus far has been catastrophic," he said.
Blangiardi said officials felt confident in the stability of the dams on the island, but that it was hard to predict how much rain would come and what it might do.
On Maui, officials issued an evacuation advisory for some Lahaina neighbourhoods after nearby retention basins neared capacity. Parts of those neighbourhoods were burned by the massive wildfire that destroyed much of Lahaina in 2023.
Officials have been watching dam levels since a storm last week dumped heavy rain across the state, which led to catastrophic flooding that washed away roads and homes. After the worst of it, a similar but weaker storm was forecast to bring more rain through this weekend.
"It's going to be a very touch-and-go day," Hawaii Governor Josh Green said in a social media post.
Most of the state was under a flood watch, with Haleiwa and Waialua in northern Oahu under a flash flood warning, according to the National Weather Service.
Parts of Oahu received up to 30 cm of rain overnight, further saturating the ground after the storm last weekend. Kaala, the island's highest peak, got nearly 40cm in the past day, NWS said.
Winter storm systems known as "Kona lows," which feature southerly or southwesterly winds that bring in moisture-laden air, were responsible for the deluges.
The intensity and frequency of heavy rains in Hawaii of have increased amid human-caused global warming, experts say.
The state has said the Wahiawa dam has "high hazard potential," and that a failure "will result in probable loss of human life".
The earthen dam was built in 1906 to increase sugar production for the Waialua Agricultural Company, which eventually became a subsidiary of Dole Food Company. It was reconstructed following a collapse in 1921.
The state has sent Dole four notices of deficiency about the dam since 2009.
"The dam continues to operate as designed with no indications of damage," Dole said in an emailed statement.
