Towering flash floods and an imminent dam failure in the northern part of Oahu triggered evacuation warnings in Hawaii on Friday, as the state continued contending with a powerful storm this week.

The waters came on quickly in the middle of the night, and videos on social media captured inundated streets and cars being swallowed by the muddy floodwaters.

As heavy rains continued to batter Hawaii, the Wahiawā dam on Oahu had water pouring over its spillway at 1,500 gallons per second on Friday morning. Oahu Emergency Management warned that the dam “may collapse or breach at any time”.

Honolulu officials told residents in an emergency message to leave the area downstream of the dam.

Emergency sirens blared along Oahu’s famed North Shore, where rising waters also damaged homes. Honolulu officials issued a “LEAVE NOW” evacuation order at 5.35am on Friday for Waialua and Haleiwa: “Extremely dangerous flooding and Wahiawa dam is high.”

It remained unclear how many residents would be able to evacuate, given that many roads were already inundated. A shelter set up at Waialua high school lost power early on Friday morning and had to be evacuated itself.

North Shore state representative Amy Perruso told Honolulu Civil Beat that emergency services were having trouble reaching people, and many residents’ vehicles were under water. “There’s no exit possibility for a lot of folks right now,” Perruso said.

The Honolulu emergency department said in the early hours of Friday: “If you are trapped, go to the highest level. Stay out of attics without a way to the roof.”

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.

Josh Green, the Hawaii governor, said in a social media post that the Hawaii national guard had been activated to respond to the flooding. “The storm of course is very severe right now, particularly on the northern part of Oahu,” he said, describing chest-high flood waters. “It’s going to be a very touch-and-go day.”

The flash flooding came only days after a multi-day storm brought more than a foot of rain to areas across Hawaii.

Most of the state was under a flood watch, with northern Oahu under a flash-flood warning, according to the National Weather Service, which reported “widespread life-threatening flash flooding”, particularly in Haleiwa and Waialua.

One shelter at Waialua high and intermediate school is being evacuated because of flooding, said Ian Scheuring, a spokesperson for Honolulu. There were about 185 people and 50 pets who had sought shelter there but are being bussed to another evacuation center.

As she prepared to evacuate to a friend’s home on higher ground, Waialua resident Kathleen Pahinui told the Associated Press in a phone interview that the ageing dam is a concern every time it rains.

“Just pray for us,” she said. “We understand there’s more rain coming.”

Molly Pierce, spokesperson for the Honolulu department of emergency management, said the evacuation order covers more than 4,000 people, though the number could be higher.

Officials issued a warning for the dam during heavy rain last week, but the water level receded as rain subsided.

“The water is actively running over the spillway right now,” she said.

The state regulates 132 dams across Hawaii, most of them built as part of irrigation systems for the sugar cane industry, according to a 2019 infrastructure report by the American Society of Civil Engineers.

In 2006, seven people were killed when the Ka Loko dam on the island of Kauai collapsed and water rushed downhill.