Following a forecast of "catastrophic" rainfall, flash floods and landslides, Hawaii Gov. David Ige declared a state of emergency for the tropical state.
“Now is the time to make sure you have an emergency plan in place and supplies ready should you need to move away from rising water,” Ige said in a press conference Monday night.
The city of Honolulu recorded its wettest December day on Monday after receiving 8 inches of rainfall. It also recorded the most single-day rainfall since 1958, when it received 15.32 inches.
"A kona low northwest of the islands will continue to bring the threat of heavy rain and a few thunderstorms with gusty winds to Kauai County and Oahu today, and potentially tonight," the National Weather Service Forecast Office in Honolulu said.
While Monday's flash flood warning for Oahu has since expired, there is still an active flood watch for the island.
Due to the heavy downpour, the Oahu Department of Emergency Management reported cases of vehicle rescues, water evacuation requests, inundated homes and road closures.
"The H-1 freeway and several roads in town are experiencing considerable flooding, so limit travel wherever possible," the department said.
Images and videos from Twitter showed indoor flooding and cars and buses driving in and stranded in floodwater.
ABC News' Daniel Peck contributed to this report.