Hurricane Lane has soaked Hawaii's Big Island, dumping 30cm of rain in as many hours as the state endures its first hurricane since 1992.
The National Weather Service warned that some areas could see up to 30in before the system passes, with bands of rain extending 564km from the hurricane's centre.
The Category 4 storm is not projected to make a direct hit on the islands, but officials have warned that even a lesser blow could do significant harm.
Steve Goldstein, a meteorologist at the National Oceanic and Atmospheric Administration in Washington, said: "You do not need a direct strike to have major impacts from a hurricane this strong."
The centre of the storm was predicted to move close to or over portions of the main islands later on Thursday or on Friday, bringing dangerous surf of 6m and a storm surge of up to 1.2m, forecasters said.
As of 8am local time, the hurricane was 467km south of Honolulu and moving north-west at 11km/h. Maximum winds had weakened slightly to 210km/h, the National Weather Service said.
Pablo Akira Beimler, who lives on the coast in Honokaa on the Big Island, said roads had been cut off by landslides.
"Rain has been non-stop for the last half-hour or so and winds are just starting to pick up," he said as he posted videos online of trees being blown sideways. "Our usually quiet stream is raging right now."
Forecast track for Hurricane Lane and current hurricane warning for much of Hawaii. Hurricane Lane will likely bring life threatening conditions across Hawaii through Saturday with damaging winds, dangerous surf, coastal storm surge and intense flooding rains.
Hurricane Lane will bring life threatening conditions across Hawaii through Saturday with damaging winds, dangerous surf, coastal storm surge and INTENSE FLOODING RAINS. The heavy rains will lead to landslides and hazardous travel! pic.twitter.com/OU0Jac3Jbh
"We essentially have one way in and out of our towns so sheltering in place is the priority," he said on Twitter.
I have authorized an emergency disaster declaration to provide Hawaii the necessary support ahead of #HurricaneLane. Our teams are closely coordinating with the state and local authorities. You are in our thoughts! https://t.co/lrhwbdUavj
Elsewhere, two campers were reported trapped overnight in Waipio Valley, along the Big Island's northern coast. The campers called authorities but emergency crews could not mount a rescue operation.
"We can't go in because the roads – there's a river of water down there," Hawaii County managing director Wil Okabe said.
In addition, there are reports of boulders falling into a park in Hilo on the east side of the island.
Shelters opened on Wednesday on the Big Island and on the islands of Maui, Molokai and Lanai.
Officials have also been working to help Hawaii's sizeable homeless population, many of whom live near beaches and streams that could flood.
Hawaii Emergency Management Agency administrator Tom Travis said there is not enough shelter space state-wide and he advised people who are not in flood zones to stay at home.
Authorities also warned that the shelters are not designed to withstand winds greater than about 64km/h and that for most people they should be a "last resort".
Public schools on the islands have been closed for the rest of the week, and local government workers have been told to stay at home unless they are essential employees.
Meteorologist Chevy Chevalier said Lane may weaken to a Category 3 by Thursday afternoon but that would still be a major hurricane.
The central Pacific gets fewer hurricanes than other regions, with only about four or five named storms a year. Hawaii rarely gets hit, and the last major storm there was Iniki in 1992.